Question

3. Calculate the pH of a buffer solution made by adding 25.5 g of NaCH3CO2 (MW:82.034g/mol), and 0.550M HCH3CO2 (Ka=1.8x10-5) to make 500 mL of the buffer. a. Identify the acid and the base (write the formula and the ID next to it) b. Will Na+ affect the pH of the solution? Why? c. What is your prediction for the calculated pH compared to the pKa of the sample? Why? d. What is the pH of the solution? Was your prediction correct?

Answer 1

The pH of a buffer is determined by two factors:-

1) Ka of the weak acid and

2) Ratio of weak base [A^-] to weak acid [HA] in solution.

1) K_a tells us what proportion of HA will be dissociated into H⁺ and A^- in solution. The more the H⁺ ions are created, the more acidic and lower will be the pH of the resulting solution.

2) Ratio of A^- to HA of a buffer affects the pH. If a buffer has more base than acid, more OH^-ions will be present and the pH will rise. If a buffer has more acid than base, more H⁺ ions will be present and the pH will decrease.

predicting the pH

since the pka is low it means, the pH of buffer will be more towards acidic, and also since the concentration of the salt is greater than acid.... this will also increase the pH

concentration of Na⁺ concentraion of CH₃COO^- concentraion of OH^- (if Na+ from outside are added)

thus pH depends on the concentraion of concentrion of the Na+