Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

1.2.2Physical Chemistry

Empirical and Molecular Formula — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 9 of 102

📚 AQA AS 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 6Two Types of Formula

Empirical formula
  • Empirical formula The simplest whole-number ratio of atoms of each element in a compound, such as CH₂ for ethene.
Molecular formula
  • Molecular formula The actual number of atoms of each element in one molecule, such as C₂H₄ for ethene.
Relationship
  • Relationship The molecular formula is a whole-number multiple of the empirical formula.
Same ratio
  • Same ratio Different compounds, such as ethene and propene, can have the same empirical formula but different molecular formulae.

Part 2 of 6Finding an Empirical Formula

Start from masses or percentages by mass.

1
Write the mass (or percentage) of each element present in the compound.
2
Divide each mass by the Ar of that element to get the number of moles of atoms.
3
Divide all of the mole values by the smallest of them.
4
If a ratio is close to x.5, multiply all values by 2 (or by 3 for x.33 and x.67) to get whole numbers.
5
Write the formula using the whole-number ratio as subscripts.

Part 3 of 6Finding the Molecular Formula

Finding the Molecular Formula
Step 1
  • Step 1 Calculate the empirical formula mass by adding the Ar values of the atoms in the empirical formula.
Step 2
  • Step 2 Divide the relative molecular mass by the empirical formula mass to find the multiple.
Step 3
  • Step 3 Multiply every subscript in the empirical formula by this whole number.
Check
  • Check The multiple must be a whole number; if it is not, there is an error in the calculation.

Part 4 of 6Combustion Data

CO₂ mass
  • CO₂ mass The mass of CO₂ gives the mass of carbon: multiply moles of CO₂ by 12.0.
H₂O mass
  • H₂O mass The mass of water gives hydrogen: each mole of H₂O contains 2 mol of H atoms.
Oxygen
  • Oxygen The oxygen in the compound is found by subtracting the masses of C and H from the sample mass.
Then proceed
  • Then proceed Convert these masses to moles and find the simplest ratio exactly as before.

Part 5 of 6Worked Example

Question

  • A compound contains 40.0 % carbon, 6.7 % hydrogen and 53.3 % oxygen by mass, and has Mr = 180.
  • Find its empirical and molecular formulae (Ar: C = 12.0, H = 1.0, O = 16.0).

  1. Moles in 100 g: C = 40.0 ÷ 12.0 = 3.33; H = 6.7 ÷ 1.0 = 6.7; O = 53.3 ÷ 16.0 = 3.33.
  2. Divide by the smallest (3.33): C : H : O = 1 : 2 : 1, so the empirical formula is CH₂O.
  3. Empirical formula mass = 12.0 + 2.0 + 16.0 = 30.0.
  4. Multiple = 180 ÷ 30.0 = 6, so the molecular formula is C₆H₁₂O₆.

Answer

  • Empirical formula CH₂O;
    • molecular formula C₆H₁₂O₆

Part 6 of 6Exam Checklist & Tips

1.2.2 – Key Points for the Exam

  • Empirical formula The simplest whole-number ratio of atoms of each element in a compound.
  • Molecular formula The actual number of atoms of each element in a molecule.
  • Mole ratio The ratio of the amounts, in moles, of the elements present.
  • Percentage composition The percentage by mass of each element in a compound.

Common Mistakes

  • Dividing percentages by the Ar of the wrong element, or forgetting to divide by Ar at all.
  • Rounding ratios such as 1.33 down to 1 instead of multiplying every value by 3.
  • Giving the empirical formula when the question asks for the molecular formula.
  • Dividing Mr by the wrong empirical formula mass, so the multiple is not a whole number.

Last updated:

\n