Calculate [H+] in a solution that is 0.34 M in NaF and 0.58 M in HF. (Ka = 7.2 x 10^–4)
A) 0.58 M
B) 4.2 x 10^–4 M
C) 1.2 x 10^–3 M
D) 2.0 x 10^–2 M
E) 1.1 x 10^–4 M
Solution: This is a buffer solution since it has a weak acid HF and its conjugate base NaF (F-) in approximately close amounts.
The equation for calculating pH of a buffer is called Henderson-Hasselbach equation which is
pH = pKa + log ([A-]/[HA]) where HA is the weak acid and A- is its conjugate base. pKa=-log(Ka)
pH = -log(7.2 x 10^–4) + log (.34M/.58M)
pH = 2.91
[H+] = 10^-pH
[H+] = 10^-2.91 = 1.2 x 10^–3 M
Choice C is the correct answer