Question

92gm of sodium sulphate is added to 1kg water what is the molality of Na+ ion

Answer 1

Weight of sodium sulphate = 92gm

molecular weight of sodium sulphate , Na2SO4 = 2 × 23 + 32 + 16 × 4
= 46 + 32 + 64
= 142gm/mol

number of mole of sodium sulphate , (Na2SO4) = 92gm/142gm/mol = 0.647 mol

weight of water, (solvent) = 1kg

now, molality = number of mole solute/weight of solvent in kg

so, molality of Na+ ion = 0.647mol/1kg
= 0.647m

Answer 2

The weight of Na2SO4 is given as 92gm

And 1Kg water

So first of all we calculates the moles of compounds as shown in formula

Moles = mass/ molar mass

92= mass

Molar mass of Na2SO4 = 142.04g/mole

= mass / mol.mass

92/ 142.04 = 0.64770g/mole

Molarity = 0.64770 / 1kg of Water

= 0.00064m