If a strong acid is added to a buffer, the conjugate base of buffer will reacts with the H⁺ from the strong acid to form the weak acid. The H⁺ gets absorbed by the CH₃COO⁻ instead of reacting with water to form H₃O⁺ so the pH will remain constant, The equation is:
CH₃COO⁻ + HNO₃ → CH₃COOH + NO₃⁻
S.A W.A
Buffer converts strong acid to weak acid
