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Question 11.4: CALCULATING THE MOLALITY OF A SOLUTION What is the molality ...

CALCULATING THE MOLALITY OF A SOLUTION

What is the molality of a solution made by dissolving 1.45 g of table sugar (sucrose, C_{12}H_{22}O_{11}) in 30.0 mL of water? The molar mass of sucrose is 342.3 g/mol.

STRATEGY
Molality is the number of moles of solute per kilogram of solvent. Thus, we need to find how many moles are in 1.45 g of sucrose and how many kilograms are in 30.0 mL of water.

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The number of moles of sucrose is

1.45 g sucrose × \frac{1  mol  sucrose}{342.3  g  sucrose}  =  4.24  ×  10^{-3} mol sucrose

Since the density of water is 1.00 g/mL, 30.0 mL of water has a mass of 30.0 g, or 0.0300 kg. Thus, the molality of the solution is

Molality = \frac{4.24  ×  10^{-3}  mol}{0.0300  kg} = 0.141 m

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