Question

a)Using a 0.15 M Acetate buffer with a pH of 4.9, you add 0.75 mL of 0.08 M HCl to 73 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)
b)

Using a 0.15 M Acetate buffer with a pH of 4.9, you add 0.75 mL of 0.08 M NaOH to 73 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)

Answer 1

a)

pka of aceticacid = 4.76

   no of mole of acetate buffer = 0.15*73 = 10.95 mmol

   pH = 4.9

pH of acidic buffer = pka +log(salt/acid)

salt = acetateion = x

acid = aceticacid = 10.95-x

4.9 = 4.76 + log(x/(10.95-x))

acetate = x = 6.35 mmol

aceticacid = 10.95-6.35 = 4.6 mmol

after addition of HCl , pH = ?

no of mmol of HCl added = 0.75*0.08 = 0.06 mmol

pH of acidic buffer = pka +log(salt-HCl/acid+HCl)

      = 4.76 + log((6.35-0.06)/(4.6+0.06))

     = 4.89

b)
   after addition of NaOH , pH = ?

no of mmol of NaOH added = 0.75*0.08 = 0.06 mmol

pH of acidic buffer = pka +log(salt+NaOH/acid-NaOH)

      = 4.76 + log((6.35+0.06)/(4.6-0.06))

     = 4.91