This question is about reactions and uses of weak acids.
A student adds calcium metal to an aqueous solution of propanoic acid, CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3CH2COOH. A redox reaction takes place.
Write the overall equation for this reaction and explain, in terms of oxidation numbers, which element has been oxidised and which element has been reduced.
The KaK_{\text{a}}Ka values of benzoic acid and propanoic acid are shown below:
| Weak acid | Ka / mol dm−3K_{\text{a}} \text{ / mol dm}^{-3}Ka / mol dm−3 |
|---|---|
| Benzoic acid, C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6H5COOH | 6.28×10−56.28 \times 10^{-5}6.28×10−5 |
| Propanoic acid, CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3CH2COOH | 1.35×10−51.35 \times 10^{-5}1.35×10−5 |
A student adds benzoic acid to propanoic acid. An equilibrium is set up containing two acid-base pairs.
Complete the equilibrium reaction below and label the conjugate acid-base pairs as A1, B1 and A2, B2:
C6H5COOH+CH3CH2COOH⇌……………⋯+……………… \text{C}_6\text{H}_5\text{COOH} + \text{CH}_3\text{CH}_2\text{COOH} \rightleftharpoons \dots\dots\dots\dots\dots\dots + \dots\dots\dots\dots\dots\dots C6H5COOH+CH3CH2COOH⇌……………⋯+………………Use the data provided above to answer the following questions.
The student measures the pH of CH3CH2COOH(aq)\text{CH}_3\text{CH}_2\text{COOH(aq)}CH3CH2COOH(aq) as 2.932.932.93. Show that the concentration of the CH3CH2COOH(aq)\text{CH}_3\text{CH}_2\text{COOH(aq)}CH3CH2COOH(aq) is 0.102 mol dm−30.102 \text{ mol dm}^{-3}0.102 mol dm−3.
The student plans to make a buffer solution of pH 4.504.504.50 from a mixture of CH3CH2COOH(aq)\text{CH}_3\text{CH}_2\text{COOH(aq)}CH3CH2COOH(aq) and sodium propanoate, CH3CH2COONa(aq)\text{CH}_3\text{CH}_2\text{COONa(aq)}CH3CH2COONa(aq).
The student mixes 300 cm3300 \text{ cm}^3300 cm3 of 0.102 mol dm−3 CH3CH2COOH(aq)0.102 \text{ mol dm}^{-3} \text{ CH}_3\text{CH}_2\text{COOH(aq)}0.102 mol dm−3 CH3CH2COOH(aq) with 700 cm3700 \text{ cm}^3700 cm3 of CH3CH2COONa(aq)\text{CH}_3\text{CH}_2\text{COONa(aq)}CH3CH2COONa(aq).
Calculate the concentration of CH3CH2COONa(aq)\text{CH}_3\text{CH}_2\text{COONa(aq)}CH3CH2COONa(aq) needed to prepare this buffer solution of pH 4.504.504.50.
