Compounds, Molecules, and Bonds - MCAT Physical
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Which of the following is a polar molecule?
Which of the following is a polar molecule?
Of the answers, only H2O has a net dipole moment, making water the polar molecule. All the other molecules have balanced structures and no difference in electronegativity between side groups.
Of the answers, only H2O has a net dipole moment, making water the polar molecule. All the other molecules have balanced structures and no difference in electronegativity between side groups.
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How many valence electrons do boron and nitrogen have?
How many valence electrons do boron and nitrogen have?
To determine the number of electrons an atom has, you must look at which column the atom is in on the periodic table. Boron is in column 3A, so it has three valence electrons. Nitrogen is in column 5A, so it has five valence electrons.
You should be familiar with common elements, like nitrogen, without looking at the periodic table. This will save you time on the exam.
To determine the number of electrons an atom has, you must look at which column the atom is in on the periodic table. Boron is in column 3A, so it has three valence electrons. Nitrogen is in column 5A, so it has five valence electrons.
You should be familiar with common elements, like nitrogen, without looking at the periodic table. This will save you time on the exam.
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can be represented by three different Lewis diagrams. What is best term for this phenomenon?
The answer is resonance structures. When different Lewis structures can be drawn for a single molecule, the molecule exists as a composite of these structures, which are called resonance structures.
Isotopes refers to multiple nuclear compositions for a single element, based on varying numbers of neutrons. Isomers are different configurations of a given molecular formula, based on geometry and orientation. Epimers are a specific class of isomers involving a single stereocenter.
The answer is resonance structures. When different Lewis structures can be drawn for a single molecule, the molecule exists as a composite of these structures, which are called resonance structures.
Isotopes refers to multiple nuclear compositions for a single element, based on varying numbers of neutrons. Isomers are different configurations of a given molecular formula, based on geometry and orientation. Epimers are a specific class of isomers involving a single stereocenter.
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Diffusion can be defined as the net transfer of molecules down a gradient of differing concentrations. This is a passive and spontaneous process and relies on the random movement of molecules and Brownian motion. Diffusion is an important biological process, especially in the respiratory system where oxygen diffuses from alveoli, the basic unit of lung mechanics, to red blood cells in the capillaries.
Figure 1 depicts this process, showing an alveoli separated from neighboring cells by a capillary with red blood cells. The partial pressures of oxygen and carbon dioxide are given. One such equation used in determining gas exchange is Fick's law, given by:
ΔV = (Area/Thickness) · Dgas · (P1 – P2)
Where ΔV is flow rate and area and thickness refer to the permeable membrane through which the gas passes, in this case, the wall of the avlveoli. P1 and P2 refer to the partial pressures upstream and downstream, respectively. Further, Dgas, the diffusion constant of the gas, is defined as:
Dgas = Solubility / (Molecular Weight)^(1/2)
Carbon dioxide and oxygen have which type of intra-molecular bonds?
Diffusion can be defined as the net transfer of molecules down a gradient of differing concentrations. This is a passive and spontaneous process and relies on the random movement of molecules and Brownian motion. Diffusion is an important biological process, especially in the respiratory system where oxygen diffuses from alveoli, the basic unit of lung mechanics, to red blood cells in the capillaries.
Figure 1 depicts this process, showing an alveoli separated from neighboring cells by a capillary with red blood cells. The partial pressures of oxygen and carbon dioxide are given. One such equation used in determining gas exchange is Fick's law, given by:
ΔV = (Area/Thickness) · Dgas · (P1 – P2)
Where ΔV is flow rate and area and thickness refer to the permeable membrane through which the gas passes, in this case, the wall of the avlveoli. P1 and P2 refer to the partial pressures upstream and downstream, respectively. Further, Dgas, the diffusion constant of the gas, is defined as:
Dgas = Solubility / (Molecular Weight)^(1/2)
Carbon dioxide and oxygen have which type of intra-molecular bonds?
Note how this question has little to do with the passage. Often you will see questions like this on the MCAT. This is a simple, straightforward vocabulary question. The bonds between each compound are covalent. One might get confused and misunderstand the question to read inter-molecular bonds—those between different molecules—as opposed to the bonds within a molecule.
