Answer to Question #7060 in Chemistry for Emily

Question

I need 100 mL of a buffer with a pH of 5.20 and it's acetic acid or HC2H3O2. So I need to find out how many grams of acetic acid are needed to make this buffer. (Ka = 1.8 x 10-5)

What I have so far...

pH = pKa + log(base/acid)

5.20 = 4.74 + logx

x = 2.884 M

2.884 M * .1 L = .2884 mol

.2884 mol * 60.052 g = 17.32 g

i know this is wrong because the answer is 11.54 g.

can you help me find my mistake please?