Colligative Propertie Date PRE-LAB QUESTIONS I. Most persons have participated i
ID: 1044004 • Letter: C
Question
Colligative Propertie Date PRE-LAB QUESTIONS I. Most persons have participated in the fun of making homemade ice cream. (If you havent, do it soonl) A key feature of the process takes advantage of a colligative property. Describe why it is necessary to add salt to the ice that surrounds the ice cream container in order to freeze the ice cream inside. freezing point depression of 9.50°C, what is the molality of this solution? (The molal freezing point constant, K, of cyclohexane is 20.2 C/m.) 3. Cyclohexanol, C.H, OH, is sometimes used as the solvent in molecular weight determinations by freezing-paint depression, if 0.253 g of benzoic acid, GHOOOH, dissolved in 12.45 g of cycohexanol lowered the freezing point of pure eycdlohexanol by 6.55'C, what is the molal freezing-point constant, K, of this solvent? 149Explanation / Answer
1. The ice cream mixture freezes at a lower temperature than normal water. The is because of the added sugars and cream which lowers the freezing point of water. Hence surrounding the container with normal ice won't help as the temperature will not be low enough for ice cream to freeze. However, adding salt to the ice surrounding the container will also lower its freezing point. Hence lower temperature will be attainable allowing the ice cream mixture to freeze.
2. Freezing point depression, ?Tf = Kf × m; m = molality; Kf = molal freezing point constant.
Given ?Tf = 9.5°C and Kf = 20.2°C/m
m = ?Tf/Kf = 9.5/20.2 = 0.47 molal
3. 122g of benzoic acid = 1 mole
0.253g of benzoic acid = 0.253/122 = 0.002 moles
Molality of solution = (0.002 × 1000)/12.45 = 0.1606 molal
Given ?Tf = 6.55°C; Kf = ?Tf/m = 6.55/0.1606 = 40.785°C/m
4. Given ?Tf = 4.2°C, Kf = 9.8°C/m
?Tf = Kf × m
m = ?Tf/Kf = 9.8/4.2 = 2.3 molal
Molality = (moles × 1000)/(mass of solvent)
Moles = (molality × mass of solvent)/1000
= 2.3×10.18/1000 = 0.0237 moles
Amount of solute added = 0.68g
Molecular weight = (amount of solute)/moles
= 0.68/0.0237 = 28.69 g/mol