Chem 1C – Chapter 17 – CLAS

e-mail ⇒ benzene4president@gmail.com website ⇒ clas.sa.ucsb.edu/staff/terri/ drop-in ⇒ M/W 10-11, and F 10-12 and 2-31. What is the molality of an aqueous solution that is 12.5% methanol by weight (molar mass of CH3OH is 32.05 g/mol).

2. Calculate the molality and mole fraction of an aqueous solution that is 8 M NaCl, the density of the solution is 1.18 g/ml.

3. An aqueous solution of acetic acid has a molality of 9.2 m and a density of 1.05 g/mL. Calculate the molarity of the solution.

4. Which of the following statements is/are correct?

a. Acetone (CH3C(O)CH3) does not hydrogen bond to itself but can hydrogen bond with waterb. Ion-dipole forces result when dissolving an ionic compound into waterc. Solutes tend to be more soluble at higher temperaturesd. Molarity, molality and mole fraction are independent of temperaturee. Oxygen gas is more soluble in water than carbon monoxide at any one temperature and pressure

5. A schematic for the dissolution of a solute into a solvent is shown below:

Based on this schematic, which of the following statements is/are true? a. H1 is negative b. Step 2 is endothermic c. Generally, heat is released during Step 3d. Two of these statements are true e. Statements (a), (b) and (c) are all true

6. Rank the following aqueous solutions in order of increasing vapor pressure:

a. 0.1 M C6H12O6 b. 0.1 M KBr c. 0.05 M Na2SO4 d. 0.05 M CH3OH

7. Calculate the vapor pressure of a solution (in torr) made by dissolving 159 g of ethylene glycol (HOCH2CH2OH – 62.08g/mol) in 500 g of water at 27 °C. At 27 °C the vapor pressure of pure water is 26.7 torr.

8. At 25 °C the vapor pressures of pure toluene (C7H8 – 92.15 g/mol) and pure benzene (C6H6 – 78.12 g/mol) are 28 torr and 95 torr respectively.

a. What is the vapor pressure of a solution prepared by mixing 40 g of toluene with 15 g of benzene at 25 °C? b. What is the mole fraction of the benzene in the vapor above the solution?

9. What composition of a pentane and hexane solution at 25 °C would give a vapor pressure of 350 torr? At 25 °C the vapor pressures of pure pentane and hexane are 511 torr and 150 torr respectively.

10. After substance A is mixed with substance B the solution feels hotter than before they were mixed. What deviation from Raoult’s Law (if any) would be expected for this solution?

a. no deviationb. positive deviation – A and B form stronger forces than pure A and Bc. positive deviation – A and B form weaker forces than pure A and Bd. negative deviation – A and B form stronger forces than pure A and Be. negative deviation – A and B form weaker forces than pure A and B

11. Predict if the following solutions will be ideal or non-ideal. What type of deviation from Raoult’s Law would you expect for the non-ideal solutions?a. carbon tetrachloride (CCl4) and dichloromethane (CH2Cl2)b. water and acetone (CH3COCH3)c. carbon tetrachloride and benzene (C6H6)

12. The vapor pressures of several solutions of water and butanol were determined at various compositions and the

data is given below:

a. What is the vapor pressure of a solution with XH2O according to Raoult’s Law?b. Are the solutions of water and butanol ideal?c. Which of the above solutions would have the lowest boiling point?

13. Rank the following aqueous solutions by their boiling points, and freezing points.

a. 0.2 M CH2Ob. 0.15 M LiFc. 0.08 M (NH4)2SO4

14. Calculate the boiling point and freezing point of a solution made by dissolving 110 g of K3PO4 (212.3g/mol) in 800 g of water at 1 atm. For water kb = 0.51 °Ckg/mol and kf = 1.86 °Ckg/mol.

15. A solution contains 3.75 g of a nonvolatile hydrocarbon in 95 g of acetone. The boiling points of pure acetone and the solution are 55.9 °C and 56.5 °C respectively. What is the molar mass of the hydrocarbon? For acetone the kb = 1.71 °Ckg/mol.

16. Calculate the osmotic pressure of a solution made by dissolving 83 g of NaF in enough water to make 1.2 L of solution at 30 °C.

17. A solution that contains 29 g of non-volatile/non-ionizing solute in 126 g of water has a vapor pressure of 723.4 torr at 100 °C. What is the molar mass of the solute?

18. A solid mixture contains MgCl2 (molar mass= 95.218 𝑔 𝑚𝑜𝑙) and NaCl (molar mass = 58.443 𝑔 𝑚𝑜𝑙). When 0.500 g of this solid is dissolved in enough water to form 1.000 L of solution, the osmotic pressure at 25.0˚C is observed

to be 0.3950 atm. What is the mass percent of MgCl2 in the solid? (Assume ideal behavior for the solution.)

Solubility ⇒ the maximum amount of solute that can dissolve in a given amount of solvent at any one T and P

Solubility Considerations

1. Polarity of solute and solvent ⇒ “like dissolves like” 2. Temperature ⇒ Substances are more soluble at high T’s if ΔSdiss > 0 (typical for solid solutes) and less soluble at

high T’s if ΔSdiss < 0 (typical for gas solutes) 3. Pressure ⇒ gases are more soluble at high P’s ⇒ C = kP (Henry’s Law is an example of Le Chatlier’s principle)

Concentrations Variations:

Molarity ⇒ M = n soluteL solution   Molality ⇒ m =

n solutekg solvent   Mole fraction ⇒ Xa =

n an total 

Colligative properties ⇒ the properties of a solution with a non-volatile solute relative to a pure solventAs the concentration of solute particles ↑vapor pressure ↓ “depression” ⇒ Pa = Xa Pa°freezing point ↓ “depression” ⇒ ΔTf = - kf i mboiling point ↑ “elevation” ⇒ ΔTb = kb i mosmotic pressure ↑ ⇒ π = iMRT

i = vant Hoff factor = moles of solute particlesmoles of solute

Ideal vs. Non-Ideal Solutions

a. ΔHsol’n ~ 0 ⇒ Ideal solution – the actual vapor pressures will agree with Raoult’s lawb. ΔHsol’n > 0 ⇒ Non-Ideal solution – the forces in the solution are weaker than in the pure substances resulting in higher

VPs than expected from Raoult’s Lawc. ΔHsol’n < 0 ⇒ Non-Ideal solution – the forces in the solution are stronger than in the pure substances resulting in lower VPs than expected from Raoult’s Law

Osmosis ⇒ the diffusion of water thru a semipermeable membrane

Osmotic pressure (π) ⇒ the minimum pressure that Reverse osmosis ⇒ increasing the concentration must be applied to keep osmosis from gradient by the movement of water thru a occurring ⇒ π= iMRT ⇒ as the concentration semi-permeable membrane by applying gradient ↑ π↑ a pressure that is > the osmotic pressure