GRE Subject Test: Chemistry : General Chemistry

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #151 : General Chemistry

What happens when a solution of \displaystyle NaCl is added to a solution of \displaystyle AgNO_3?

Possible Answers:

A synthesis reaction

A decomposition reaction

A combustion reaction

A combination reaction

A double displacement reaction

Correct answer:

A double displacement reaction

Explanation:

The chemical reaction that would occur is provided below:

\displaystyle AgNO_{3(aq)}+NaCl_{(aq)}\rightarrow NaNO_{3(aq)} + AgCl_{(s)}

This type of reaction is called a double displacement reaction in which there is an exchange of ions between two compounds. This type of reaction usually results in the formation of a precipitate or gas. In terms of the positive ions, the \displaystyle Ag^{+} switch places \displaystyle Na^{+} to form two new products.

Example Question #152 : General Chemistry

Which of the following types of reactions best describes the following reaction: 

\displaystyle C_{3}H_{8}\ +\ 5O_{2}\rightarrow\ 3CO_{2}\ +\ 4H_{2}O

Possible Answers:

Combustion

Synthesis

Catalytic

Alkylation

Correct answer:

Combustion

Explanation:

The type of reaction given is termed a combustion reaction. A combustion reaction is one that involves a substance reacting with molecular oxygen. For combustion reactions involving a hydrocarbon as given in the example, the major products are carbon dioxide and water. The word combustion is a synonym for burn.

Example Question #153 : General Chemistry

Screen shot 2016 02 27 at 6.21.15 pm

For the reaction given, identify the type of reaction taking place.

Possible Answers:

Combination reaction

Synthesis reaction

Decomposition reaction

Displacement reaction

Correct answer:

Displacement reaction

Explanation:

A displacement reaction is one in which an atom, ion or molecule moves from one compound to another to replace an atom, ion or molecule in another compound. In the reaction given, \displaystyle SO_{4}^{2-} ion moves from \displaystyle CuSO_{4} to \displaystyle Zn(s) to form \displaystyle ZnSO_{4}.

Example Question #4 : Reactions With Inorganic Compounds

Screen shot 2016 02 27 at 6.20.58 pm

Determine the percentage yield of the reaction given if 12.6 grams of \displaystyle Ba(NO)_{3} is reacted excess \displaystyle Na_{2}SO_{4} to give 10.4 grams of \displaystyle BaSO_{4}.

Possible Answers:

\displaystyle 83.5 \%

\displaystyle 2.0 \%

\displaystyle 92.0 \%

\displaystyle 77.0 \%

Correct answer:

\displaystyle 92.0 \%

Explanation:

In chemistry, percentage yield is a term used to quantify the efficiency of a chemical reaction. In order to calculate this quantity, the value for the actual yield and theoretical yield is needed. The actual yield is the amount of product obtained during a chemical reaction. The theoretical yield is the maximum amount of product possible to be obtain by a chemical reaction which is based on the amount of reactants used. Below is the formula needed to calculate the percentage yield.

Convert the number of grams of \displaystyle Ba(NO_{3})_{2} to moles:

\displaystyle 12.6 g\ Ba(NO_{3})_{2}\times \frac{mole}{261.37g}=0.0482\ moles\ Ba(NO_{3})_{2}

Based on the chemical equation 1 mole of \displaystyle BaSO_{4} is produced for every 1 mole of \displaystyle Ba(NO_{3})_{2} reacted.

\displaystyle moles\ of Ba(NO_{3})_{2}=moles\ of\ BaSO_{4}

Convert the moles of \displaystyle BaSO_{4} to grams:

\displaystyle 0.0482\ moles\ BaSO_{4} \times\ \frac{233.43g}{moles}=11.3g\ BaSO_{4}

\displaystyle Percentage\ Yield =\frac{actual\ yield}{theoretical\ yield}\times100

\displaystyle Percentage\ Yield =\frac{10.4g}{11.3g}\times100=92.0 \%

Example Question #153 : General Chemistry

Screen shot 2016 02 27 at 6.20.52 pm

Determine the percentage yield of the reaction given if 21.0 grams of \displaystyle NaOH is reacted with excess \displaystyle HCl to give 10.2 grams of \displaystyle NaCl.

