Question

Four different solutions (I, II, III, and IV) are labeled on the pH scale below.

A p H scale is shown, fading from red at the left end to blue at the right. p H is shown in even increments from 0 to 14. Points marked are: Roman numeral 1 at 1 on the scale, Roman 2 at 6, Roman 3 at 11, and roman 4 at 13.

Why would solution IV be considered the most concentrated base out of the four substances?
It has the highest concentration of hydrogen ions.
It has the highest concentration of hydronium ions.
It has the highest concentration of oxide ions.

Answer 1

Neither of the three choices is correct as written. Solution IV (pH 13) is the most basic because it has the highest concentration of hydroxide ions (OH−).

Explanation: high pH means low [H+] (or H3O+) and, at 25 °C, pOH = 14 − pH. For pH 13, pOH = 1 so [OH−] = 10−1 M, which is larger than the [OH−] in the other solutions. Oxide ions (O2−) are not the relevant species in aqueous bases.