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