Note how this question has little to do with the passage. Often you will see questions like this on the MCAT. This is a simple, straightforward vocabulary question. The bonds between each compound are covalent. One might get confused and misunderstand the question to read inter-molecular bonds—those between different molecules—as opposed to the bonds within a molecule.
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Which of the following is not the correct geometric configuration for the given molecule?
Which of the following is not the correct geometric configuration for the given molecule?
Recall the following relationships between geometry and number of pairs of electrons on the central atom.
2: linear
3: trigonal planar
4: tetrahedral
5: trigonal bipyriamidal
6: octahedral
To visualize the geometry, we need to think of how many electron pairs are on the central atom. Drawing Lewis dot diagrams may be helpful here. None of the answer choices has lone central electron pairs, with the exception of water, so the number of atoms bound to the central atom is the same as the number of central electron pairs.
The only one that does not match up with the correct geometry is SF6, which is actually octahedral since it has six central electron pairs. In a water molecule, the central oxygen has six valence electrons, plus one from each bond with hydrogen, for a total of eight central electrons and four central electron pairs. So, this geometry is a variation on the tetrahedral form (bent), in which two central electron pairs are not bound.
Recall the following relationships between geometry and number of pairs of electrons on the central atom.
2: linear
3: trigonal planar
4: tetrahedral
5: trigonal bipyriamidal
6: octahedral
To visualize the geometry, we need to think of how many electron pairs are on the central atom. Drawing Lewis dot diagrams may be helpful here. None of the answer choices has lone central electron pairs, with the exception of water, so the number of atoms bound to the central atom is the same as the number of central electron pairs.
The only one that does not match up with the correct geometry is SF6, which is actually octahedral since it has six central electron pairs. In a water molecule, the central oxygen has six valence electrons, plus one from each bond with hydrogen, for a total of eight central electrons and four central electron pairs. So, this geometry is a variation on the tetrahedral form (bent), in which two central electron pairs are not bound.
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Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.
When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.
For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.
When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.
Dipole moment = charge * separation distance
Water is commonly called the universal solvent, in part due to its relatively strong molecular dipole moment. What is true of the oxygen atom in water's O-H bonds?
Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.
When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.
For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.
When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.
Dipole moment = charge * separation distance
Water is commonly called the universal solvent, in part due to its relatively strong molecular dipole moment. What is true of the oxygen atom in water's O-H bonds?
The oxygen atom in water has two lone pairs, though they are not as often drawn into depictions of the molecule as they are in nitgrogenous molecules, such as ammonia. They, along with the two bonds formed with hydrogen in stable water, make up the satisfied octet of oxygen.
The oxygen atom in water has two lone pairs, though they are not as often drawn into depictions of the molecule as they are in nitgrogenous molecules, such as ammonia. They, along with the two bonds formed with hydrogen in stable water, make up the satisfied octet of oxygen.
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Which is not a property of covalent compounds?
Which is not a property of covalent compounds?
The incorrect statement is a property of ionic compounds rather than covalent. Recall that electronegativity is a measure of the ability of an atom to draw electrons to itself. Ionic compounds are formed by elements with very different electronegativities, since elements with different electronegativities will tend to form positive and negative ions (that is, they give away or gain electrons easily). In contrast, covalent bonds are formed by elements which are close in electronegativity and could exist as stable free molecules. All other statements are true of covalent compounds.
The incorrect statement is a property of ionic compounds rather than covalent. Recall that electronegativity is a measure of the ability of an atom to draw electrons to itself. Ionic compounds are formed by elements with very different electronegativities, since elements with different electronegativities will tend to form positive and negative ions (that is, they give away or gain electrons easily). In contrast, covalent bonds are formed by elements which are close in electronegativity and could exist as stable free molecules. All other statements are true of covalent compounds.
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Which of the folling carbon-carbon bonds exhibits the stongest bond?
Which of the folling carbon-carbon bonds exhibits the stongest bond?
The suffix -yne tells us that this is an aklyne and contains a carbon-carbon triple bond. Ethyne has the stongest carbon-carbon bond because it has the shortest bond length. It is important to remember that as the bond length gets shorter, the bond strength increases, and vice versa.
