Question

Determine the pH of each of the following two-component solutions.

0.270 M NH4NO3 and 0.101 M HCN

7.5×10⁻² M RbOH and 0.130 M NaCl

9.2×10⁻² M HClO4 and 2.0×10⁻² M KOH

0.110 M NaClO and 5.50×10⁻² M KI

Answer 1

0.270 M NH4NO3 and 0.101 M HCN

The solution contains two weak acids NH₄⁺ and HCN with quite equal pKa’s (9.25 and 9.21). Ignore NO₃^- an ineffective base.

pH = pKa/2 -1/2logC_acid) = 9.2/2 – log (0.270+0.101) = 4.6 + 0.2 = 4.8

7.5×10⁻² M RbOH and 0.130 M NaCl

Consider only the strong base RBOH

pOH = -log7.5×10⁻² = 1.12

pH = 14-pOH = 12.88

9.2×10⁻² M HClO4 and 2.0×10⁻² M KOH

It is a mixture of strong acid and aa strong base. After neutralization

[H+] = 9.2×10⁻² M - 2.0×10⁻² M = 7.2×10⁻² M

pH = -log 7.2×10⁻² = 1.15

0.110 M NaClO and 5.50×10⁻² M KI

I- is an ineffective base (conjugated to a strong acid). This solution contains the weak base ClO-.

pOH = (pKb x Cbase)^1/2 = pKb/2 – 1/2logCbase = 3.75 + 0.48 = 4.23

pH = 14-pOH = 9.77