Multiple Choice
A solution of water (Kb=0.512 °C/m) and glucose boils at 103.56 °C. What is the molal concentration of glucose in this solution? Assume that the boiling point of pure water is 100.00 °C.
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14. Solutions
Boiling Point Elevation
Multiple Choice
Some people add salt to water, claiming that the salt raises the boiling point of the water, which makes the food cook faster. If 323 g of NaCl is added to a 2.54 L pot of water, what is the new boiling point of the solution?
A
101.3°C
B
102.0°C
C
99.8°C
D
100.5°C
Verified step by step guidance1
Determine the molality of the NaCl solution. First, calculate the number of moles of NaCl using its molar mass. The molar mass of NaCl is approximately 58.44 g/mol. Use the formula: \( \text{moles of NaCl} = \frac{\text{mass of NaCl}}{\text{molar mass of NaCl}} \).
Calculate the molality of the solution. Molality (m) is defined as the number of moles of solute per kilogram of solvent. Assume the density of water is approximately 1 g/mL, so 2.54 L of water is 2540 g. Use the formula: \( m = \frac{\text{moles of NaCl}}{\text{mass of water in kg}} \).
Use the boiling point elevation formula to find the change in boiling point. The formula is \( \Delta T_b = i \cdot K_b \cdot m \), where \( i \) is the van't Hoff factor for NaCl (which is 2, since NaCl dissociates into two ions: Na\(^+\) and Cl\(^-\)), \( K_b \) is the ebullioscopic constant for water (0.512 °C kg/mol), and \( m \) is the molality.
Calculate the change in boiling point (\( \Delta T_b \)) using the values obtained in the previous steps.
Add the change in boiling point (\( \Delta T_b \)) to the normal boiling point of water (100°C) to find the new boiling point of the solution.
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