Table of contents

18. Aqueous Equilibrium

Henderson-Hasselbalch Equation

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Multiple Choice

Using the Henderson-Hasselbalch equation, what is the pH of a buffer system that contains 0.87 M dimethylamine ((CH3)2NH) and 0.68 M dimethylammonium ion ((CH3)2NH2+)? The Kb of dimethylamine is 5.9 × 10⁻⁴.

pH = 9.78

pH = 10.22

pH = 8.50

pH = 11.00

Verified step by step guidance

Start by understanding the Henderson-Hasselbalch equation, which is used to calculate the pH of a buffer solution:

pH = pK a + \frac{\log}{\frac{[A -]}{[HA]}}

Since we are given the Kb of dimethylamine, we need to convert it to Ka using the relation:

K w = K a \times K b

, where

K w

is the ion product of water,

1.0 \times 10^{- 14}

Calculate

K a

using the formula:

K a = \frac{K w}{K b}

. Substitute the values:

\frac{1.0 \times 10^{- 14}}{5.9 \times 10^{- 4}}

Once you have

K a

, calculate

pK a

using the formula:

pK a = - \log (K a)

Finally, substitute the values into the Henderson-Hasselbalch equation:

pH = pK a + \log (\frac{0.87}{0.68})

to find the pH of the buffer system.

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