Chem Calculate the pH

Question

Working thru problems to get ready for final. Any help on the method for solving would be great.

Ka for formic acid is 1.7 x 10-4 at 25oC. A buffer is made by mixing 505 mL of 0.503 M formic acid, HCHO2, and 466 mL of 0.476 M sodium formate, NaCHO2. Calculate the pH of this solution at 25oC after 92.9 mL of 0.154 M HCl is added to this buffer. Assume that the volumes of the solutions are additive.

Answer 1

use thw Hendersen /Haselbach relationship

pH = pKa + log base/acid

pKa = 3.77  

next get mmoles of each form

505mL x .503M  = 254.02 mmoles of  acid form

466mL x .476M  = 221.82 mmoles

you will be adding 92.9 mL x 0.154 M HCl = 14.3 mmoles

so by adding this amount of H+ you will decrease the base form by 14.3 mmoles and increase the acid form by 14.3 mmoles  so the pH will be

pH = 3.77 + log {268.3 / 207.52} = 3.882