How many atoms are there altogether in one molecule of H₂SO₄?

7. 2 hydrogen + 1 sulfur + 4 oxygen.

What is the formula of calcium chloride? (Ions: Ca²⁺ and Cl⁻.)

CaCl₂. Two 1− ions balance one 2+ ion.

CH₄ + ?O₂ → CO₂ + 2H₂O. Which number balances the equation?

2. There are four oxygen atoms on the right, so two O₂ molecules are needed.

What is the empirical formula of C₄H₈?

CH₂. Divide both numbers by 4.

What is the formula of magnesium hydroxide? (Ions: Mg²⁺ and OH⁻.)

Mg(OH)₂. Two hydroxide ions balance one Mg²⁺; brackets show the 2 applies to all of OH.

Formulae and equations · Cambridge IGCSE Co-ordinated Sciences (9

In plain words

A formula is a recipe: H₂O says two hydrogen atoms and one oxygen. An equation is the whole cooking instruction, and atoms can't appear or vanish: whatever you start with, you must end with. Balancing an equation is just making the atom count the same on both sides.

Three things to know

A formula shows how many of each atom there are. For an ionic compound the charges must balance: Mg²⁺ and Cl⁻ give MgCl₂.
To balance an equation, change only the numbers in front of the formulae. State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
The empirical formula is the simplest whole-number ratio of the atoms. The molecular formula is the actual number in one molecule.

Worked example

Balance: Al + O₂ → Al₂O₃.

Oxygen: 2 on the left, 3 on the right. The lowest number both go into is 6, so use 3O₂ and 2Al₂O₃.
That gives 4 aluminium atoms on the right, so put 4Al on the left.
4Al + 3O₂ → 2Al₂O₃.

Tips and tricks

Only change the big numbers in front. Changing a small number changes the substance: H₂O is water, H₂O₂ is not.
Count each kind of atom on both sides at the end. It takes ten seconds and catches most mistakes.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Formulae and equations: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

How many atoms are there altogether in one molecule of H₂SO₄?
- 3
- 6
- 7 (the answer)
- 8
2 hydrogen + 1 sulfur + 4 oxygen.
Which state symbol means dissolved in water?
- (s)
- (l)
- (g)
- (aq) (the answer)
It is short for aqueous.
What is the formula of calcium chloride? (Ions: Ca²⁺ and Cl⁻.)
- CaCl
- CaCl₂ (the answer)
- Ca₂Cl
- Ca₂Cl₂
Two 1− ions balance one 2+ ion.
CH₄ + ?O₂ → CO₂ + 2H₂O. Which number balances the equation?
- 1
- 2 (the answer)
- 3
- 4
There are four oxygen atoms on the right, so two O₂ molecules are needed.
What is the empirical formula of C₄H₈?
- C₂H₄
- CH₂ (the answer)
- CH₄
- C₄H₈
Divide both numbers by 4.
What is the formula of magnesium hydroxide? (Ions: Mg²⁺ and OH⁻.)
- MgOH
- Mg₂OH
- Mg(OH)₂ (the answer)
- MgO₂H
Two hydroxide ions balance one Mg²⁺; brackets show the 2 applies to all of OH.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

4 questions, 8 marks. Write your answers on paper, then check them.

Formulae and equations

Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 8 marks · papermunch.org

Name ______________________________ Date ______________

Balance the equation: Mg + O₂ → MgO.[2]
Show answerHide answer
2Mg + O₂ → 2MgO.
I got this one
Work out the formula of sodium sulfate. (Ions: Na⁺ and SO₄²⁻.)[2]
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Na₂SO₄.
I got this one
Balance the equation: N₂ + H₂ → NH₃.[2]
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N₂ + 3H₂ → 2NH₃.
I got this one
Give the empirical formulae of glucose, C₆H₁₂O₆, and ethane, C₂H₆.[2]
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CH₂O and CH₃.
I got this one

Answers: Formulae and equations

1. 2Mg + O₂ → 2MgO.
2. Na₂SO₄.
3. N₂ + 3H₂ → 2NH₃.
4. CH₂O and CH₃.