Question

IA 290.0 mL buffer solution is 0.280 M in acetic acid and 0.280 M in sodium acetate.

What is the initial pH of this solution?

What is the pH after addition of 0.0150 mol of HCl?

What is the pH after addition of 0.0150 mol of NaOH?

Answer 1

this is a buffer so

pH = pKa + log(Acetate/acetic acid)

since (Acetate/acetic acid) = 1

then

pH = pKa

pKa for acetic acid = 4.75

therefore

pH = 4.75 initially

a)

after adding 0.015 mol of acid

mol of acetic acid initially = MV = (0.28*0.29) = 0.0812

mol of acetate initially = MV =  (0.28*0.29) = 0.0812

after 0.015

mol of acid increases, acetate reacts so decreases

pH = pKa + log(Aacetate/acid)

pH = 4.75 + log ( (0.0812 - 0.015) / (0.0812+0.015) )

pH =4.5876

now...

after addition of NaOH

acetate is formed, since acid is reacting

pH = 4.75 + log ( (0.0812 + 0.015) / (0.0812-0.015) )

pH = 4.912