If the density of some lake water is 1.25 g mL-1 and contains 92 g of Na+ ions per kg of water, calculate the molality of Na+ ions in the lake.

If the density of some lake water is 1.25 g mL^-1and contains 92 g of Na⁺ ions per kg of water, calculate the molality of Na⁺ ions in the lake.

Solution :

Density of lake water = 1.25 g mL^-1
Amount of Na⁺ ion in one kg of water = 92 g Atomic mass of Na⁺= 23 g mo1-1 Let V ml of lake water be taken.

Let take mass of solvent(water) = 1 Kg

Then mass of Na⁺ ions  = 92 g

Number of moles of  = 92/23 = 4  (molar mass of Na is 23g/mol)

Plug the values we get

Molality = 4 /1 = 4m

Solution molality of Na ⁺ ions = 4m