1. What's the concentration of hydronium ions if a water-base solution has a temperature of 25°C (Kw = 1.0x10-14), with a concentration of hydroxide ions of 2.21x10-6 M? A. 3.1x10-6 M
B. 4.52 X10-9 M
C. 2.8x10-8 M
D. 1.6x10-9 M

B. 4.52 X10-9 M

Explanation:

Our goal for this question is to calculate the concentration of hydronium ions produced by water in a vessel with a concentration of hydroxide ions of . So, our first approach can be the ionization reaction of water:

If we write the Keq expression for this reaction we will have:

Now, water is the universal solvent, so, Keq has a special name. In the equilibrium problems for water we have to use "Kw" instead of "Keq":

From this equation, we know the Kw value () and the concentration of the hydroxide ions ([2.21X10^-^6~M]). If we replace these values into the equation we can solve for :

I hope it helps!