Textbook Question
An aqueous KNO3 solution is made using 55.3 g of KNO3 diluted to a total solution volume of 2.00 L. Calculate the molality of the solution. (Assume a density of 1.05 g>mL for the solution.)
14. Solutions
Molality
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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?
A
0.67 molal
B
0.45 molal
C
0.56 molal
D
0.75 molal
Verified step by step guidance1
Calculate the moles of CaCl2: First, determine the molar mass of CaCl2 by adding the atomic masses of calcium (Ca) and chlorine (Cl). The molar mass of CaCl2 is the sum of the atomic mass of Ca (approximately 40.08 g/mol) and twice the atomic mass of Cl (approximately 35.45 g/mol each). Use the formula: \( \text{Moles of CaCl2} = \frac{\text{mass of CaCl2}}{\text{molar mass of CaCl2}} \).
Convert the mass of water to kilograms: Since molality is defined as moles of solute per kilogram of solvent, convert the mass of water from grams to kilograms by dividing by 1000. Use the formula: \( \text{mass of water in kg} = \frac{\text{mass of water in g}}{1000} \).
Calculate the molality of the solution: Molality (m) is calculated using the formula \( m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}} \). Substitute the moles of CaCl2 and the mass of water in kilograms into this formula.
Understand the concept of molality: Molality is a concentration unit that expresses the moles of solute per kilogram of solvent. It is temperature-independent because it is based on mass, not volume.
Verify the calculation: Double-check each step to ensure that the molar mass, moles of solute, and mass of solvent are correctly calculated and used in the molality formula. This helps confirm the accuracy of the final result.
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