1. Which formula gives the simplest atomic ratio?
- Displayed formula
- Molecular formula always
- Empirical formula
Check the answer and reasoning
Empirical formula
An empirical formula reduces atom counts to their simplest whole-number ratio.
CHEM / AS level
Amounts, gases and solutions
Stoichiometry converts measured masses, volumes and concentrations into amounts. Empirical formulae use the simplest whole-number ratio, while molecular formulae also need molar mass.
Learning objectives
- Explain amounts, gases and solutions using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
Stoichiometry converts measured masses, volumes and concentrations into amounts. Empirical formulae use the simplest whole-number ratio, while molecular formulae also need molar mass.
The ideal gas equation relates pressure, volume, amount and temperature with consistent SI units. Real gases deviate most strongly at high pressure and low temperature because particle volume and attractions matter.
Equations and relationships
Ideal gas model; T in kelvin and matching units for R.
Worked example
Find moles in 25.0 cm³ of 0.200 mol/dm³ solution.
Follow the solution
Convert volume to 0.0250 dm³, then n=cV=0.200×0.0250.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.
Check your understanding
2. Find moles in 25.0 cm³ of 0.200 mol/dm³ solution.
- 0.00500 mol
- 5.00 mol
- 0.500 mol
Check the answer and reasoning
0.00500 mol
Convert volume to 0.0250 dm³, then n=cV=0.200×0.0250.
3. May Celsius temperature be inserted directly in pV=nRT?
- Yes
- No
- Only below 100°C
Check the answer and reasoning
No
Absolute temperature in kelvin is required because the model uses a physical zero.