Possible Answers:

\displaystyle 16.6 \%

\displaystyle 53.0 \%

\displaystyle 33.2 \%

\displaystyle 64.3 \%

Correct answer:

\displaystyle 33.2 \%

Explanation:

In chemistry, percentage yield is a term used to quantify the efficiency of a chemical reaction. In order to calculate this quantity, the value for the actual yield and theoretical yield is needed. The actual yield is the amount of product obtained during a chemical reaction. The theoretical yield is the maximum amount of product possible to be obtain by a chemical reaction which is based on the amount of reactants used. Below is the formula needed to calculate the percentage yield.

Convert the number of grams of \displaystyle NaOH to moles:

\displaystyle 21.0 g\ NaOH\times \frac{mole}{40.0g}=0.525\ moles\ NaOH

Based on the chemical equation 1 mole of \displaystyle NaCl is produced for every 1 mole of \displaystyle NaOH reacted.

\displaystyle moles\ of\ NaOH=moles\ of\ NaCl

Convert the moles of \displaystyle NaCl to grams:

\displaystyle 0.525\ moles\ \times\ \frac{58.44g}{moles}=30.7g\ NaCl

\displaystyle Percentage\ Yield =\frac{actual\ yield}{theoretical\ yield}\times100

\displaystyle Percent\ Yield =\frac{10.2g}{30.7g}\times100=33.2 \%

Example Question #1 : Reactions With Inorganic Compounds

Screen shot 2016 02 27 at 6.21.03 pm

Determine the theoretical yield in grams of \displaystyle Al_{2}(SO_{4})_{3} produced if 5.6 grams of \displaystyle Al(OH)_{3} is reacted with excess \displaystyle H_{2}SO_{4}.

Possible Answers:

\displaystyle 1.1

\displaystyle 4.7

\displaystyle 4.6

\displaystyle 12.3

Correct answer:

\displaystyle 12.3

Explanation:

Convert the number of grams of \displaystyle Al(OH)_{3} to moles:

\displaystyle 5.6 g\ Al(OH)_{3}\times \frac{mole}{78.00g}=0.0718\ moles\ Al(OH)_{3}

Based on the chemical equation 1 mole of \displaystyle Al_{2}(SO_{4})_{3} is produced for every 2 mole of \displaystyle Al(OH)_{3} reacted.

\displaystyle 2\ moles\ of Al(OH)_{3}=1\ mole\ of\ Al_{2}(SO_{4})_{3}

Therefore: \displaystyle 0.0718\ moles\ Al(OH)_{3}\times \frac{1\ moles\ Al_{2}(SO_{4})_{3}}{2\ moles\ Al(OH)_{3}}=0.0359\ moles\ Al_{2}(SO_{4})_{3}

Convert the moles of \displaystyle Al_{2}(SO_{4})_{3} to grams:

\displaystyle 0.0359\ moles\ Al_{2}(SO_{4})_{3} \times\ \frac{342.15g}{moles}=12.3g\ Al_{2}(SO_{4})_{3}

Example Question #5 : Reactions With Inorganic Compounds

Screen shot 2016 02 27 at 6.21.09 pm

For the reaction given, identify the type of reaction taking place.

Possible Answers:

Combination reaction

Decomposition reaction

Double displacement reaction

Synthesis reaction

Correct answer:

Double displacement reaction

Explanation:

A displacement reaction is one in which an atom, ion or molecule moves from one compound to another to replace an atom, ion or molecule in another compound. In the reaction given, \displaystyle NO_{3}^{-} ion moves from \displaystyle AgNO_{3} to \displaystyle Na_{2}CO_{3}. Also, \displaystyle CO_{3}^{2-}ion moves from \displaystyle Na_{2}CO_{3} to \displaystyle AgNO_{3}.

