Answer:
The total volume of the buffer is 75,0mL
Step-by-step explanation:
A buffer is a mixture of a weak acid with its conjugate base or a weak base with its conjugate acid.
The buffer than Stu and Doc made is:
CH₃COOH ⇄ CH₃COO⁻ + H⁺
Doc add 30.0 mL of 0.0635 M CH₃COOH while Stu add 45.0 mL of 0.0285 M NaCH₃COO (CH₃COO⁻)
Thus, the total volume of the buffer is:
30,0mL + 45,0mL = 75,0 mL
I hope it helps!