The suffix -yne tells us that this is an aklyne and contains a carbon-carbon triple bond. Ethyne has the stongest carbon-carbon bond because it has the shortest bond length. It is important to remember that as the bond length gets shorter, the bond strength increases, and vice versa.
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What type of bond does O2 have?
What type of bond does O2 have?
O2 has a nonpolar covalent bond. Nonpolar covalent bonds are bonds formed between atoms that have the same (or nearly the same) electronegativity. Since both oxygen atoms have the same electronegativity, they will have a nonpolar covalent bond between them.
O2 has a nonpolar covalent bond. Nonpolar covalent bonds are bonds formed between atoms that have the same (or nearly the same) electronegativity. Since both oxygen atoms have the same electronegativity, they will have a nonpolar covalent bond between them.
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What type of bonds form between the two hydrogen atoms and one oxygen atom within a single water molucule?
What type of bonds form between the two hydrogen atoms and one oxygen atom within a single water molucule?
The intramolecular bonds within a water molecule are polar covalent bonds. O-H bonds are polar covalent since there is more electron density around the oxygen atom than there is around the hydrogen atom, making the oxygen atom slightly negative and the hydrogen atom slighty positive. This polarity allows for intermolecular hydrogen bonding; water does not exhibit intramolecular hydrogen bonding.
This bond is not ionic since the oxygen does not completly steal the electron away from the hydrogen (the electronegativity difference between the two atoms is between 0.4 and 1.7 on the Pauling scale).
The intramolecular bonds within a water molecule are polar covalent bonds. O-H bonds are polar covalent since there is more electron density around the oxygen atom than there is around the hydrogen atom, making the oxygen atom slightly negative and the hydrogen atom slighty positive. This polarity allows for intermolecular hydrogen bonding; water does not exhibit intramolecular hydrogen bonding.
This bond is not ionic since the oxygen does not completly steal the electron away from the hydrogen (the electronegativity difference between the two atoms is between 0.4 and 1.7 on the Pauling scale).
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Which of the following molecules contains polar, covalent bonds?
I. 
II. 
III. 
IV. 
V. 
Which of the following molecules contains polar, covalent bonds?
I.
II.
III.
IV.
V.
is an ionic compound, while 
is a nonpolar covalent compound. Remember that polarity results from a difference in the electronegativities of the atoms involved in the bond. Too great of a difference will result in an ionic bond; two of the same atoms will have zero difference, resulting in a nonpolar bond.
, 
, and 
contain polar covalent bonds. In the first two, oxygen will carry a slight negative charge, leaving sulfate and phosphorus with slight positive charges. In 
, chlorine will carry a slight negative charge, leaving the carbon slightly positive. In 
and 
, symmetry helps to balance the polar bonds, resulting in an overall nonpolar molecule, even though the individual bonds are nonpolar.
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Which of the following molecules contains the shortest bond between nitrogen and oxygen?
Which of the following molecules contains the shortest bond between nitrogen and oxygen?
This question is testing your ability to draw Lewis dot structures and your knowledge of how resonance effects bond length. The N-O bond with the greatest pi-bond character will be the shortest; thus, we are looking for a double- or triple-bond between nitrogen and oxygen.
Hydroxylamine (
) only contains single bonds, which have the least pi-bond character.
The nitrite and nitrate ions both have a double bond between the nitrogen and oxygen, but also one or more single bonds between these elements. This means that, because of resonance, the N-O bonds in these molecules will be averaged and our average bond order will be somewhere between single and double. Nitrate will have one double bond and two single bonds, for an average bond order of 1.33. Nitrite has one single bond and one double bond, for a bond order of 1.5.
The nitrosyl ion, however, will contain a triple bonds between the nitrogen and oxygen atoms, giving it the greatest pi-bond character. This bond will contain more energy and be shorter than the bonds in the other answer options.
This question is testing your ability to draw Lewis dot structures and your knowledge of how resonance effects bond length. The N-O bond with the greatest pi-bond character will be the shortest; thus, we are looking for a double- or triple-bond between nitrogen and oxygen.
