$36$ g of glucose ($M = 180$ g/mol) is dissolved in $200$ g of water. The molality of the solution is:
A$0.5$ mol/kg
B$2.0$ mol/kg
C$0.1$ mol/kg
D$1.0$ mol/kg
Answer & Solution
Correct answer: D. $1.0$ mol/kg
Moles of glucose: $\dfrac{36}{180} = 0.2$ mol.
Mass of solvent in kilograms: $\dfrac{200}{1000} = 0.2$ kg.
Molality: $m = \dfrac{0.2}{0.2} = 1.0$ mol/kg.
Note the use of solvent mass (not solution mass) in the denominator: that distinction separates molality from molarity. For very dilute aqueous solutions the two are numerically close, but they diverge for concentrated solutions.
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