Question

Calculate the pH of 75mL of the undiluted buffer solution after the addition of 15mL of 3M HCl.

The undiluted buffer solution is prepared by mixing 100mL of 1.000M sodium acetate with 100 mL of1.0798M acetic acid.

I calculated the moles of acetic acid to be 1.0798M x (.1L/.2L) x 0.075L = 0.0405

moles sodium acetate: 1.000M (.1/.2) x 0.075L = 0.0375

moles HCL = 3.00 x 0.015 = 0.045 mols

However if i plug in the value PH=4.75 + log (-0.0075/0.0855), I get a math error for the log value.. What did i do wrong?