Question

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

Part A

7.0×10⁻² M in HNO3 and 0.185 M in HC7H5O2

Part B

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

Part C

8.5×10⁻² M in HF and 0.230 M in HC6H5O

Part D

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

Answer 1

1. 7.0×10⁻² M in HNO3 and 0.185 M in HC7H5O2

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

By itself, 7.0x10^-2 M HNO3 would have a [H+] = 7.0x10^-2

Let HA represent benzoic acid and X= the H+ contribution from benzoic acid
A^-1 will also = X
6.3x10^-5 = (X+ 0.070)X)/(0.185 - X)
6.3x10^-5 = X^2 + 0.070 X / (0.185 - X)
X^2 + (0.070 + 0.000063) X - 1.117x10^-5 = 0
X^2 + 0.070X - 1.117x10^-5 = 0 ( solve the quadratic equation )
X = 0.00016 which is insignificant compared to 0.070

pH = -log(0.070) = 1.15

Part B

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

both HBr and HCIO4 are strong acids . so they ionize 100 % add the concentrations of both and take the pH.

1.5×10⁻² M + 2.0×10⁻² M = 3.5 ×10⁻² M

pH= log 3.5 ×10⁻² M

= 1.456

Part C

8.5×10⁻² M in HF and 0.230 M in HC6H5O