GRE Subject Test: Chemistry : GRE Subject Test: Chemistry

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

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Example Questions

Example Question #4 : Acid Base Reactions

Consider the following reaction:

\(\displaystyle CH_3COOH_(_a_q_)\:+\:H_2O_(_l_)\rightleftharpoons CH_3COO^-_(_a_q_)\:+\:H_3O^+_(_a_q_)\)

 Which of the following changes will increase the pH of this solution? 

Possible Answers:

Adding sodium acetate

Decreasing the volume of \(\displaystyle H_2O\)

Increasing the acetic acid concentration

Increasing the pKa of \(\displaystyle CH_3COOH\)

Correct answer:

Adding sodium acetate

Explanation:

To answer this question you need to use Le Chatelier’s principle. Adding sodium acetate to the solution will cause it to dissociate as follows:

\(\displaystyle NaCH_3COO \rightleftharpoons CH_3COO^- \:+\: Na^+\)

The dissociation reaction will produce more acetate ions. According to Le Chatelier’s principle, the increase in acetate ions will shift the equilibrium of the reaction (given in the question) to the left. This means that \(\displaystyle CH_3COO^-\)and \(\displaystyle H_3O^+\) will be utilized to form \(\displaystyle CH_3COOH\). This will cause a decrease in the amount of hydronium ions in solution. Recall that pH is increased when the concentration of hydrogen ions (or hydronium ions, \(\displaystyle H_3O^+\)) is decreased; therefore, adding sodium acetate will increase the pH of the solution.

Increasing acetic acid concentration will shift the equilibrium to the right and produce more hydronium ions, thereby decreasing the pH. Recall that you can never change the pKa of an acid. The pKa of acetic acid is around 4.75, and it cannot be altered. Le Chatelier’s principle only applies when there is a change in amount of aqueous or gaseous substances; liquid and solid substances will not shift the equilibrium. Changing the volume of liquid water will not change the concentration of hydronium ions.

Example Question #1 : Acid Base Reactions

Which of the following acids is polyprotic?

Possible Answers:

None of these

\(\displaystyle HNO_{3}\)

\(\displaystyle H_{2}SO_{4}\)

\(\displaystyle HCl\)

\(\displaystyle CH_{3}CH_{2}COOH\)

Correct answer:

\(\displaystyle H_{2}SO_{4}\)

Explanation:

Sulfuric acid (\(\displaystyle H_{2}SO_{4}\)) is considered a polyprotic acid because it has two ionizable protons in its molecular formula. The protons dissociate in an aqueous solution according to the acid-base equilibria below:

\(\displaystyle H_{2}SO_{4} \leftrightarrow H^{+} + HSO_{4}^{-}\)

\(\displaystyle HSO_{4}^{-} \leftrightarrow H^{+} + SO_{4}^{2-}\)

All the other acids listed in the answer choices are monoprotic.

Example Question #4 : Acid Base Chemistry

Which of the following acids is considered polyprotic?

Possible Answers:

All of these

\(\displaystyle HOC_6H_5\)

\(\displaystyle HF\)

\(\displaystyle H_{2}CO_3\)

\(\displaystyle HCN\)

Correct answer:

\(\displaystyle H_{2}CO_3\)

Explanation:

Carbonic acid (\(\displaystyle H_{2}CO_{3}\)) is considered a polyprotic acid because it has two ionizable protons (\(\displaystyle H\) atoms) in its molecular formula. The protons dissociate in an aqueous solution according to the acid-base equilibria below:

\(\displaystyle H_{2}CO_{3} \leftrightarrow H^{+} + HCO_{3}^{-}\)

\(\displaystyle HCO_{3}^{-} \leftrightarrow H^{+} + CO_{3}^{2-}\)

The other acids are all monoprotic acids.

Example Question #1 : Acid Base Reactions

Considering the Ka for \(\displaystyle HClO_2\) is \(\displaystyle 1.1*10^{-2}\), what is the Kb for \(\displaystyle ClO_2^{-}\)?

