Question

A biochemist prepares two stocks of sodium phosphate buffer by titrating phosphoric acid with sodium hydroxide. She prepares a 0.10 M solution of sodium phosphate buffer at pH 2.15, and she prepares a 0.10 M solution of sodium phosphate buffer at pH 7.20. If she mixes 50. mL of the pH 2.15 buffer with 30. mL of the pH 7.20 buffer, what will be the pH of the resulting solution? Recall that the pKas of phosphoric acid are 2.15, 7.20, and 12.35.

Answer 1

For 0.10 M solution of sodium phosphate buffer at pH 2.15,

50. mL of 0.10 M solution of sodium phosphate buffer at pH 2.15 will contain of

and of

222

For 0.10 M solution of sodium phosphate buffer at pH 7.20,

30. mL of 0.10 M solution of sodium phosphate buffer at pH 7.20 will contain of

and of

When 50. mL of the pH 2.15 buffer are mixed with 30. mL of the pH 7.20 buffer, the resulting solution will have

of

0.0025 moles of and 0.0015 moles of

0.0015 moles of will combine with 0.0015 moles of to form 0.0030 moles of .

moles of will remain.

0.0040+0.0030=0.0070 moles of are now present.

When 50. mL of the pH 2.15 buffer are mixed with 30. mL of the pH 7.20 buffer, the resulting solution will have pH of 2.99