Starter quiz
- What is the chemical formula for water?
- 'H₂O' ✓
- How many particles are in one mole of any substance?
- 6.02 × 10²³ ✓
- 23 × 10⁶
- 6.23 × 10²⁰
- 3.60 × 10²
-
- Calculate the relative formula mass (Mr) of carbon dioxide (CO₂).
- '44' ✓
- If 2 moles of hydrogen gas react with 1 mole of oxygen gas, how many moles of water are produced?
- '2' ✓
- What is the mass (g) of 2 moles of carbon dioxide (CO₂)?
- '88 g' ✓
- If a compound is made of 30 g of carbon and 70 g of oxygen, what is the percentage by mass of carbon?
- '30%' ✓
Exit quiz
- Which of the following best describes the term 'empirical formula'?
- The formula that shows the total number of atoms in a molecule.
- The simplest whole number ratio of atoms of each element in a compound. ✓
- The formula that describes the types of chemical bonds in a compound.
- The formula used exclusively in organic chemistry.
-
- Write the empirical formula and molecular formula for a compound with a molecular formula of C₆H₁₂O₆.
- 'CH₂O' ✓
- The empirical formula of a compound is NH₃ and its relative formula mass (Mr) is 34. What is its molecular formula?
- N₂H₆ ✓
- NH₃
- N₂H₄
- NH₂
-
- True or false? Empirical formula shows the simplest ratio of how many atoms there are of each element in a substance; molecular formula shows the exact numbers of atoms of each element.
- True ✓
- False
- Given the following information, calculate the empirical formula of a compound: 2.4 moles of magnesium and 4.8 moles of oxygen.
- MgO
- MgO₂ ✓
- Mg₂O₄
- Mg₂O
-
- Given 40 g of carbon reacts with 10 g of hydrogen to form a compound, calculate the empirical formula of the compound. Use the relative atomic masses (Ar): Carbon = 12, Hydrogen = 1.
- CH
- CH₄ ✓
- C₄H
- C₄H₄
-
Worksheet
Presentation
Video
Lesson Details
Key learning points
- Empirical formula is the simplest whole number ratio of atoms of each element in a compound.
- Using the masses of elements reacted together, or their percentage compositions, calculate the empirical formula.
- Knowing the relative formula mass of a substance, and its empirical formula, calculate the molecular formula.
- Know that we are using moles, and molar ratios to calculate formulae
Common misconception
Pupils easily confuse empirical and molecular formula.
Provide multiple examples of formula and challenge pupils to distinguish them as molecular or empirical formula; if the former, push pupils to suggest the molecule's empirical formula.
Keywords
Empirical formula - A substance's empirical formula shows the simplest whole number ratio of atoms of each element in a compound.
Relative formula mass - The relative formula mass of a substance is the sum of the relative atomic masses of all the atoms in a formula.
Molecular formula - A substance's molecular formula shows the actual number of atoms of each element in a molecule of a compound.
Mole - A mole of something is 6.02 × 10²³ of it. The mass of a mole of a substance is its relative mass expressed in grams.