Ideal gas law lab answers for AP Chemistry (free PDF download)

The method and the results are on the ideal gas law lab. Do the analysis yourself first, then check the answers below.

Quick answer: The ideal gas law, PV = nRT, links pressure, volume, amount and absolute temperature of a gas. These answers go with the AP Chemistry lab on ideal gas law: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.

Ideal gas law lab answers for AP Chemistry (free PDF download)

Model answers use the sample data. Your own values may differ slightly, but the method does not change: subtract the vapour pressure of water, convert to kelvin and keep units consistent. Give results to three significant figures.

Level: AP Chemistry · Curriculum: AP Chem Unit 3

Contents

Question 1 Write the equation

Mg + 2HCl → MgCl₂ + H₂. The ratio is 1 : 1, so n(H₂) = n(Mg).

Question 2 Amount of hydrogen

n = 0.0250 / 24.305 = 1.03 × 10⁻³ mol.

Question 3 Pressure of hydrogen

P(H₂) = 100.80 − 2.49 = 98.31 kPa.

Question 4 Calculate R

V = 0.0256 L. R = 98.31 × 0.0256 / (1.0286 × 10⁻³ × 294.15) = 2.5167 / 0.30256 = 8.32 L·kPa/(mol·K).

Question 5 Mean value

Mean = (8.32 + 8.35 + 8.24) / 3 = 8.30. Difference = (8.314 − 8.30)/8.314 × 100 = 0.2 percent.

Question 6 Graph

A straight line through the origin, because V = (RT/(P·M)) × mass. Gradient = about 1.02 mL/mg, so RT/(P·M) can be used to find R.

Question 7 Sources of error

Gas dissolving in water, or hydrogen lost before the bung fits, makes V too small and R too small. Reading the volume with unequal water levels changes the pressure inside. Each of these changes R in a known direction.

Question 8 Improvement

Use a gas syringe so no water vapour or dissolving occurs, and a larger Mg mass so the volume is larger; the percentage uncertainty in V and mass then falls. This improves precision.

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