Chemistry

Solution and Colloids

5,181 words, 9 sections

Solutions and colloids is the chapter that explains what happens when one substance disappears into another, and what that disappearance does to the solvent around it. The official PMDC syllabus table does not carry the heading, so a student will not find the chapter by name, but the papers do not share that omission: the bank holds 73 distinct questions on this material, taken from 52 official papers between 2003 and 2025 and set by Balochistan, DUHS, Federal, KMU, NUMS and UHS, with 505 active questions and a mean item difficulty of 2.3. The subject belongs with the official units on Liquids and on Solids, and that is where it should sit on a revision plan, because vapour pressure, boiling point and freezing point are properties of the liquid state that a dissolved solute interferes with. A student who has the chapter under control can turn any stated amount into the concentration unit the question wants, decide whether a solution is saturated or will precipitate, work a colligative calculation in either direction, and classify a colloid by its two phases.

Concentration is the amount of solute present in a stated amount of solvent or solution, and the paper expects you to move between five measures without hesitation. Mass percent, written $w/w$ , is the mass of solute in 100 grams of solution, so a 5 per cent sodium chloride solution holds 5 grams of salt in 100 grams of solution and 95 grams of water. Molarity is the number of moles of solute per cubic decimetre of solution, $M = \frac{\text{moles of solute}}{\text{volume of solution in dm}^3}$ . Molality is the number of moles of solute per kilogram of solvent, $m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}}$ . Mole fraction is the ratio of the moles of one component to the total moles of the solution, $X_A = \frac{n_A}{n_A + n_B}$ , and the mole fractions of all components always add to one. Parts per million, per billion and per trillion are used when the solute is present in trace amounts, and ppm is the amount of solute per million parts of solution, $\text{ppm} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 10^6$ .

The single distinction that decides most questions is the one between the first two. Molarity divides by the volume of the solution, and volume expands and contracts with temperature, so molarity is temperature dependent. Molality divides by the mass of the solvent, and mass does not change on heating or cooling, so molality is temperature independent. Mole fraction is temperature independent for the same reason, since it counts particles rather than measuring space. A question that asks which concentration unit changes with temperature is asking about molarity and nothing else.

Worked example

Find the molarity of a solution made by dissolving 4.0 grams of sodium hydroxide in enough water to make 250 cubic centimetres of solution. The molar mass of $NaOH$ is $23 + 16 + 1 = 40\ \text{g mol}^{-1}$ , so the number of moles is $\frac{4.0}{40} = 0.10\ \text{mol}$ . Convert the volume: $250\ \text{cm}^3 = 0.250\ \text{dm}^3$ . Then $M = \frac{0.10}{0.250} = 0.40\ \text{mol dm}^{-3}$ . Run the same figure the other way and a $0.5\ \text{mol dm}^{-3}$ solution holds $0.5 \times 40 = 20\ \text{g}$ of $NaOH$ per cubic decimetre. The step students get wrong is stopping at the mass without converting it to moles, which reports 4.0 or 16 as the concentration; the molar mass has to come out of the calculation before the division.

7 more sections in this chapter

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Introduction

1. Concentration unitsConcentration is the amount of solute present in a stated amount of solvent or solution, and the paper expects you to move between five measures without hesitat