Question

15) What is the hydronium ion concentration [H3O + ] of a 0.125 M hydrocyanic acid solution with Ka = 4.9 × 10-10? The equation for the dissociation of hydrocyanic acid is: HCN(aq) + H2O(l) ⇌ H3O+(aq) + CN- (aq)

Answer 1

The hydronium ion concentration [H₃O⁺]=7.826 x 10⁻⁶

Further explanation

In general, the weak acid ionization reaction

HA (aq) ---> H⁺ (aq) + A⁻ (aq)

Ka's value

`\large {\boxed {\bold {Ka \: = \: \frac {[H ^ +] [A ^ -]} {[HA]}}}}`

Reaction

HCN(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CN⁻ (aq)

0.125

x x x

0.125-x x x

`\tt Ka=([H_3O^+][CN^-])/([HCN^-])`

`\tt 4.9* 10^(-10)=(x^2)/(0.125-x)\approx (x^2)/(0.125)\\\\x^2=6.125* 10^(-11)\\\\x=7.826* 10^(-6)`