Solutions

A solution is a homogeneous mixture of two or more components, and most chemical and biological processes, from the action of medicines to the transport of nutrients in plants, take place in solutions. In this chapter you will study the types of solutions and the ways of expressing their concentration: mass percentage, parts per million, mole fraction, molarity and molality. You will learn how temperature and pressure affect the solubility of solids and gases, and apply Henry's law to examples such as carbonated drinks, scuba diving and the bends. Raoult's law relates the vapour pressure of a solution to its composition. You will then distinguish ideal from non-ideal solutions showing positive and negative deviations, and learn about minimum and maximum boiling azeotropes. The central idea of the chapter is colligative properties, which depend only on the number of solute particles: relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure. You will use them to find molar masses, understand reverse osmosis, and explain abnormal molar masses through the van't Hoff factor.

Detailed Worksheet: Solutions Section A - Definitions (10 marks) 1. Define molarity and molality. Which of the two does not change with temperature, and why? (2 marks) 2. State Henry's law and write its mathematical form. How does the value of KH change with temperature? (2 marks) 3. State Raoult's law for a solution of two volatile liquids and for a solution containing a non-volatile solute. (2 marks) 4. What are azeotropes? Give one example each of a minimum boiling and a maximum boiling azeotrope. (2 marks) 5. Define osmotic pressure and the van't Hoff factor. What is the value of i for K2SO4 assuming complete dissociation? (2 marks) Section B - Calculations and Applications (15 marks) 6. 18 g of glucose (C6H12O6, M = 180 g mol^-1) is dissolved in 500 g of water. Calculate (i) the molality of the solution and (ii) the mole fraction of glucose (H2O = 18 g mol^-1). (3 marks) 7. Henry's law constant for N2 in water at 293 K is 76.48 kbar. If N2 exerts a partial pressure of 0.987 bar, calculate the number of millimoles of N2 that dissolve in 1 litre (55.5 mol) of water. (3 marks) 8. Calculate the boiling point of a solution prepared by dissolving 9 g of glucose (M = 180 g mol^-1) in 250 g of water. Kb for water = 0.52 K kg mol^-1 and the boiling point of water = 373.15 K. (3 marks) 9. 1.00 g of a non-electrolyte solute dissolved in 50 g of benzene lowered its freezing point by 0.40 K. Kf for benzene is 5.12 K kg mol^-1. Find the molar mass of the solute. (3 marks) 10. 200 cm3 of an aqueous solution of a protein contains 1.26 g of the protein. The osmotic pressure of the solution at 300 K is 2.57 x 10^-3 bar. Calculate the molar mass of the protein (R = 0.083 L bar mol^-1 K^-1). Why is osmotic pressure preferred over other colligative properties for macromolecules? (3 marks) Section C - Diagrams (10 marks) 11. Draw vapour pressure versus mole fraction graphs for (i) an ideal solution, (ii) a solution showing positive deviation and (iii) a solution showing negative deviation. Give one example of each. (4 marks) 12. Draw a vapour pressure versus temperature diagram showing the elevation of boiling point of a solution. Label the pure solvent curve, the solution curve, Tb0, Tb and 1 atm. (3 marks) 13. Draw a labelled diagram showing osmosis and reverse osmosis through a semipermeable membrane, as used in desalination of sea water. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. 2 g of benzoic acid (C6H5COOH, M = 122 g mol^-1) dissolved in 25 g of benzene shows a freezing point depression of 1.62 K. Kf for benzene is 4.9 K kg mol^-1. Calculate the observed molar mass, the van't Hoff factor and the percentage association of benzoic acid if it forms dimers. Explain why the molar mass is abnormal. (5 marks) 15. Explain, on the basis of intermolecular forces, why a mixture of ethanol and acetone shows positive deviation while a mixture of chloroform and acetone shows negative deviation from Raoult's law. Predict the sign of the enthalpy and volume of mixing in each case and the type of azeotrope formed. (5 marks) 16. Explain the following using colligative properties or Henry's law: (i) salt is spread on icy roads, (ii) a raw mango shrinks in concentrated salt solution, (iii) climbers at high altitude suffer from anoxia, (iv) intravenous fluids must be isotonic with blood and (v) soft drink bottles are sealed under high pressure. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show all formulae, substitutions and units in numericals. Draw neat labelled graphs.