Possible Answers:

\(\displaystyle 4.8*10^{-11}\)

\(\displaystyle 5*10^{-3}\)

\(\displaystyle 9.1*10^{-13}\)

\(\displaystyle 2.1*10^{-10}\)

\(\displaystyle 6.3*10^{-12}\)

Correct answer:

\(\displaystyle 9.1*10^{-13}\)

Explanation:

The equilibrium governing the dissolution of \(\displaystyle HClO_2\) in water is:

\(\displaystyle HClO_{2}(aq)\ +\ H_{2}O(l)\rightleftharpoons\ H_{3}O^{+}(aq)\ +\ HClO_{2}^{-}(aq)\)

\(\displaystyle HClO_2\) is the conjugate acid of \(\displaystyle ClO_2^{-}\). In other words, \(\displaystyle ClO_2^{-}\) is the conjugate base of \(\displaystyle HClO_2\) .

Using the relationship, \(\displaystyle K_{w}=K_{a}K_{b}\), we can calculate the Kb.

Rearrange the equation and solve:

\(\displaystyle K_{b}=\frac{K_{w}}{K_{a}}=\frac{1.0*10^{-14}}{1.1*10^{-2}}=9.1*10^{-13}\) 

Example Question #1 : Acid Base Reactions

Considering the Ka for \(\displaystyle HBrO\) is \(\displaystyle 2.5*10^{-9}\), what is the Kb for \(\displaystyle BrO^-\)?

Possible Answers:

\(\displaystyle 4.4\)

\(\displaystyle 2*10^{-8}\)

\(\displaystyle 3*10^{-1}\)

\(\displaystyle 1*10^{-9}\)

\(\displaystyle 4*10^{-6}\)

Correct answer:

\(\displaystyle 4*10^{-6}\)

Explanation:

The equilibrium governing the dissolution of \(\displaystyle HBrO\) in water is:

\(\displaystyle HBrO(aq)\ +\ H_{2}O(l)\rightleftharpoons\ H_{3}O^{+}(aq)\ +\ BrO^{-}(aq)\)

\(\displaystyle HBrO\) is the conjugate acid of \(\displaystyle BrO^-\). In other words, \(\displaystyle BrO^-\) is the conjugate base of \(\displaystyle HBrO\).

Using the relationship, \(\displaystyle K_{w}=K_{a}K_{b}\), we can calculate the Kb.

Rearrange the equation and solve:

\(\displaystyle K_{b}=\frac{K_{w}}{K_{a}}=\frac{1.0*10^{-14}}{2.5*10^{-9}}=4*10^{-6}\) 

Example Question #11 : Acid Base Chemistry

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Based on the equilibrium shown, what does \(\displaystyle Cl^{-}\) act as? 

Possible Answers:

An acid

A radical

A cation

A base

A catalyst

Correct answer:

A base

Explanation:

A base is a substance that can accept a proton. The conjugate base of an acid is formed when the acid donates a proton. In this case, \(\displaystyle Cl^{-}\) is the conjugate base to the acid \(\displaystyle HCl\). This is because \(\displaystyle HCl\) donates a hydrogen ion to the organic molecule to form \(\displaystyle Cl^{-}\), the conjugate base.

Example Question #2 : Acid Base Reactions

Which of the following is the weakest acid?

Possible Answers:

\(\displaystyle HBr\)

\(\displaystyle HF\)

\(\displaystyle HClO_{4}\)

\(\displaystyle HI\)

\(\displaystyle HCl\)

Correct answer:

\(\displaystyle HF\)

Explanation:

\(\displaystyle HF\) (hydrofluoric acid) is the weakest acid. Fluoride ion is the most electronegative ion. Among the other halogens, its atomic radius is smaller, and therefore bonds more strongly with hydrogen and therefore does not completely dissociate in solution as compared to \(\displaystyle HBr\), \(\displaystyle HI\), and \(\displaystyle HCl\). Perchloric acid is a strong acid and dissociates completely in solution.

Example Question #12 : Acid Base Chemistry

\(\displaystyle HCl_{(aq)}+KOH_{(aq)}\rightarrow\ H_{2}O_{(l)}+KCl_{(aq)}\)

What is the type of reaction given?

Possible Answers:

Redox Reaction

Sublimation Reaction

Synthesis Reaction

Catalytic reaction

Neutralization Reaction

Correct answer:

Neutralization Reaction

Explanation:

The reaction given is called a neutralization reaction because the acid and base components react to counterbalance each other making them chemically neutral. This type of reaction occurs between an acid and base to form a water and a salt. A neutralization reaction between a strong acid and a strong base react to form a neutral solution of pH 7.

