Strong and Weak Acids/Bases

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AP Chemistry › Strong and Weak Acids/Bases

Questions 1 - 3

Which of the following is a weak base?

$NH_{3}$

$CH_{3}COOH$

Explanation

Strong acids and bases are those that ionize completely in solution. In contrast, weak acids and bases ionize only partially in solution.

, hydrogen fluoride, is an example of a very strong acid. In solution, it dissolves completely to form $H^{+}$ and $F^{-}$ .

, sodium hydroxide, is an example of a very strong base. In solution, it dissolves completely to form $Na^{+}$ and $OH^{-}$ .

$CH_{3}COOH$ , acetic acid, is an example of a weak acid. In solution, it dissolves partially to form $CH_{3}COO^{-}$ and $H^{+}$ .

$NH_{3}$ , ammonia, is an example of a weak base. In solution, it accepts a proton to form $NH_{4}^{+}$ and $OH^{-}$ .

$KOH(aq)\ +\ HCN(aq)\ \rightleftharpoons\ H_{2}O(l)\ +\ KCN(aq)$

reacts with in aqueous solution according to the chemical equation provided. If it took $29.0\ mL$ of a $0.12\ M$ solution to titrate $30.0\ mL$ of an solution, what was the concentration of original solution?

$0.12\ M$

$0.24\ M$

$11.0\ M$

$0.0030\ M$

Explanation

Using the concentration of as a conversion factor, we can convert the volume of to moles by dimensional analysis:

$\frac{0.12\ moles}{L}\times0.029\ L=0.0035\ moles\ KOH$

At the equivalence point of the titration:

$moles\ of\ KOH=moles\ of\ HCN=0.0035\ moles$

$Molarity=\frac{0.0035\ moles}{0.030\ L}=0.12\ M$

$CH_{3}COOH(aq)\ +\ H_{2}O(l)\rightleftharpoons\ H_{3}O^{+}(aq)\ +\ CH_{3}COO^{-}(aq)$

Determine the acid dissociation constant expression for the given reaction.

$K_{a}=\frac{[H_{3}O^{+}][CH_{3}COO^{-}]}{[CH_{3}COOH]}$

$K_{a}=\frac{[H_{3}O^{+}][CH_{3}COO^{-}]}{[CH_{3}COOH][H_{2}O]}$

$K_{a}=\frac{[CH_{3}COOH]}{[H_{3}O^{+}][CH_{3}COO^{-}]}$

$K_{a}=\frac{[CH_{3}COOH][H_{2}O]}{[H_{3}O^{+}][CH_{3}COO^{-}]}$

Explanation

Acid dissociation constant which is denoted as $K_{a}$ is the equilibrium constant for the ionization of an acid. Therefore, the numerator contains the product of the concentrations of the substances on the product side of the chemical equation. The denominator contains the product of the concentrations of the substances on the reactant side of the chemical equation. $H_{2}O$ is omitted in the acid dissociation constant expression because as the solvent it is in excess and therefore the change in its concentration is negligible in comparison to the other substances in solution.

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