Question

one kg of hard water contain 30mg of magnesium sulphate, then the degree of hardness of water is

Answer 1

30 mg MgSO4 means 30/120 millimoles of MgSO4 i.e. 0.25 millimoles of MgSO4

Thus weight of equal number of moles of CaCO3 = 0.25 *100 = 25 mg

Thus degree of hardness = 25*1000000/1000000 = 25

Answer 2

The degree of hardness of water is 25 ppm

Explanation:

Mass of water = 1 kg

Density of water = 1 kg/litre

Therefore, volume of hard water = 1 litre

Weight of MgSO₄ = 30 mg

Molecular weight of  MgSO₄ = 120 g/mol

Valency = 2

Equivalent weight of  MgSO₄ = 120/2 = 60

Gram equivalent of  MgSO₄ = gram equivalent of CaCO₃

Weight of MgSO₄ /Equivalent weight of MgSO₄ = Weight of CaCO₃/Equivalent weight of CaCO₃

or, 30/60 = Weight of CaCO₃/50

or, Weight of Weight of CaCO₃ = 25 mg

Thus, in 1 L of water, hardness as CaCO₃ = 25 mg/L

Therefore, the degree of hardness of water = 25 ppm

Hope this answer is helpful.

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