Question

The Following questions pertain to a 2.2M solution of hydrocyanic acid at 25°C. pKa = 9.21 at 25°C. Find the concentrations of all species present in the solution at equilibrium. Find the pH of the solution. Identify the strongest base in this system.

Answer 1

1) Chemical reaction: HCN + H₂O → CN⁻ + H₃O⁺.
c(HCN) = 2,2 M = 2,2 mol/L.
pKa(HCN) = 9,21.
Ka = 6,16·10⁻¹⁰.
[CN⁻] = [H₃O⁺] = x.
[HCN] = 2,2 M - x.
Ka = [CN⁻] · [H₃O⁺] / [HCN].
6,16·10⁻¹⁰ = x² / 2,2 M -x.
Solve quadratic equation: [CN⁻] = [H₃O⁺] = 0,0000346 M.
[HCN] = 2,2 M - 0,0000346 M = 2,199 M.

2) pH = - log[H₃O⁺].
pH = -log( 0,0000346 M).
pH = 4,46.
Hydrocyanic acid and hydronium ion (H₃O⁺) are acids. Cyanide anion (CN⁻) is the strongest base in the system, cyanide anion accept protons in chemical reaction.
pKb = pKw - pKa.
pKb = 14 - 9,21 = 4,79.

Answer 2

The strongest base in the system is the cyanide ion.

The following steps must be followed here;

• Write the equation
• Set up the table
• Make relevant calculations from the table

The calculation of the pH of the solution, the strongest base in the system are shown in the file attached. The pH is the negative logarithm of the hydrogen ion concentration of the solution as shown.

Hence, the strongest base in the system is the cyanide ion.

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