Starter quiz
- Which of the following is the correct formula for carbon dioxide?
- CO
- CO₂ ✓
- C₂O
- C₂O₂
- 2CO
-
- Calculate the relative molecular mass (Mr) of carbon dioxide (CO₂).
- '44' ✓
- If two molecules of hydrogen gas react with one molecule of oxygen gas, how many molecules of water are produced?
- one
- two ✓
- three
- four
-
- A reaction requires three molecules of nitrogen gas and nine molecules of hydrogen gas. What is the simplest ratio of nitrogen to hydrogen needed for this reaction?
- 3:9
- 1:3 ✓
- 9:3
- 6:2
-
- If one unit of carbon dioxide has a mass of 44 grams, what would be the mass of two units of carbon dioxide?
- '88 g' ✓
- If a compound is made of 30g of carbon and 70g 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 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 120g of carbon reacts with 10g of hydrogen to form a compound, calculate the simplest ratio of carbon:hydrogen in the compound. Use the relative atomic masses (Ar): carbon = 12, hydrogen = 1.
- 1:1 ✓
- 1:4
- 4:1
- 4:4
-
- A compound is 85.7% carbon and 14.3% hydrogen by mass. Calculate its empirical formula. Use the relative atomic masses (Ar): carbon = 12, hydrogen = 1.
- CH
- C₂H₃
- CH₂ ✓
- C₃H₄
-
Worksheet
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Presentation
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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.
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.
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