GCE A-Level - Higher 2 (H2) · Chemistry (9476)
5 multiple-choice questions marked as you go, and 2 written questions with worked solutions. All on Acid dissociation constants, Ka and the use of pKa.
Two weak monoprotic acids, \( HA \) and \( HB \), have \( pK_a \) values of 3.50 and 4.80 respectively at 298 K. Which of the following statements is correct for aqueous solutions of these acids with an equal concentration of 0.10 mol dm\(^{-3}\)?
Methylamine, \( CH_3NH_2 \), is a weak base with a \( pK_b \) of 3.36 at 298 K. Using the assumption that the concentration of the base at equilibrium is approximately equal to its initial concentration, what is the pH of a 0.10 mol dm\(^{-3}\) aqueous solution of methylamine?
The acid dissociation constant, \( K_a \), for the ammonium ion, \( NH_4^+ \), is \( 5.6 \times 10^{-10} \) mol dm\(^{-3}\) at 298 K. Given that the ionic product of water, \( K_w \), is \( 1.0 \times 10^{-14} \) mol\(^2\) dm\(^{-6}\), what is the \( pK_b \) of its conjugate base, ammonia (\( NH_3 \)), at this temperature?
A 0.020 mol dm\(^{-3}\) aqueous solution of a weak monoprotic acid, \( HX \), is found to be 4.5% dissociated at 298 K. What is the value of the acid dissociation constant, \( K_a \), for this acid?
A weak monoprotic acid, (\(HA\)), has an acid dissociation constant, (\(K_a\)), of (\(2.0 \times 10^{-5}\)) mol dm\(^{-3}\) at 298 K. Using the standard approximation that the extent of dissociation is negligible, what is the pH of a 0.050 mol dm\(^{-3}\) aqueous solution of (\(HA\))?
Monochloroacetic acid, \( ClCH_2COOH \), has a \( pK_a \) value of 2.86 at 298 K. Calculate the acid dissociation constant, \( K_a \), of this acid and explain how its strength compares to ethanoic acid (\( pK_a = 4.76 \)).
Write your answer out first, then check it against the worked solution.
An aqueous solution of a weak monoacidic base, \( B \), has a concentration of \( 0.150 \text{ mol dm}^{-3} \) and a \( pH \) of \( 11.20 \) at \( 298 \text{ K} \). Calculate the base dissociation constant, \( K_b \), for this base at this temperature, assuming that the concentration of the base at equilibrium is approximately equal to its initial concentration. (Take \( K_w = 1.00 \times 10^{-14} \text{ mol}^2 \text{ dm}^{-6} \))
Write your answer out first, then check it against the worked solution.
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