Question

a mass of 5.96 g of hydrated disodium hydrogenphosphate, na2hPO4.7h2O, was dissolved in 250 cm³ water at 298 K. Calculate the molality of the solute in the resulting solution, given that the mass density of water at this temperature is 0.997g cm^-3

Answer 1

Solution :-

Molar mass of Na2HPO4.7H2O = 268.07 g/mol

Mass of solute Na2HPO4.7H2O = 5.96 g

Volume of water = 250 cm3

Density of water = 0.997 g/cm3

Mass of water = volume x density

                         = 250 cm3 * 0.997 g/cm3

                         = 249.25 g

249.25 g * 1 kg/ 1000 g = 0.24925 kg

Lets calculate the moles of solute

Moles= mass/ molar mass

Moles of Na2HPO4.7H2O = 5.96 g / 268.07 g per mol

                                               = 0.0222 mol

Now lets calculate the molality

Molality = moles of solute / kg solvent

               = 0.0222 mol / 0.24925 kg

               = 0.0891 m

Therefore the molality of the solute is 0.0891 m