What is the percentage by mass of water of crystallization in copper(II) tetraoxosulfate(V… — JAMB UTME

What is the percentage by mass of water of crystallization in copper(II) tetraoxosulfate(VI) pentahydrate ( $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ )? $[\text{Cu} = 64, \text{S} = 32, \text{O} = 16, \text{H} = 1]$

A
$7.2\%$
$36.0\%$ Answer
C
$56.3\%$
D
$64.0\%$

Answer

The percentage by mass of water of crystallization in

\text{CuSO}_4 \cdot 5\text{H}_2\text{O}

36.0\%

The total molar mass of

\text{CuSO}_4 \cdot 5\text{H}_2\text{O}

250\text{ g/mol}

and the mass contributed by the five water molecules is

90\text{ g/mol}

. Dividing

90

250

and multiplying by

100\%

yields

36.0\%

Step-by-Step Solution

Calculate the molar mass of water (

\text{H}_2\text{O}

) and the total mass of five moles of water.

Molar mass of

\text{H}_2\text{O} = (2 \times 1) + 16 = 18\text{ g/mol}

. For

5\text{H}_2\text{O}

, mass

= 5 \times 18 = 90\text{ g/mol}

Water of crystallization in the formula consists of five water molecules per formula unit.

Calculate the total molar mass of hydrated copper(II) tetraoxosulfate(VI) (

\text{CuSO}_4 \cdot 5\text{H}_2\text{O}

Molar mass

= 64 + 32 + (4 \times 16) + 90 = 64 + 32 + 64 + 90 = 250\text{ g/mol}

The percentage composition must be based on the complete formula weight of the hydrated compound.

Calculate the percentage by mass of water of crystallization.

\text{Percentage of } \text{H}_2\text{O} = \left(\frac{90}{250}\right) \times 100\% = 36.0\%

Percentage composition by mass is the mass of the component divided by the total molar mass of the compound multiplied by 100.

Key Concept

Percentage Water of Crystallization in Hydrated Salts

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