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

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