Shin U.

asked • 04/12/21

Calculate the molality of a 100-gram 25% by mass sucrose [C12H22O11] in water.

how to calculate its molality

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J.R. S. answered • 04/12/21

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Molality = moles sucrose / kg solvent.

First, we will find the mass of sucrose and the mass of water.

100 g solution x 25% sucrose = 25 g sucrose

This leaves 75 g water.

Converting 25 g sucrose to moles we have...

25 g sucrose x 1 mol sucrose / 342 g = 0.073 moles sucrose

Converting 75 g water to kg, we have...

75 g x 1 kg / 1000 g = 0.075 kg


Molality = 0.073 moles / 0.075 kg = 0.98 molal

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J.R. S.

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You asked how we got 75 g for the mass of water. The total mass of solution is 100 g. This is made up of sucrose and water. The mass of the sucrose was determined to be 25 g. That means the rest of the mass must be water. 100 g - 25 g = 75 g water.
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04/13/21

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