IGCSE STOICHIOMETRY - NOTES

Mr. Hisham Mahmoud

Part 5 Calculating Masses in Reactions Notes and explanation

Lesson 1.Downloadable PDF

Lesson 1 . Youtube Video(22 minutes)

Part 5 Calculating Masses in Reactions Questions

Lesson 2. Downloadable PDF

Calculating Masses

Relative atomic mass

Individual atoms have a tiny mass.

For example, an atom of Carbon has a mass of about 2 × 10⁻²³ g (that is 0.000 000 000 000 000 000 000 02 g).

As numbers like this are awkward to use, scientists measure the mass of atoms relative to each other. They use a scale where the mass of a ¹²C atom is defined as being exactly 12.

On this scale, an atom of ²⁴Mg has a relative mass of 24 and is twice as heavy as a ¹²C atom, whereas an atom of ¹H has a relative mass of 1 and is 12 times lighter than a ¹²C atom.

In effect, the relative mass of a single atom equals the mass number of that atom (mass number is the number of protons plus the number of neutrons).

Many elements are made up of a mixture of atoms of different isotopes. For example, 75% of chlorine atoms are ³⁵Cl with relative mass 35 and the remaining 25% are ³⁷Cl atoms with relative mass 37. The average relative mass of chlorine atoms is 35.5 as there are more chlorine atoms with relative mass 35 than 37.

The relative atomic mass (Ar) of an element is the average mass of atoms of that element taking into account the mass and amount of each isotope it contains on a scale where the mass of a ¹²C atom is 12.

Relative formula mass

The relative formula mass (Mr) of a substance is the sum of the relative atomic masses of all the atoms shown in the formula. It is often just called formula mass

For example, the formula of water is H₂O and so the relative formula mass is the sum of the relative atomic mass of two hydrogen atoms (2 × 1) and one oxygen atom (16) which adds up to 18. This and other examples are shown in the next table