Multiple Choice
A solution is prepared by dissolving 23.7 g of CaCl2 in 375 g of water. The density of the resulting solution is 1.05 g/mL. What is the molality of CaCl2 in this solution?
14. Solutions
Molality
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Concentrated perchloric acid is an aqueous solution that is 70% HClO4 by mass. If the density of this solution is 1.670 g/mL, what is its molality?
A
11.9 mol/kg
B
7.5 mol/kg
C
9.8 mol/kg
D
13.5 mol/kg
Verified step by step guidance1
Determine the mass of HClO4 in 100 g of solution. Since the solution is 70% HClO4 by mass, there are 70 g of HClO4 in 100 g of solution.
Calculate the mass of water in the solution. Since the total mass of the solution is 100 g and the mass of HClO4 is 70 g, the mass of water is 100 g - 70 g = 30 g.
Convert the mass of water from grams to kilograms, as molality is expressed in moles of solute per kilogram of solvent. Therefore, 30 g of water is 0.030 kg.
Calculate the number of moles of HClO4. Use the molar mass of HClO4 (approximately 100.46 g/mol) to convert the mass of HClO4 to moles: \( \text{moles of HClO4} = \frac{70 \text{ g}}{100.46 \text{ g/mol}} \).
Determine the molality of the solution by dividing the moles of HClO4 by the mass of water in kilograms: \( \text{molality} = \frac{\text{moles of HClO4}}{0.030 \text{ kg of water}} \).
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