Question

Adding salt to water when boiling vegetables or pasta makes the water boil at a higher temperature. How much would the boiling point of water be increased if 49.2 g of NaCl is added to 500 mL of water? (Remember that the density of water is 1 g/mL and kb for water is 0.51 oC/m)

Answer 1

Adding a non-volatile solute in solvent, increases the boiling point of solvent. This is referred as elevation in boiling point. The increase in boiling point is mathematically expressed as
ΔTb = Kb X m ....... (1)
where, Kb = molal elevation constant = 0.51 oC/m
m = molality of solution =
`\frac{\text{weight of solute (g)}}{\text{Molecular weight X Weight of solvent (Kg)}}`

Given: weight of solute = 49.2 g
volume of solution = 500 ml
Density of water = 1 g/ml

∴ weight of solvent = 500 g = 0.5 kg

Also,we know that molecular weight of NaCl = 58.5

∴m = molality of solution =
`\frac{\text{49.2}}{\text{58.5 X 0.5}}`
= 1.68 m

The molality of solution is 1.68 m ...... (2)

Substituting 2 in 1 we get
ΔTb = Kb X m = 0.51 x 1.68 = 0.8568 oC

Thus, increase in boiling point of water is 0.8568 oC
Final boiling point of solution = 100 + 0.8565 = 100.8568 oC