Question

A compound has a pKa of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. The resulting solution is pH:

(if using the henderson hasselbach equation, how do you go from log [A-]/[HA] = 0.6 to [A-]/[HA]=4 ?)

6.5

6.8

7.2

7.4

7.5

Answer 1

if using the henderson hasselbach equation, then

The rule to be followed is if , then

The buffer is 100 mL of 1.0 M solution.

Hence, the solution is 0.8 M in conjugate base and 0.2 M in weak acid.

100 mL of solution will contain and

Now 30 mL of 1.0 M hydrochloric acid is added. It corresponds to

0.03 moles of HCl will react with 0.03 moles of to form 0.03 moles of HA. Total number of moles of HA moles HA. moles of will remain. Since the number of moles of HA is equal to the number of moles of , the pH of the solution will be equal to pKa. It will be 7.4.