Write the K_a expression for the following equilibrium of the weak acid, HCN, and water:

HCN(aq) + H₂O (l) ↔ H₃O⁺ (aq)  + CN^-  (aq)

7.K_a =_______________

 

[H₃O⁺] [CN^-]

K_a = ^{____________________}

[HCN]

or

[H⁺] [CN^-]

K_a = ^{___________________}

[HCN]

Step-by-step explanation

This is the chemical formula for weak acid equilibrium:

HA(aq) + H2O(l) ---> H3O+(aq) + A^-(aq)

where:

HA = weak acid

A = aqueous anions

In the formula of HCN and water, the HA is HCN while A^- is CN^-.

HCN(aq) + H2O (l) ↔ H3O+ (aq) + CN- (aq)

Note: When writing an equilibrium constant expression for weak acid or K_a, the concentration of water is not included.

The equilibrium constant for weak acids (K_a) has the formula:

[H₃O⁺] [A^-]

K_a = ^{____________________}

[HA]

or

[H⁺] [A^-]

K_a = ^{____________________}

[HA]

therefore; we can write the K_a expression of HCN in water as:

[H₃O⁺] [CN^-]

K_a = ^{____________________}

[HCN]

or

[H⁺] [CN^-]

K_a = ^{____________________}

[HCN]