Hydroxylamine (
The nitrite and nitrate ions both have a double bond between the nitrogen and oxygen, but also one or more single bonds between these elements. This means that, because of resonance, the N-O bonds in these molecules will be averaged and our average bond order will be somewhere between single and double. Nitrate will have one double bond and two single bonds, for an average bond order of 1.33. Nitrite has one single bond and one double bond, for a bond order of 1.5.
The nitrosyl ion, however, will contain a triple bonds between the nitrogen and oxygen atoms, giving it the greatest pi-bond character. This bond will contain more energy and be shorter than the bonds in the other answer options.
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When covalent bonds are formed __________.
When covalent bonds are formed __________.
Covalency is a form of electron sharing that lets an atom fulfill the octet rule. The sharing may be unequal, in which case the more electronegative species more strongly attracts the electrons than the weaker, less electronegative species, creating a polar covalent bond. In the Lewis dot model, however, we draw the electrons in freeze-frame, equally distributed between the various atoms.
Covalency is a form of electron sharing that lets an atom fulfill the octet rule. The sharing may be unequal, in which case the more electronegative species more strongly attracts the electrons than the weaker, less electronegative species, creating a polar covalent bond. In the Lewis dot model, however, we draw the electrons in freeze-frame, equally distributed between the various atoms.
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Which of the following represents a triple bond?
Which of the following represents a triple bond?
A sigma bond is a single covalent bond, involving an electron pair located between the two bonding atoms. A pi bond occurs when the p orbitals above and below the bonding atoms overlap, or when the p orbitals to the left and right overlap. In any covalent bond, the first bond formed is a sigma bond and any additional bonds must be pi bonds. Initial orbital overlap always comes from the sigma, or s, subshell; subsequent overlap comes from the pi, or p, subshells.
A sigma bond is a single covalent bond, involving an electron pair located between the two bonding atoms. A pi bond occurs when the p orbitals above and below the bonding atoms overlap, or when the p orbitals to the left and right overlap. In any covalent bond, the first bond formed is a sigma bond and any additional bonds must be pi bonds. Initial orbital overlap always comes from the sigma, or s, subshell; subsequent overlap comes from the pi, or p, subshells.
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Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.
When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.
For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.
When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.
Dipole moment = charge * separation distance
A scientist is investigating the polar nature of several compounds. He compares the vapor pressure of water to the vapor pressure of an assortment of low molecular weight hydrocarbons. What is he most likely to find?
Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.
When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.
For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.
When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.
Dipole moment = charge * separation distance
A scientist is investigating the polar nature of several compounds. He compares the vapor pressure of water to the vapor pressure of an assortment of low molecular weight hydrocarbons. What is he most likely to find?
The strong polarity of water relative to hydrocarbons means that water will have a more difficult time breaking out of its liquid phase, and into its gas phase to generate a vapor pressure. Substances with a high vapor pressure generally have weaker intermolecular bonds and a lower boiling point.
The strong polarity of water relative to hydrocarbons means that water will have a more difficult time breaking out of its liquid phase, and into its gas phase to generate a vapor pressure. Substances with a high vapor pressure generally have weaker intermolecular bonds and a lower boiling point.
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Which of the following molecules contain intramolecular hydrogen bonds?
Which of the following molecules contain intramolecular hydrogen bonds?
The question is asking for intramolecular hydrogen bonds, meaning which of the following molecules will contain hydrogen bonds between the atoms within a single molecule. Hydrogen bonds exist only between a hydrogen and a nitrogen, oxygen, or flourine. Although acetone and dimethyl ether contain an oxygen that can make hydrogen bonds, the molecules themselves do not contain hydrogen bonds. These compounds form intermolecular hydrogen bonds only.
Para-Nitrophenol, similarly, will form intermolecular hydrogen bonds. The para positioning of substituents prevents them from interacting within a single molecule. Ortho-nitrophenol allows for such itneractions by having substituents on adjacent carbons. The hydrogen of the phenol and the oxygen of the nitro will form a hydrogen bond within a single molecule, therefore, ortho-nitrophenol is the only molecule present that contains intramolecular hydrogen bonds since it can form hydrogen bonds within itself.
