Henderson Hasselbalch equation Where did I go wrong?

[shenzys]

Homework Statement

Q) Calculate the volume of 0.1M HCl you would need to add to 200ml of 0.15M sodium acetate solution to obtain a buffer of pH 4.2.
Ka = 1.74 * 10^-5 moldm-3

Homework Equations

pH = pKa + log [salt]/[acid]

The Attempt at a Solution

Log[salt]/[acid] = pH - pKa

[salt]/[acid] = 0.275

Equation: [salt] = 0.275 [acid]

[salt] = 200*0.15/200+v

[acid] = (0.1*V / 200+v) - (200*0.15/200+v)

Substituting into above equation:

200*0.15/200+v = 0.275(0.1*V/200+v) - 0.275(200*0.15/200+v)

30 = 0.0275v - 8.25

38.25= 0.0275v

V = 38.25/0.0275v

V = 1390 cm^3

This is obviously wrong! Where did I go wrong?

 

[Borek]

Moles of salt + moles of acid should be constant.

 

[shenzys]

Moles of salt + moles of acid should be constant.

I still don't understand... could you explain a bit more please - sorry to bother you, I am just really desperate!

 

[Borek]

Take a look at the protonation reaction. When you add strong acid to acetate amount of acetic acid grows, but amount of acetate goes down.

Could be you tried to account for that, but you did it wrong way.