Hardness in water is the sum of divalent cations which is equal to Ca²⁺ and Mg²⁺. Hardness is expressed as mg/L of CaCO₃. Soft water has hardness less than about 60 mg/L of CaCO_3; moderately hard water 75-150, and hard water 150-300, very hard water > 300. Total Permanent Hardness = Calcium Hardness + Magnesium Hardness

The ratio of molar masses are:

M_CaCO3 = 100.1 = 2.5             M_CaCO3 = 100.1 = 4.1

M_Ca          40.1                      M_Mg       24.3

Total permanent water hardness expressed as equivalent CaCO3 can be calculated with the following formula:

[CaCO₃] = 2.5 [Ca²⁺] + 4.1 [Mg²⁺]

Calculate the hardness of water for the following two situations and tell me if it is hard or soft water.

a. Mg²⁺ = 5 mg/L ; Ca²⁺= 15 mg/L

b. Mg²⁺ = 10 mg/L ; Ca²⁺= 50 mg/L