Which one of the following solutions will have the greatest concentration of hydroxide ions?

a) 0.100 M hydrochloric acid

b) 0.100 M magnesium hydroxide

c) 0.100 M ammonia

d) 0.300 M rubidium hydroxide

e) 0.250 M calcium hydroxide

e = 0.250 M calcium hydroxide

it will produce 0.5 M [OH⁻].

Explanation:

A = 0.100 M HCl

HCl is an acid it would produce negligible amount of OH⁻.

B = 0.100 M magnesium hydroxide

Mg (OH) ₂ + H₂O → Mg²⁺ + 2OH⁻

The ration of Mg (OH) ₂ and OH⁻ is 1:2.

[OH⁻] = 2 * 0.100 = 0.200 M

C = 0.100 M ammonia

NH₃ + H₂O → NH₄OH → NH₄⁺ + OH⁻

The Kb expression will be written as,

Kb = [NH₄⁺] [OH⁻] / [NH₃]

Kb = 1.8 * 10 ⁻⁵

Now we will put the values:

1.8 * 10 ⁻⁵ = [NH₄⁺] [OH⁻] / 0.100

[NH₄⁺] [OH⁻] = x²

1.8 * 10 ⁻⁵ = x² / 0.100

x² = 1.8 * 10 ⁻⁵ * 0.100

x² = 1.8 * 10 ⁻⁶

x = 1.34 * 10 ⁻³

[OH⁻] = 1.34 * 10 ⁻³ M

d = 0.300 M rubidium hydroxide

RbOH + H₂O → Rb⁺ + OH⁻

The ratio of RbOH and OH⁻ is 1:1.

[OH⁻] = 0.300 M

e = 0.250 M calcium hydroxide

Ca (OH) ₂ + H₂O → Ca²⁺ + 2OH⁻

The ration of Ca (OH) ₂ and OH⁻ is 1:2.

[OH⁻] = 2 * 0.250 = 0.5 M