Question

Use the Acid-Base Table to determine the pKa of the weak acid H2O. Express your answer to two decimal places.

pKa = _________

Answer 1

Answer on the Question #63813, Chemistry / General chemistry

Use the Acid-Base Table to determine the pKa of the weak acid H2O. Express your answer to two decimal places.

pKa=__________

Solution:

Dissociation of the weak acid like water occurs by the following reaction:

$$H _ {2} O _ {(a q)} = O H _ {(a q)} ^ {-} + H _ {(a q)} ^ {+}$$

The law of mass action for this equation would be expressed as:

$$K _ {a} = \frac {a _ {O H ^ {-}} \cdot a _ {H ^ {+}}}{a _ {H _ {2} O}} = \frac {[ O H ^ {-} ] \cdot [ H ^ {+} ]}{[ H _ {2} O ]}$$

The product of $[OH^{-}]\cdot [H^{+}] = 1\cdot 10^{-14}$ and concentration of water $[H_2O] = 55.33M,$ thus

$$K _ {a} = \frac {1 \cdot 1 0 ^ {- 1 4}}{5 5 . 3 3} = 2 \cdot 1 0 ^ {- 1 6}$$

Therefore:

$$p K _ {a} = - \log (K _ {a}) = - \log (2 \cdot 1 0 ^ {- 1 6}) = 1 5. 6 9$$

Answer: pKa=15.69

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