Ar and Mr

Let’s try counting the number of rice particles in a bucket of rice.

Is it easy?

Relative atomic mass, Ar

Atoms are very tiny particles.

Atoms have very small masses.For example,mass of hydrogen atom = 1.4 X 10-24 gmass of lead atom = 2.9 x 10-22gMass of carbon – 12 atom = 1.7 X 10-23g

It is not practical to use actual masses of atoms in calculations.

Chemists often compare masses of different atoms with each other.

In 1961…• Scientists decided to adopt carbon – 12 as

the standard for defining the relative atomic mass of the other elements.

• Scientists gave carbon – 12 a basic mass value of 12 units.

• Masses of all other atoms are compared to one-twelfth ( 1/12) of carbon – 12.

Why carbon – 12?

• Carbon – 12 means the carbon has a mass number (protons + neutrons) of 12.

• Carbon – 12 has a mass of 12 units which can be accurately determined.

• Large abundance of carbon – 12

Relative Atomic Mass

The relative atomic mass of any atom is the number of times the mass of one atom of an element is greater than 1/12 of the mass of one carbon-12 atom.

Relative atomic mass =Mass of 1/12 of an atom of carbon-12

Mass of one atom of the element

Relative Atomic Mass

The symbol for relative atomic mass is Ar.

Relative atomic mass is a ratio and has no unit.

The relative atomic mass of each element is given in the Periodic Table.

What is the relative atomic mass, Ar, of oxygen in the periodic table?

16

One atom of oxygen is 16 times heavier than 1/12 of an atom of carbon-12.

Relative Atomic Mass of Some Elements

Some Ar values are not whole numbers.

Element Relative atomic mass, Ar

Hydrogen 1

Sodium 23

Copper 64

Chlorine 35.5

Why are some Ar values not whole numbers?

This is because such elements occur as mixtures of isotopes.

For example, chlorine exists in two isotopic forms: chlorine-35 and chlorine-37.

How do we derive the Ar of chlorine?

Hence, relative atomic mass of chlorine = (0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5

A sample of chlorine is made up of 75% of chlorine-35 atoms and 25% of chlorine-37 atoms.

Relative Molecular Mass, Mr

Many elements and compounds exist as molecules. For example, chlorine exists as molecules.

ClCl

Hence, we use relative molecular mass instead of relative atomic mass.

The relative molecular mass (Mr) of an element or compound is the mass of a molecule, compared to 1/12 the mass of one atom of carbon-12.

Like Ar, it is a ratio and therefore has no unit.

Relative Molecular Mass, Mr

Mr

Mass of 1 molecule

Mass of 1/12 of a carbon-12 atom=

• Relative molecular mass of a molecule = sum of relative atomic mass, Ar of all atoms in a molecule

Relative Molecular Mass, Mr

How do we calculate the relative molecular mass of chlorine gas (Cl2)?

Ar of Cl = 35.5

Mr of Cl2 = 35.5 X 2 = 71

How do we calculate the relative molecular mass of ammonia (NH3)?

Ar of N = 14 Ar of H = 1

Mr of NH3 = 14 + 1 3 = 17

Relative Formula Mass, Mr Many substances such as water are covalent and exist as molecules.

However, substances like sodium chloride, NaCl, are ionic and do not exist as molecules.

The ‘relative molecular mass’ of an ionic compound is more accurately known as relative formula mass, Mr.

Relative Formula Mass, Mr Like relative molecular mass, - relative formula mass has the symbol Mr and no units. - relative formula mass, Mr, is the sum of all the relative atomic masses

For example, the relative formula mass of sodium chloride (NaCl) is 23 + 35.5 = 58.5.

Relative Formula Mass Many substances such as water are covalent and exist as molecules.

However, substances like sodium chloride are ionic and do not exist as molecules.

The ‘relative molecular mass’ of an ionic compound is more accurately known as relative formula mass.

Relative Formula Mass

Like relative molecular mass, relative formula mass has the symbol Mr and relative formula mass has no units.

For example, the relative formula mass of sodium chloride (NaCl) is 23 + 35.5 = 58.5.

Calculating Mr of MgSO4

Ar of Mg = 24

Ar of S = 32

Mr of MgSO4 = 24 + 32 + (16 4) = 120

Ar of O = 16

Compound Chemical Formula

Relative formula mass, Mr

Magnesium sulphate

MgSO4 (1 X 26) + (1 X 32) + (4 X 16) = 120

Calcium nitrate Ca(NO3)2 (1 X 40) + (2 X 14) + (6 X 16) = 164

Copper (II) sulphate crystals

CuSO4. 5H2O (1 X 64) + (1 X 32) + (9 x 16) + (10 x 1) = 250