Not given in the exam, recall it.
What it is for
General gas law is used in Heat (SPM Physics Form 4). Keep the SI units in every line of working. Heat
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| p₁ | initial pressure | Pa |
| V₁ | initial volume | m³ |
| T₁ | initial absolute temperature | K |
| p₂ | final pressure | Pa |
| V₂ | final volume | m³ |
| T₂ | final absolute temperature | K |
Rearrangements
V₂ = p₁V₁T₂ / (T₁p₂)p₂ = p₁V₁T₂ / (T₁V₂)T₂ = p₂V₂T₁ / (p₁V₁)
Worked example
Common trap
Understanding the general gas law
The general gas law p₁V₁ / T₁ = p₂V₂ / T₂ connects two states of a fixed mass of gas. p is pressure in pascals (Pa), V is volume in cubic metres (m³) or any consistent unit, and T is the absolute temperature in kelvin (K). The subscripts 1 and 2 mark the initial and final states.
It combines Boyle's, Charles's and the pressure law into one relation, since the quantity pV/T stays constant for a fixed amount of gas. To find one unknown, rearrange, for example V₂ = p₁V₁T₂ / (T₁p₂), p₂ = p₁V₁T₂ / (T₁V₂), or T₂ = p₂V₂T₁ / (p₁V₁).
Temperature must always be in kelvin.
A worked example, step by step
A gas at p₁ = 1.5 × 10⁵ Pa, V₁ = 3.0 L and T₁ = 300 K is compressed to V₂ = 1.5 L and warmed to T₂ = 350 K. Find the new pressure p₂.
Write the rearranged formula: p₂ = p₁V₁T₂ / (T₁V₂). Substitute with units: p₂ = (1.5 × 10⁵ Pa × 3.0 L × 350 K) / (300 K × 1.5 L).
Top line: 1.5 × 10⁵ × 3.0 × 350 = 1.575 × 10⁸. Bottom line: 300 × 1.5 = 450.
Divide: p₂ = 1.575 × 10⁸ / 450 = 3.5 × 10⁵ Pa. The litres cancel, leaving pascals.
The pressure rises to 3.5 × 10⁵ Pa, quoted to two significant figures.
Common mistakes and how this is tested
The biggest trap is leaving temperature in degrees Celsius. Always convert with K = °C + 273 before substituting.
Volume, by contrast, can stay in litres or cm³ as long as both sides use the same unit.
Other errors: mixing pressure units (Pa on one side, kPa on the other); swapping which state is 1 and which is 2; and applying the law to a changing mass of gas, which it does not allow.
Under calculate, show the substitution with units and the kelvin conversion. Explain may ask why a sealed can bursts when heated, or why a gas cools as it expands.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)