How to find pH of a buffer solution?

A buffer is prepared by mixing 50.4 mL of 0.050 M sodium bicarbonate and 10.8 mL of 0.10 M NaOH.

(a) What is the pH?



(b) How many grams of HCl must be added to 25.0 mL of the buffer to change the pH by 0.07 units?

Answers in gramsHow to find pH of a buffer solution?
I'll do a, you do b.



Ka (H2CO3) = 4.3E-7 and Ka (HCO3-) = 5.6E-11



a)

NaHCO3 + NaOH ---%26gt; Na2CO3 + H2O



50.4 mL * 0.050 M = 2.5 mmol NaHCO3

10.8 mL * 0.100 M = 1.08 mmol NaOH



2.5 mmol NaHCO3 will react completely with 1.08 mmol NaOH to produce 1.08 mmol Na2CO3 (2.5-1.08=1.42 mmol NaHCO3 leftover). So you would have a buffer solution with:

1.42 mmol NaHCO3

1.08 mmol Na2CO3



Use the Henderson-Hasselbalch equation:



pH = pKa + log[A-/HA]



pH = -log[5.6E-11] + log[1.08/1.42]



pH = 10.25 - 0.12 = 10.13