Question

Using a 0.20 M phosphate buffer with a pH of 7.6, you add 0.72 mL of 0.46 M NaOH to 55 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)

Answer 1

The buffer we are dealing with is composed mainly by the following species: and it has a pK_a = 7.21 To calculate the relation between HPO₄^-2 and H₂PO^4- , we can use the hendersson hasselbach equation:

Where 7.21 is the pK_a, [A-] = HPO₄^-2 and [AH]= H₂PO^4-

use the hendersson hasselbach equation as given 0.20 M phosphate buffer and  0.46 M NaOH

pH = pK_a + log ([Base] / [ Acid]) = 7.21 + log (0.46/0.20)

pH = 7.21 + log (2.3) = 7.21 + 0.361 = 7.57

Therefore,the new pH of the solution = 7.57 (three significant figures)