Question

pH of a 0.01 M HCN solution for which pka is 4 is
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Answer 1

Explanation:

If pKa=4, Ka= 1E-4

As its a weak acid we can assune that the concentration of hydrogen ion equals the concentration of cyanide ions.

So using the Ka expression for HCN, the concentration of hydrogen ion = sqr root(0.01 × 1E - 4)= 0.001 mol/dm^3

Hence pH = -log(0.001) = 3

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Answer 2

If pka=4, Ka 1E - 4

As it is a weak acid we can assume that the concentration of hydrogen ions equals the concentration of cyanide ions.

So using the Ka expression HCN, the concentration of hydrogen ions = square root(0.01 x 1E - 4)= 0.001 mol/dm^3

Hence ph = -log(0.001) = 3

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