This question is about reactions and uses of weak acids.
A student adds magnesium ribbon to an aqueous solution of ethanoic acid, CH3COOH\text{CH}_3\text{COOH}CH3COOH. 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 methanoic acid and ethanoic acid are shown below:
| Weak acid | Ka / mol dm−3K_{\text{a}} \text{ / mol dm}^{-3}Ka / mol dm−3 |
|---|---|
| Methanoic acid, HCOOH\text{HCOOH}HCOOH | 1.78×10−41.78 \times 10^{-4}1.78×10−4 |
| Ethanoic acid, CH3COOH\text{CH}_3\text{COOH}CH3COOH | 1.75×10−51.75 \times 10^{-5}1.75×10−5 |
A student adds methanoic acid to ethanoic acid. An equilibrium is set up containing two conjugate acid-base pairs.
Complete the equilibrium reaction below and label the conjugate acid-base pairs as A1, B1 and A2, B2:
HCOOH+CH3COOH⇌……………⋯+……………… \text{HCOOH} + \text{CH}_3\text{COOH} \rightleftharpoons \dots\dots\dots\dots\dots\dots + \dots\dots\dots\dots\dots\dots HCOOH+CH3COOH⇌……………⋯+………………Use the data provided above to answer the following questions.
The student measures the pH of CH3COOH(aq)\text{CH}_3\text{COOH(aq)}CH3COOH(aq) as 2.842.842.84. Show that the concentration of the CH3COOH(aq)\text{CH}_3\text{COOH(aq)}CH3COOH(aq) is approximately 0.120 mol dm−30.120 \text{ mol dm}^{-3}0.120 mol dm−3.
The student plans to make a buffer solution of pH 4.604.604.60 from a mixture of CH3COOH(aq)\text{CH}_3\text{COOH(aq)}CH3COOH(aq) and sodium ethanoate, CH3COONa(aq)\text{CH}_3\text{COONa(aq)}CH3COONa(aq).
The student mixes 250 cm3250 \text{ cm}^3250 cm3 of 0.120 mol dm−3 CH3COOH(aq)0.120 \text{ mol dm}^{-3} \text{ CH}_3\text{COOH(aq)}0.120 mol dm−3 CH3COOH(aq) with 550 cm3550 \text{ cm}^3550 cm3 of CH3COONa(aq)\text{CH}_3\text{COONa(aq)}CH3COONa(aq).
Calculate the concentration of CH3COONa(aq)\text{CH}_3\text{COONa(aq)}CH3COONa(aq) needed to prepare this buffer solution of pH 4.604.604.60.
