Question

The pKa of HF is 3.17.

a. Calculate the pH of a 1.00 L solution that is 1.00M HF and 1.50M NaF.

b. What is the pH of this solution after addition of 50.0ml of 10.0M HCl.

Answer 1

(a) Solution of HF and NaF will form buffer solution. To calculate the pH of buffer solution Henderson–Hasselbalch equation will be used which is given as follow:

pH = pKa + log ([base] / [acid])

pH = 3.17 + log (1.50/1) = 3.35

On addition of HCl, concentration of NaF will decrease and concentration of HF will increase.

Total volume = 1.05 L

Moles of HCl added = molarity x volume = 10 x 0.05 = 0.5 moles

Now, number of moles of HCl = 1 + 0.5 = 1.5 moles

[HCl] = number of moles / volume of solution (in L) = 1.5 / 1.05 = 1.43 M

Moles of NaF = 1.50 - 0.5 = 1

[NaF] = 1/1.05 = 0.95 M

Now calculate pH,

pH = 3.17 + log (0.95/1.43) = 2.99