Question

a mass of 5.96 g of hydrated disodium hydrogenphosphate, na2hPO4.7h2O, was dissolved in 250 cm3 water...

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

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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

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