The question is asking for intramolecular hydrogen bonds, meaning which of the following molecules will contain hydrogen bonds between the atoms within a single molecule. Hydrogen bonds exist only between a hydrogen and a nitrogen, oxygen, or flourine. Although acetone and dimethyl ether contain an oxygen that can make hydrogen bonds, the molecules themselves do not contain hydrogen bonds. These compounds form intermolecular hydrogen bonds only.
Para-Nitrophenol, similarly, will form intermolecular hydrogen bonds. The para positioning of substituents prevents them from interacting within a single molecule. Ortho-nitrophenol allows for such itneractions by having substituents on adjacent carbons. The hydrogen of the phenol and the oxygen of the nitro will form a hydrogen bond within a single molecule, therefore, ortho-nitrophenol is the only molecule present that contains intramolecular hydrogen bonds since it can form hydrogen bonds within itself.
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Which of the following is not true of hydrogen bonds?
Which of the following is not true of hydrogen bonds?
Hydrogen bonds are only formed between molecules with polar covalent bonds, and not in nonpolar moelcules. They result from the electromagnetic attraction between hydrogen (which is slightly positively charged) and an atom of opposite (negative) charge, namely the negatively charged end of a polar molecule. All the other statements are accurate.
Hydrogen bonds are only formed between molecules with polar covalent bonds, and not in nonpolar moelcules. They result from the electromagnetic attraction between hydrogen (which is slightly positively charged) and an atom of opposite (negative) charge, namely the negatively charged end of a polar molecule. All the other statements are accurate.
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Rank the following compounds in order of increasing polarity, starting with the most non-polar compound.
, 
, 
, 
Rank the following compounds in order of increasing polarity, starting with the most non-polar compound.
Polarity is determined by differences in electronegativity between the two atoms involved in a bond. A large difference in electronegativity will result in a more polar compound. Symmetry, however, can balance net polarities in bonds, can cancel the differences.
has tetrahedral geometry, and since the four groups attached to the central silicon atom are identical, this molecule has no net dipole moment due to its symmetry. It is the most non-polar compound.
In comparing 
and 
, recall that carbon, nitrogen, and oxygen are in the same row of the periodic table, but carbon is farther from oxygen than nitrogen. This means there is a greater electronegativity difference in 
than 
, and 
is going to be more polar.
Finally, 
is an ionic compound, so it is going to be the most polar of all four compounds.
Polarity is determined by differences in electronegativity between the two atoms involved in a bond. A large difference in electronegativity will result in a more polar compound. Symmetry, however, can balance net polarities in bonds, can cancel the differences.
In comparing
Finally,
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Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils.
Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here.
In this law, 
is the mole fraction of the solvent, 
is the vapor pressure of the pure solvent, and 
is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.
A scientist is studying a series of compounds at standard conditions. Of the compounds listed below, which is likely to have the highest vapor pressure?
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils.
Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here.
In this law,
A scientist is studying a series of compounds at standard conditions. Of the compounds listed below, which is likely to have the highest vapor pressure?
In this example, methane, 
, has the lowest molecular weight. Hydrocarbons that have the lowest molecular weight have the least opportunity for van der Waals forces to keep them from moving into the gaseous state; thus, they have the greatest tendency to form vapor and have the greatest vapor pressure. Recognize that the greater the intermolecular forces, the higher the boiling point and lower the vapor pressure.
Sodium chloride is a solid salt. Solids do in fact have vapor pressures, but the ionic structure of this salt makes it very low.
In this example, methane,
Sodium chloride is a solid salt. Solids do in fact have vapor pressures, but the ionic structure of this salt makes it very low.
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Which hydrocarbon has the highest melting point?
Which hydrocarbon has the highest melting point?
Melting points of hydrocarbons are determined by two main factors: length of the carbon chain and degree of saturation. Longer carbon chains will have higher melting points, and chains with more saturated bonds have higher melting points.
Of the given answers, 
has the longest carbon chain and is fully saturated. It will thus have the highest melting point.
Melting points of hydrocarbons are determined by two main factors: length of the carbon chain and degree of saturation. Longer carbon chains will have higher melting points, and chains with more saturated bonds have higher melting points.
Of the given answers,
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