If [H_3O^+]=5.0xx10^-3*mol*L^-1, what is pH and what is pOH of this solution?

1 Answer
Apr 17, 2017

pH=2.30............

Explanation:

.......and pOH=11.70

These acid base reactions are governed by the equilibrium:

2H_2O(l) rightleftharpoons H_3O^(+) + HO^-.

And at 298*K, K_w=[H_3O^+][HO^-]

log_10K_w=log_10[H_3O^+]+log_10[HO^-]

And since we know that K_w=10^-14, then..........

-14=log_10[H_3O^+]+log_10[HO^-]

OR 14=-log_10[H_3O^+]-log_10[HO^-]

But, by definition, -log_10[H_3O^+]=pH, and -log_10[H_3O^+]=pOH

And thus, BY DEFINITON, pH+pOH=14 under standard conditions. If you can remember this relationship, then these sorts of problems become almost trivial.

And thus, for your problem, pH=-log_10(0.0050)=-(-2.30)=2.30.

pOH=14-2.30=11.70.

And can you tell me what is [HO^-] here? It should be small......