Starter quiz
- What does an empirical formula represent?
- The exact number of atoms in a molecule.
- The simplest whole number ratio of atoms in a compound. ✓
- The total mass of a compound.
- The physical properties of a compound.
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- Which empirical formula indicates a 1 : 1 ratio of metal to nonmetal ions?
- CaCl₂
- NaBr ✓
- MgO₂
- Al₂O₃
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- The empirical formula demonstrates the simplest whole number ratio of elements, which aligns with the ratios in ______ chemical equations.
- 'balanced' ✓
- The relative atomic mass of elements is crucial in determining the empirical formula because it ...
- identifies isotopes.
- helps calculate the ratio of atoms present in a compound. ✓
- determines chemical reactivity.
- indicates physical state changes.
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- If 12 g of carbon reacts with 32 g of oxygen, what is the empirical formula of the resulting compound?
- CO
- CO₂ ✓
- C₂O
- C₃O₂
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- 32.70 g of sulfur, 2.04 g of hydrogen, and 65.30 g of oxygen combine to form a compound. Determine the empirical formula.
- SH₂O₄ ✓
- SH₄O₂
- S₂H₄O
- S₄H₂O
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Exit quiz
- 24 g of magnesium reacted with oxygen, producing 40 g of magnesium oxide. Calculate the mass of oxygen that reacted.
- '16 g' ✓
- Why is it important to periodically open the crucible during the heating of magnesium to produce magnesium oxide?
- To let the magnesium cool down.
- To allow oxygen to react with the magnesium. ✓
- To prevent the magnesium from reacting.
- To observe the magnesium's colour change.
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- The combined mass of magnesium and oxygen that reacted is equal to the mass of magnesium oxide produced. This demonstrates the ______ of mass, by accounting for all reactant and product masses.
- 'conservation' ✓
- Lifting the crucible lid too often, when heating magnesium to form magnesium oxide, can lead to which of the following experimental errors?
- A higher final mass.
- A lower final mass. ✓
- The same final mass.
- A more accurate empirical formula.
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- Given that 48 g of magnesium reacted with oxygen and produced 80 g of magnesium oxide, calculate the mass of oxygen that reacted and deduce the empirical formula of magnesium oxide.
- MgO ✓
- Mg₂O₃
- MgO₂
- 2Mg2O
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- In an experiment, 5.4 g of aluminium reacts with 4.8 g of oxygen to form aluminium oxide. What is the simplest whole number ratio of aluminium to oxygen (Al : O)?
- '2 : 3' ✓
Worksheet
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Lesson Details
Key learning points
- Small masses of a substance, including magnesium, can be easily heated in a crucible.
- The crucible needs to be opened periodically to allow air (i.e. oxygen) to enter the apparatus.
- Lifting a crucible lid too often or for too long may lead to loss of product (i.e. lower final mass recorded).
- Using practical data and conservation of mass, chemists can calculate unknown reactant masses.
- Reactant masses can determine the empirical formula of a substance, including the ionic ratio of a metal and oxygen.
Common misconception
Pupils think that simply ensuring a fair test will improve data collecting in a practical.
Evaluating a practical is an often overlooked skill. Challenge pupils to focus on the equipment chosen and/or the method followed to consider how better data might be collected.
Keywords
Empirical formula - A substance's empirical formula shows the simplest whole number ratio of atoms of each element in a compound.
Relative atomic mass - Relative atomic mass is the mean mass of an atom relative to 1/12th the mass of an atom of carbon–12.
Evaluate - To evaluate something is to make a judgement about it with respect to a particular purpose.
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.
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