Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.2.3Chemical Reactions

Ideal Gas Law and Solutions — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 5 parts Lesson 16 of 18 Unit quiz

📚 AP CHEMISTRY Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read

Part 1 of 5Gases in Reactions

When a gas is a reactant or product, the mole ratio links it to every other species and the ideal gas law converts moles to volume.

1
Write and balance the equation, because the coefficients give the mole ratio that connects the known substance to the unknown one.
2
Convert the given mass of the known substance into moles by dividing by its molar mass in g mol⁻¹.
3
Multiply by the coefficient ratio (moles wanted ÷ moles known) to obtain the moles of the gas of interest.
4
Rearrange PV = nRT to V = nRT/P, using temperature in kelvin and the value of R whose units match your pressure and volume units.
5
Round the final volume to the same number of significant figures as the least precise measured value in the question.

Part 2 of 5Choosing the Gas Constant

The correct value of R depends on the units you are using, so check units before substituting.

R in joules
  • R in joules 8.314 J mol⁻¹ K⁻¹ is used when pressure is in pascals and volume in cubic metres, or in energy calculations.
R in atmospheres
  • R in atmospheres 0.08206 L atm mol⁻¹ K⁻¹ is the usual choice when pressure is in atm and volume is in litres.
R in torr
  • R in torr 62.36 L torr mol⁻¹ K⁻¹ is used when pressure is quoted in torr and volume in litres.
Temperature
  • Temperature Always convert °C to K by adding 273.15 (273 is acceptable), since the gas law needs absolute temperature.
Pressure unitVolume unitR value
atmL0.08206 L atm mol⁻¹ K⁻¹
torrL62.36 L torr mol⁻¹ K⁻¹
Pam³8.314 J mol⁻¹ K⁻¹

Part 3 of 5Solutions in Reactions

For aqueous reactions the link between volume and moles is molarity, M = n/V.

Solutions in Reactions
Molarity
  • Molarity Moles of solute per litre of solution, so moles = M × V when the volume is expressed in litres.
Unit conversion
  • Unit conversion Volumes in mL must be divided by 1000 before multiplying by molarity to give moles.
Mole ratio
  • Mole ratio Coefficients in the balanced equation give the ratio of moles reacting, not the ratio of volumes or concentrations.
Finding volume
  • Finding volume To find the volume of a solution needed, divide the required moles by its molarity.
Starting point
  • Starting point Begin with the species for which you know both volume and concentration, then work towards the unknown.

Part 4 of 5Worked Example

Question

  • Zinc reacts with hydrochloric acid: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
  • What volume of hydrogen is collected from 6.54 g of zinc at 0.950 atm and 25 °C (298 K)?

  1. Molar mass of Zn = 65.38 g mol⁻¹, so n(Zn) = 6.54 ÷ 65.38 = 0.100 mol.
  2. The ratio ZnH₂ is 1 : 1, so n(H₂) = 0.100 mol.
  3. V = nRT/P = (0.100 mol × 0.08206 L atm mol⁻¹ K⁻¹ × 298 K) ÷ 0.950 atm = 2.57 L.

Answer 2.57 L of hydrogen (3 significant figures)

Part 5 of 5Exam Checklist & Tips

4.2.3 – Key Points for the Exam

  • Ideal gas law PV = nRT, relating the pressure, volume, amount and absolute temperature of an ideal gas.
  • Molarity Concentration in moles of solute per litre of solution, written M.
  • Mole ratio The ratio of coefficients in a balanced equation, used to convert between substances.
  • Gas constant, R The proportionality constant in PV = nRT, with a numerical value that depends on units.
  • Limiting precision The answer cannot be more precise than the least precise measurement used.

Common Mistakes

  • Using temperature in °C in PV = nRT instead of converting to kelvin first.
  • Picking R = 8.314 when pressure is in atm and volume is in litres, giving a wrong magnitude.
  • Forgetting to convert mL to L before calculating moles from molarity.
  • Using the ratio of molarities or volumes instead of the ratio of balanced-equation coefficients.

Last updated:

\n