A 1.0L buffer solution contains 0.100 molHC2H3O2 and 0.100 molNaC2H3O2. The value of Ka for HC2H3O2 is 1.8×10−5. Calculate the pH of the solution upon the addition of 10.0 mL of 1.00 MHCl to the original buffer.

Express the your answer to two decimal places.
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Starting pH = pKa because the acid and conjugate base have equal concentrations.
pH = pKa = -log(1.8*10–5) = 4.74

0.0100 L • 1.00 M = 0.0100 mol HCl, which reacts with the conjugate base, producing 0.0100 mol of weak acid.

You now have 0.110 mol weak acid and 0.090 mol conjugate base. Use the Henderson-Hasselbach equation to solve for pH now.

pH = 4.74 + log(0.090/0.110) = 4.65

The equation technically says to use concentrations of the acid and base, but that doesn’t actually matter, because it would give the same answer.