DETERMINATION oF MOLAR EXPERIMENT 23 MASS FROM FREEZING POINT DEPRESSION EQUIPME
ID: 481092 • Letter: D
Question
DETERMINATION oF MOLAR EXPERIMENT 23 MASS FROM FREEZING POINT DEPRESSION EQUIPMENT AND CHEMICALS: Equipment 600 mL beaker Chemicals Cyclohexane (C Stir plate 100 ml. graduated cylinder Clamp Nonvolatile Unknowns Freezing point apparatus Spin vane 0.1 C Thermometer INTRODUCTION: When a solute is introduced into a pure solvent the newly formed solution will have certain physical properties that are from the solvent. As a of the presence of the the pressure of the solution will be lower than that of the pure solvent. As the vapor pressure is lowered there is a resulting increase in the amount of energy required to reach the boiling point of the solution. Conversely, the solution must cool to a lower temperature before a state change from liquid to solid occurs. These changes in physical properties vapor pressure lowering, boiling point elevation and freezing point depression are collectively referred to as colligative properties. These colligative properties, which also include osmotic pressure, do not depend on the nature of the nonvolatile solute in the solution but rather on the concentration of solute present. Because the properties are independent of the nature of the solute, they may be used to determine other properties of the solute such as molecular weight. The effect of a nonvolatile, nonelectrolyte solute on a pure solvent will be examined by finding the freezing point depression of a cyclohexane unknown so solution. The freezing point of a pure sample of cyclohexane will be determined. Introducing a nonelectrolyte solute should in solution formed to freeze at a lower temperature. This temperature will then be experimentally determined As stated the The molality of the solution dependent on the quantity of solute particles present in solution. (moles solute particles per kilogram of solvent) is directly proportional to the change in temperature between the freezing point of the pure solvent and that of the ATfss Kim The molal freezing point depression constant, K for cyclohexane is equal to 20.00 cfmolal By experimentally determining the freezing point depression of a ohexanel unknown solution, the molal concentration is easily calculated. In turn the molar mass of the unknown olute can be calculated as well Chemistry 132 EXPERIMENT23Explanation / Answer
1. Unknown no. 9
2. Volume of pure cyclohexane = 30 mL
3. Mass of pure cyclohexane = Volume * Density
= 30 * 0.779 = 23.37 g
= 0.0234 kg
4. Mass of unknown = 0.501 g
5. Freezing point of pure cyclohexane = 6.5 °C
6. Freezing point of unknown = 3.6 °C
7. T = 6.5 – 3.6 = 2.9 °C
8. Kf = 20 °C/molal
T = Kf * molality
Molality of solution, m = 2.9 / 20 = 0.145 mol/ kg solvent
9. Moles unknown = m * mass of cyclohexane
= 0.145 * 0.0234 = 0.00339 moles
10. Molar mass of unknown = Mass of unknown / Moles of unknown
= 0.501 / 0.00339 = 147.85 g/mol
Q1.
Sources of error:
a)Small amount of impurities present
b)Thermometer inaccuracies
Q2.
T will remain same when thermometer consistently shows 1.5 °C lower.
Therefore molar mass determined will also remain same.
Q3.
T = Freezing point of solvent - Freezing point of solution
T will be higher if freezing point of solution is read 0.3 °C lower than actual value.
Subsequently, molality and moles of solute calculated will be higher.
Molar mass = Mass / Moles
Molar mass calculated would be lower than actual value.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.