Consider the following general reaction (acetic acid + water in equilibrium with

Question

Consider the following general reaction (acetic acid + water in equilibrium with hydronium + acetate): HC2H3O2 (aq) + H2O (l) H3O+ (aq) + C2H3O2– (aq) Kc = 1.8 x 10–5 (at 25 ºC) If a solution initially contains (4.0x10^-1) M of HC2H3O2, (and no H3O+ or C2H3O2– ),what is the equilibrium concentration of H3O+ (aq) in the mixture? In the appropriate blanks, provide the equilibrium [H3O+ (aq)], in the solution, in M, to two significant figures and to the appropriate power (e.g. Write only the values as underlined, e.g. 5.1x10-3, do not write anything else).

Explanation / Answer

Kc = [H3O+] [C2H3O2-] / [HC2H3O2] (Eq 1)

At any point as the H3O+ is generated by loosing a H+ from from HC2H3O2

[H3O+] = [C2H3O2-] (Eq 2)

So from equation 1 & 2

Kc = [H3O+] [H3O+] / [HC2H3O2]

Kc = [H3O+]2 / [HC2H3O2]

So

[H3O+] = {[HC2H3O2] * Kc}1/2 (Eq 3)

[H3O+] = {4 * 10 -1 * 1.8 * 10 -5 }1/2

[H3O+] = 2.7 * 10-3

So equilibrium concentration of H3O+ is 2.7 * 10-3

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