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Question
A solution of glucose in water is labelled as 10% (W/W).
Calculate:
a. Molality
b. Molarity of the solution.
[Given: Density of solution is 1.20 g mL-1 and molar mass of glucose is 180 g mol-1 ]
Solution
Given: Glucose solution is 10% W/W, Density of solution = 1.20 g mL-1
To find: a. Molality b. Molarity
`1. "Molality" = "Number of moles of solute"/"Mass of solvent in kg"`
`2. "Molarity" = "Number of moles of solute"/"Volumeof solution in L"`
Calculation: 10 % W/W of glucose solution means 100 g of solution contains 10 g of glucose.
Mass of glucose = 10 g
Mass of solution = 100 g
Mass of H2O = 100 – 10 = 90 g = 0.090 kg
Molar mass of glucose = 180 g mol-1
`"Number of moles of glucose" = " Massof glucose"/"Molar massof glucose"`
`=(10 g)/(180 g mol^-1)=0.0556 mol`
From formula (1),
`"Molality of glucose solution "="Number of molesof glucose"/"Massof H2Oin kg"`
= 0.0556 mol/0.090 kg
= 0.618 m
`b. "Density of solution" =" Massof solution"/"Volumeof solution"`
Volume of solution = 100g/1.20g mL-1 = 83.33 mL = 0.08333 L
`"Molarity of glucose solution "= "Number of molesof glucose"/"Volume of solution in L"`
= 0.0556 mol/0.08333 L
= 0.67 M
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