Q.At 25°C and 760 mm of Hg pressure a gas occupies 600 mL volume. What will be its pressure at a height where temperature is 10°C and volume of the gas is 640 mL.

Step 1 – Identify the process

All three variables ($p$, $V$, $T$) change and the amount of gas is fixed, so use the combined gas law:

Step 2 – Convert temperatures to kelvin

Step 3 – List the data

Step 4 – Solve for $p_2$

$p_2 = p_1 \times \frac{V_1}{V_2} \times \frac{T_2}{T_1}$ …

Use the combined gas law $\dfrac{p_1V_1}{T_1}=\dfrac{p_2V_2}{T_2}$ when $p$, $V$ and $T$ all change between two states of a fixed …

• Forgetting to convert both Celsius temperatures to kelvin.
• Mixing up which ratio ($V_1/V_2$ vs $V_2/V_1$) goes into the formula — rearrange from $p_1V_1/T_1=p_2V_2/T_2$ symbolically first, don't guess. …