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Writing Formulae

While a compound can be named once all the reactants are known, it is also necessary to know how to write the exact formula, given the ingredients. Here is a step-by-step guide to follow:

Write down the elements, their valencies and their chemical symbols.

Sodium   Chloride
+1   -1
Na   Cl

Place the valencies as a subscript of the combining element, removing the charge.

See image 1.

This provides the correct ratio of elements, so that the charges balance.


Remove any '1's.

= NaCl

This works for any ionic compound. This next example, shows elements with different charges.

Lithium   Oxide
+1   -2
Li   O

See image 2.

Li2O1 = Li2O

In a formula, radicals operate as a group so they are treated in the same manner as the single elements in the examples above. Use parentheses to keep the radical as a unit.

Aluminium   Sulfate
+3   -2
Al   SO4

See image 3.

The formula for aluminium sulfate is Al2(SO4)3

This also works for covalent compounds if the valency of the elements is known.

Silicon   Oxygen
+4   -2
Si   O

See image 4.

= Si2O4
Since the formula represents a ratio of elements in a molecule, it is necessary to reduce the numbers to the smallest possible ratio. In this case, both subscript numbers can be divided by two to give the formula SiO2, silicon dioxide.

Note that some elements can form more than one type of compound, with the valency represented by Roman numerals in parentheses after the element. Use this to construct a formula in the same way.

Carbon (IV)   Oxide
+4   -2
C   O

See image 5.

= C2O4
Put the elements in the smallest possible ratio to get CO2 and it will be seen that carbon (IV) oxide is also carbon dioxide.

Balancing Equations

See animation 1.

A chemical equation represents the way in which reactants form to make new substances. For this convention, the reactants and the products must be known. Eventually, however, it should be possible to infer the products when the reactants are known and vice versa.

A word equation represents the ingredients of the reaction.

For example:

magnesium + hydrochloric acid magnesium chloride + hydrogen

The reactants and products are written in words. The '+' symbol shows the separate ingredients, while the ' ' symbol stands for 'reacts to form'.

Chemical equations go a bit further by being more specific about the formulae involved. Balancing a chemical equation involves simple mathematics and chemical knowledge, often including the state of the reactants and the products in standard laboratory conditions.

An example of the above as a more specific chemical equation:

Mg + HCl MgCl + H

By observing the products, it can be seen that the magnesium chloride is unstable in its current state, and the hydrogen cannot exist as a single molecule. In order to balance the compound, the formula should be worked out in the usual manner:

Magnesium   Chloride
+2   -1
Mg   Cl

See image 6.

Mg1Cl2 = MgCl2

Note that a hydrogen molecule needs at least two hydrogen atoms to exist. Rewrite the equation:

Mg + HCl MgCl2 + H2

Now it can be seen that the reactants no longer balance the products of the equation. To amend this, account for the minimum amount of atoms of each element that this equation requires to be correct by matching the products with the reactants.

On both sides, there is only one atom of magnesium, so no balancing is required. There are, however, two atoms of hydrogen and two atoms of chlorine on the product side but only one of each on the reactant side, therefore indicating that you need to balance the equation.

Rewrite the equation:

Mg + 2HCl MgCl2 + H2

Notice the '2' in front of the hydrochloric acid on the reactant side. This number is the coefficient and refers to all the atoms in the compound. Since 'HCl' is used to denote the smallest possible ratio of hydrogen to chlorine in hydrochloric acid the molecule (and not individual elements that make up the molecule), the multiple indicates the ratio of the molecule to the other reactants in the equation; in this case, '2HCl'.

The coefficient always comes before the molecule and the molecule does not need to be in parentheses. Use coefficients for any type of molecule, including monatomic and single-element molecules.

Separate the ingredients on the reactant side - one atom of magnesium, two atoms of hydrogen and two atoms of chlorine - and compare this with the ingredients on the product side - one atom of magnesium, two atoms of chlorine and two atoms of hydrogen - note that they match up. The equation balances, therefore the correct chemical equation for 'magnesium + hydrochloric acid magnesium chloride + hydrogen' is 'Mg + 2HCl MgCl2 + H2'.

Here is another example to work through:

Step 1: Start with the word equation

copper sulfate + sodium hydroxide copper hydroxide + sodium sulfate

Step 2: Convert the word equation into chemical symbols

CuSO4 + NaOH CuOH + NaSO4

Step 3: Examine the products and balance the individual molecules

Copper   Hydroxide
+2   -1
Cu   OH



+1   -2
Na   SO4


Step 4: Rewrite the equation to gauge which reactants need balancing

CuSO4 + NaOH Cu(OH)2 + Na2SO4

Step 5: Balance the equation by adding coefficients in the correct amount

CuSO4 + 2NaOH Cu(OH)2 + Na2SO4

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Question 1/5

1. What does a word equation represent?

The coefficient

An unbalanced equation

The ingredients of the reaction

The way in which reactants form to make new substances