Question

How many moles of HCl need to be added to 200.0 mL of a 0.200 M solution of NaF to make a buffer with a pH of 3.60? (Ka for HF is 6.8 x 10-4)

Answer 1

We can use the following equation

pH = pKa + log [base / acid]

pH of a buffer solution is 3.6

Ka of HF = 6.8 x 10-4

The pKa of HF = -Log Ka = -Log 6.8 x 10-4 =   3.167

Moles of NaF = 200 ml x 0.2 M / 1000 ml = 0.04 Moles

We can substitute and write the equation in the following way

3.6 = 3.17 + Log [0.04-x / x]

[0.04-x / x] = 2.6915

X =[0.04-x] /2.6915

X = 0.01486 - 0.37515 X

X + 0.37515 X =  0.01486

1.37515 X = 0.01486

X = 0.01486 / 1.3715

X = 0.01083 Moles

Hence 0.01083 Moles of HCl is required to make buffer solution having a pH of 3.6 using 200 mL of 0.2 M NaF