As an example, let's calculate [OH^{}] given that [H_{3}O^{+}]
= 4.5 x 10^{5} M. Starting with the water constant equation [H_{3}O^{+}]·[OH^{}]
= K_{w}, we can figure that [OH^{}] = K_{w}/[H_{3}O^{+}].
Then, substitute the known values for K_{w} and [H_{3}O^{+}] to
get that [OH^{}] is equal to 1.0 x 10^{14} divided by 4.5 x 10^{5}.
That comes out to be 2.2 x 10^{10} M for the concentration of hydroxide
ion. 
Problem: Given that [H_{3}O^{+}]
= 4.5 x 10^{5} M, calculate [OH^{}]. 
[H_{3}O^{+}]·[OH^{}]
= K_{w}, 
[OH^{}] = K_{w}/[H_{3}O^{+}]. 
[OH^{}] = (1.0 x 10^{14}) ÷
(4.5 x 10^{5}) 
[OH^{}] = 2.2 x 10^{10}
M 
