Molecular formula

What is Molecular Formula

The molecular formula tells us the actual number of atoms of each element present in the compound. The molecular formula is always a simple multiple of the empirical formula. To calculate the molecular formula of a compound, you firstly calculate its empirical formula, and then compare the formula mass of the empirical formula with the molecular mass given in the question. If the two values match, the empirical formula and molecular formula are the same. If the two values differ, you divide the molecular mass by the empirical formula mass to deduce the factor by which each element in the empirical formula must be multiplied to produce the molecular formulae.

The experimental and molecular formulae of a compound can be obtained experimentally as shown in the following worked examples.

Example

Hydrated copper sulphate crystals have the chemical formula CuSO₄.xH₂O. In an experiment to determine the value of x, 8.57g of the hydrated crystals was heated until it reached a constant mass of 4.79g. Use these results to determine the value of x in CuSO₄.xH₂O.

1. Write down the masses of CuSO₄ and H₂O in the crystals. Calculate the missing mass by subtracting the mass of CuSO₄ from the total mass of the hydrated crystals:

Mass of CuSO₄ = 4.79g

Mass of H₂O = 8.57 – 4.79 = 3.78g

2. Calculate the number of moles of CuSO₄ and H₂O using the equation:

number of moles = mass/molar mass

CuSO_4: 4.79/159.5 = 0.03 mol

H₂O: 3.78/18.0 = 0.21 mol

3. Divide each by the smallest number of moles to find the ratio:

CuSO_4: 0.03/0.03 = 1

H₂O: 0.21 /0.03 = 7

Therefore, the value of x is 7 and the formula of the hydrated crystals is CuSO₄.7H₂O

Example

Magnesium metal burns in oxygen gas to form magnesium oxide. The formula of this metal oxide can be determined by comparing the mass of the magnesium ribbon used to the mass of the magnesium oxide formed.

In one experiment, 2.40g of magnesium burned to form 4.00g of magnesium oxide.

Use these experimental results to deduce the empirical formula of magnesium oxide.

1. Write down the masses of magnesium and oxygen used. Calculate the mass of oxygen added by subtracting the mass of magnesium from the total mass of the magnesium oxide formed.

Mass of Mg = 2.40g

Mass of O = 4.00 – 2.40 = 1.60g

2. Calculate the number of moles of Mg and O using the equation:

number of moles = mass/molar mass

Mg: 2.40/24.0 = 0.1 mol

O: 1.60/16.0 = 0.1 mol

The number of moles of Mg and O is the same therefore they are present in magnesium oxide on a 1:1 ratio and the empirical formula is MgO.