Chemistry

Gases

4,824 words, 11 sections

Gases is unit 3 of the PMDC MDCAT chemistry syllabus, and it is the heaviest single unit in the subject. The bank holds 230 distinct questions drawn from it across 84 official papers between 2001 and 2026, set by six boards: Balochistan, DUHS, Federal, KMU, NUMS and UHS. Chemistry carries 45 of the 180 marks, and the gas laws with the ideal gas equation account for 132 of those questions on their own, with kinetic molecular theory adding 71 and the real against ideal distinction 27. Mean item difficulty is 1.83 on the bank scale, so most of these are answerable by someone who has learned the relationships rather than memorised a table of constants. A student who has this chapter under control can say which variable a given law holds fixed, move between torr, atmospheres, pascals and kelvin without hesitation, and choose correctly between $PV=nRT$ , $\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}$ and $P_t=\sum P_i$ on a question they have never seen before.

For a fixed mass of gas at constant temperature, volume is inversely proportional to pressure: $V \propto \frac{1}{P}$ and $P_1V_1=P_2V_2$ . The reason is mechanical. Pressure is the rate at which molecules strike the walls of the container, so halve the volume and every molecule has half as far to travel between collisions; collisions with the wall double in frequency and the pressure doubles. Neither temperature nor amount has moved, which is why the product $PV$ stays the same number. That product is the whole test of whether Boyle's law applies: constant $T$ , constant $n$ , therefore constant $PV$ . Plotted as $P$ against $V$ at constant temperature the curve is a rectangular hyperbola, and examiners call it an isotherm; plot $V$ against $\frac{1}{P}$ instead and the same data give a straight line through the origin.

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Introduction

1. Boyle's law: pressure and volume at fixed temperatureFor a fixed mass of gas at constant temperature, volume is inversely proportional to pressure: and . The reason is mechanical. Pressure is the rate at which mol