Question

A kilogram of solution contains following particles dissolved in water 3g of NaCl and 5g of CaCl calculate the osmolarity of the solution ( molecular weight NaCl 58.44g/mol and 110.98g/mol forCaCl).

Answer 1

Osmolarity of a solution represents number of osmols of solute particle per litre. Here the solution contains both NaCl and CaCl. So, first we need to calculate individual osmols concentration of NaCl and CaCl. Then finally adding them to get osmolarity of the solution.

We have 3g NaCl in a kg of solution. Molecular weight of NaCl is 58.44 g/mol. So, 3g will be 3/58.44 = 0.05 mol. Concentration will be 0.05 mol/ kg solution.

We know that 1 mol of NaCl gives two osmol of particles (Na⁺ and Cl^{​​​​​-}). Hence, 0.05 mol of NaCl will give 0.1 osmol in the kilogram of solution.

Similarly, we have 5g CaCl in a kg of solution. Molecular weight of CaCl is 110.98 g/mol. So, 5 g will be 5/110.98 = 0.045 mol. Concentration will be 0.045 mol/kg solution.

1 mol of CaCl (CaCl_​​​2) gives three osmol of particles ( Ca^​​​+ and two Cl^{​​​​​-}​). Hence, 0.045 mol of CaCl will give 0.135 osmol in the kilogram of solution.

Osmolarity of the solution is = 0.1 osmol/kg + 0.135 osmol/kg = 0.235 osmol/kg.

Hence, osmolarity of the solution is 0.235 osmol/kg.