CHMA10H3 Lecture Notes - Lecture 21: Freezing-Point Depression, Gas Constant, Formula Unit

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erinsea-lion742

22 Oct 2018

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UTSC

Department

Chemistry

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CHMA10H3

Professor

Marco Zimmer-De Iuliis, Xiaoan Zhang, Scott Ballantyne

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CHMA10H3 Lecture Notes - Lecture 20: Partial Pressure, Supersaturation, Gas Constant

CHMA10H3 Lecture Notes - Lecture 21: Freezing-Point Depression, Gas Constant, Formula Unit

CHMA10H3 Lecture Notes - Lecture 22: Magnetic Quantum Number, Angular Momentum

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Freezing point depression: the freezing point of a solution is lower than the freezing point of a pure solvent when a non-volatile solvent is added to it: fpsolvent - fpsolution = tf = (m)(kf) Tf = change in the temperature of the freezing point of a solution compared to the freezing point of the pure solvent. Is usually a positive value: m = molality of solution, moles of solute/kg of solvent, kf = freezing point depression constant for the respective solvent used to make the solution. Boiling point elevation: the boiling point of a solution is higher than the boiling point of a pure solvent when a non-volatile solute is added: bpsolution - bpsolvent = tb = (m)(kb) Theoretical van"t hoff factor (cid:894)i(cid:895): ratio of the moles of solute particles to the moles of the formula units that are dissolved.

Related Questions

Â± Boiling Point Elevation and Freezing Point Depression for Organic Solutions

The temperature at which a solution freezes and boils depends on the freezing and boiling points of the pure solvent as well as on the molal concentration of particles (molecules and ions) in the solution. For nonvolatile solutes, the boiling point of the solution is higher than that of the pure solvent and the freezing point is lower. The change in the boiling for a solution, ÎTb, can be calculated as

ÎTb=Kbâ m

in which m is the molality of the solution and Kb is the molal boiling-point-elevation constant for the solvent. The freezing-point depression, ÎTf, can be calculated in a similar manner:

ÎTf=Kfâ m

in which m is the molality of the solution and Kf is the molal freezing-point-depression constant for the solvent.

Part A

Cyclohexane has a freezing point of 6.50 âC and a Kf of 20.0 âC/m. What is the freezing point of a solution made by dissolving 0.463 g of biphenyl (C12H10) in 25.0 g of cyclohexane?

Express the temperature numerically in degrees Celsius.

Part B

Paradichlorobenzene, C6H4Cl2, is a component of mothballs. A solution of 2.00 g in 22.5 g of cyclohexane boils at 82.39 âC. The boiling point of pure cyclohexane is 80.70 âC. Calculate Kb for cyclohexane.

Express the constant numerically in degrees Celsius per molal.

Vapor-pressure lowering is a colligative property, as are freezing-point depression and boiling-point elevation. What is a colligative property? Why is the freezing point depressed for a solution as compared to the pure solvent? Why is the boiling point elevated for a solution as compared to the pure solvent? Explain how to calculate ∆T for a freezing-point depression problem or a boiling-point elevation problem. Of the solvents, which would have the largest freezing-point depression for a 0.50 molal solution? Which would have the smallest boiling-point elevation for a 0.50 molal solution? A common application of freezing-point depression and boiling-point elevation experiments is to provide a means to calculate the molar mass of a nonvolatile solute. What data are needed to calculate the molar mass of a nonvolatile solute? Explain how you would manipulate these data to calculate the molar mass of the nonvolatile solute.

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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 EXPERIMENT23

CHMA10H3 Lecture Notes - Lecture 21: Freezing-Point Depression, Gas Constant, Formula Unit

CHMA10H3 Lecture Notes - Lecture 20: Partial Pressure, Supersaturation, Gas Constant

CHMA10H3 Lecture Notes - Lecture 21: Freezing-Point Depression, Gas Constant, Formula Unit

CHMA10H3 Lecture Notes - Lecture 22: Magnetic Quantum Number, Angular Momentum

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