Example Question #11 : Acid Base Reactions

\(\displaystyle H_{2}SO_{4}_{(aq)}+2NaOH_{(aq)}\rightarrow\ 2H_{2}O_{(l)}+Na_{2}SO_{4}_{(aq)}\)

Based on the above balanced equation for a neutralization reaction, what is the \(\displaystyle NaOH\)concentration of a \(\displaystyle NaOH\) solution if \(\displaystyle 25.0 mL\) of \(\displaystyle 0.100 M\) \(\displaystyle H_{2}SO_{4}\) is needed to neutralize a \(\displaystyle 15 mL\) solution of \(\displaystyle NaOH\)?

Possible Answers:

\(\displaystyle 2.25M\)

\(\displaystyle 0.333M\)

\(\displaystyle 0.112M\)

\(\displaystyle 4.10M\)

\(\displaystyle 0.667M\)

Correct answer:

\(\displaystyle 0.333M\)

Explanation:

We need to convert milliliters to liters:

\(\displaystyle 25.0\ mL \times\frac{1L}{1000mL}=0.0250\ L\)

We need to determine the moles of \(\displaystyle H_{2}SO_{4}\) using dimensional analysis the concentration as a conversion factor:

\(\displaystyle Moles\ of\ H_{2}SO_{4}=0.0250\ L \times \frac{0.100\ moles}{L}=0.00250\ moles\ H_{2}SO_{4}\)

Based on the chemical equation given:

\(\displaystyle 1\ mole\ of H_{2}SO_{4}=2\ moles\ of\ NaOH\)

We can used the relationship of moles of \(\displaystyle NaOH\) and moles of \(\displaystyle H_{2}SO_{4}\) as a conversion factor to determine the moles of \(\displaystyle NaOH\):

\(\displaystyle Moles\ of\ NaOH=0.00250\ moles\ H_{2}SO_{4} \times\frac{2\ moles\ NaOH}{1\ mole\ H_{2}SO_{4}}=0.00500\ moles\ NaOH\)

Concentration in molarity can be calculated using the following formula:

\(\displaystyle Molarity\ of NaOH=\frac{moles\ of\ NaOH}{Liters\ of\ solution}\)

Let's convert liters of \(\displaystyle NaOH\) to the mL:

\(\displaystyle Liters\ of\ NaOH\ solution=15.0\ mL \times\frac{1L}{1000mL}=0.0150\ L\)

Therefore, the concentration of \(\displaystyle NaOH\) is:

\(\displaystyle Molarity\ of NaOH=\frac{0.00500\ moles\ NaOH}{0.0150\ L}=0.333\ M\)

Example Question #13 : Acid Base Chemistry

\(\displaystyle HNO_{3}_{(aq)}+NaOH_{(aq)}\rightarrow\ H_{2}O_{(l)}+NaNO_{3}_{(aq)}\)

Based on the above balanced equation for a neutralization reaction, how many moles of \(\displaystyle NaNO_{3}\) is formed if \(\displaystyle 20.0 mL\) of \(\displaystyle 0.100 M\) \(\displaystyle HNO_{3}\) is needed to neutralize a solution of \(\displaystyle NaOH\)?

Possible Answers:

\(\displaystyle 0.120mol\)

\(\displaystyle 0.00400mol\)

\(\displaystyle 0.200mol\)

\(\displaystyle 0.0450mol\)

\(\displaystyle 0.00200mol\)

Correct answer:

\(\displaystyle 0.00200mol\)

Explanation:

We need to convert milliliters of \(\displaystyle HNO_{3}\) to liters:

\(\displaystyle 20.0\ mL \times\frac{1L}{1000mL}=0.0200\ L\)

We need to determine the moles of \(\displaystyle HNO_{3}\) using dimensional analysis and the concentration as a conversion factor:

\(\displaystyle Moles\ of\ HNO_{3}=0.0200\ L \times \frac{0.100\ moles}{L}=0.00200\ moles\ HNO_{3}\)

 Based on the chemical equation given:

\(\displaystyle 1\ mole\ of HNO_{3}=1\ mole\ of\ NaNO_{3}\)

We can use the relationship of moles of \(\displaystyle HNO_{3}\) and moles of \(\displaystyle NaNO_{3}\) as a conversion factor to determine the moles of \(\displaystyle NaNO_{3}\):

\(\displaystyle Moles\ of\ NaNO_{3}=0.00200\ moles\ HNO_{3} \times\frac{1\ moles\ NaNO_{3}}{1\ mole\ HNO_{3}}=0.00200\ moles\ NaNO_{3}\)

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