[Boxxxed]

3.  A buffer solution is prepared by dissolving 1.50 g each of benzoic acid, C6H5CO2H (MW = 122 g/mol), and sodium benzoate, NaC6H5CO2 (MW=144 g/mol) in 150 mL of solution.  Ka = 6.3x10-5

a. What is the pH of this buffer solution?  (8 points)

I calculated the ph to be 4.13



c. What quantity of 2.0M HCl (in mL) must be added to the buffer to change the pH to 4.00? (Hint:Remember what ratio controls the pH of a buffer solution) (12 points)


I calculated the ratio to be 0.631 base to 1 acid. What do I do next?

[AWK]

From the first part you have moles of base and acid.
According to reaction
base + HCl = Acid + NaCl
you can see that
moles of Base - moles of HCl = new moles of base
moles of acid + moles of HCl = new moles of acid
Put these into new ratio (0.631) and find moles of HCl, then volume of 2 M HCl

[Boxxxed]

I understand some of it but I am still lost as to how to find moles of HCl.

initial moles acid = 0.01229

initial moles base = 0.01042

Would I do something like this...

0.01042-x
----------   = 0.631
0.01229+x

Would solving for x give me moles HCl?

[rabolisk]

Yes, that's correct, assuming that those numbers, 0.01042 and 0.01229 are correct.