A buffer solution contains 0.384 M KHCO3 and 0.239 M Na2CO3 . If 0.04

1.	A buffer solution contains 0.384 M KHCO3 and 0.239 M Na2CO3 . If 0.0464 moles of potassium hydroxide are added to 225.0 mL of this buffer, what is the PH of the resulting solution? (Assume the volume does not change upon adding potassium hydroxide.)
Answer» Calculate moles of bicarbonate and carbonate: HCO₃¯: (0.384 mol/L)(0.2250 L) = 0.0864 mol CO₃²¯: (0.239 mol/L)(0.2250 L) = 0.053775 mol The hydroxide reacts with the acid (the bicarbonate): HCO₃¯ decreases: 0.0864 - 0.0464 = 0.0400 mol CO₃²¯ increases: 0.053775 + 0.0464 = 0.100175 mol Need pKa of bicarbonate: K_a of HCO₃¯ is the same as the K_a2 of H₂CO₃. K_a of HCO₃¯ = 4.7 x 10¯¹¹ pK_a = 10.252 Use Henderson-Hasselbalch Equation: pH = 10.328 + log (0.100 / 0.04) pH = 10.328 + 0.398 = 10.726

A buffer solution contains 0.384 M KHCO3 and 0.239 M Na2CO3 . If 0.0464 moles of potassium hydroxide are added to 225.0 mL of this buffer, what is the PH of the resulting solution? (Assume the volume does not change upon adding potassium hydroxide.)

Answer»

Calculate moles of bicarbonate and carbonate:
HCO₃¯: (0.384 mol/L)(0.2250 L) = 0.0864 mol
CO₃²¯: (0.239 mol/L)(0.2250 L) = 0.053775 mol

The hydroxide reacts with the acid (the bicarbonate):
HCO₃¯ decreases: 0.0864 - 0.0464 = 0.0400 mol
CO₃²¯ increases: 0.053775 + 0.0464 = 0.100175 mol

Need pKa of bicarbonate:
K_a of HCO₃¯ is the same as the K_a2 of H₂CO₃.
K_a of HCO₃¯ = 4.7 x 10¯¹¹
pK_a = 10.252

Use Henderson-Hasselbalch Equation:
pH = 10.328 + log (0.100 / 0.04)
pH = 10.328 + 0.398 = 10.726

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