Question

Find the pH of each of the following solutions of mixtures of acids.

8.0×10⁻² M in HNO₃ and 0.170 M in HC₇H₅O₂

1.5×10⁻² M in HBr and 2.0×10⁻² M in HClO₄

9.5×10⁻² M in HF and 0.230 M in HC₆H₅O

0.100 M in formic acid and 4.5×10⁻² M in hypochlorous acid

Answer 1

Find the pH of each of the following solutions of mixtures of acids.

8.0×10⁻² M in HNO₃ and 0.170 M in HC₇H₅O₂

This is the mixture of strong acid and weak acid.

HC7H5O2 (benzoic acid) is a weak acid; Ka = 6.3x10^-5
HNO3 (nitric acid) is a stong acid

By itself, 8.0x10^-2 M HNO3 would have a [H+] = 8.0x10^-2

Let HA represent benzoic acid and X= the H+ contribution from benzoic acid
A^-1 will also = X

Ka = [H+][A-]/ [HA-X]

This H+ from benzoic acid is very small. Thus the pH of this mixture is depending on only strong acidHNO3:

Here;

8.0×10⁻² M in HNO₃ or H+=8.0×10⁻² M

pH = -log(8.0×10⁻² )

= 1.10