Example Question #4 : Reactions With Inorganic Compounds

Screen shot 2016 02 27 at 6.25.53 pm

Determine the percentage yield of the reaction given if 40.0 grams of \displaystyle CaCO_{3} is heated to give 15.0 grams of \displaystyle CaO.

Possible Answers:

\displaystyle 25.1 \%

\displaystyle 76.4 \%

\displaystyle 68.2 \%

\displaystyle 50.1 \%

Correct answer:

\displaystyle 68.2 \%

Explanation:

In chemistry, percentage yield is a term used to quantify the efficiency of a chemical reaction. In order to calculate this quantity, the value for the actual yield and theoretical yield is needed. The actual yield is the amount of product obtained during a chemical reaction. The theoretical yield is the maximum amount of product possible to be obtain by a chemical reaction which is based on the amount of reactants used. Below is the formula needed to calculate the percentage yield.

Convert the number of grams of \displaystyle CaCO_{3} to moles:

\displaystyle 40.0 g\ CaCO_{3}\times \frac{mole}{100.09g}=0.3996\ moles\ CaCO_{3}

Based on the chemical equation 1 mole of \displaystyle CaO is produced for every 1 mole of \displaystyle CaCO_{3} reacted.

\displaystyle moles\ of\ CaCO_{3}=moles\ of\ CaO

Convert the moles of \displaystyle CaO to grams:

\displaystyle 0.3996\ moles\ \times\ \frac{56.08g}{moles}=22.0g\ CaO

\displaystyle Percentage\ Yield =\frac{actual\ yield}{theoretical\ yield}\times100

\displaystyle Percentage\ Yield =\frac{15.0g}{22.0g}\times100=68.2 \%

Example Question #154 : General Chemistry

Screen shot 2016 02 27 at 6.20.41 pm

For the reaction given, identify the type of reaction taking place.

Possible Answers:

Displacement reaction

Combination reaction

Synthesis reaction

Decomposition reaction

Correct answer:

Displacement reaction

Explanation:

A displacement reaction is one in which an atom, ion or molecule moves from one compound to another to replace an atom, ion or molecule in another compound. In the reaction given, 2 \displaystyle Cl^{-} ions move from 2 moles of \displaystyle HCl to \displaystyle Cu(s) to form \displaystyle CuCl_{2}.

Example Question #6 : Equilibrium

\displaystyle CO(g) + 2H_{2}(g) \rightarrow CH_{3}OH(g)

\displaystyle K_{eq} = 14.5

The initial concentrations of this reaction are listed below.

\displaystyle [CH_{3}OH] = 2.5M, [CO] = 1.3M, [H_{2}] =0.6M.

Based on these initial concentrations, which statement is true? 

Possible Answers:

The ratio of products to reactants is greater than the equilibrium constant

\displaystyle Q=\frac{[CO][H_2]^2}{[CH_3OH]}

The reaction is in equilibrium

The forward rate will be greater than the reverse rate

The reaction will shift left

Correct answer:

The forward rate will be greater than the reverse rate

Explanation:

When given initial concentrations, we can determine the reaction quotient (Q) of the reaction. The reaction quotient is given by the same equation as the equilibrium constant (concentration of products divided by concentration of reactants), but its value will fluctuate as the system reacts, whereas the equilibrium constant is based on equilibrium concentrations.

By comparing the reaction quotient to the equilibrium constant, we can determine in which direction the reaction will proceed initially. 

\displaystyle Q\ \text{or}\ K_{eq}=\frac{[CH_3OH]}{[H_2]^2[CO]}

The reaction quotient with the beginning concentrations is written below.

\displaystyle \frac{[2.5]}{[0.6]^{2}[1.3]} = 5.3

This shows that the ratio of products to reactants is less than the equilibrium constant. Since Q is less than Keq in the beginning, we conclude that the reaction will proceed forward until Q is equal to Keq. In this manner, the denominator (reactants) will decrease and the numerator (products) will increase, causing Q to become closer to Keq.

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