Have you ever tried counting the number of rice particles in your bowl?

Relative Masses of Atoms and Molecules

Masses of Particles in Chemistrythe mass of a hydrogen atom is

0.000 000 000 000 000 000 000 0014g

very small values and very inconvenient to write

so, scientists do not use these actual masses in their calculation

Relative Massesscientists overcome this by

comparing the masses of atoms with the mass of a carbon-12 atom

it is a ratio and has no unit

Relative Atomic Mass

The relative atomic mass (Ar) of an element is

the average mass of one atom of the element when compared with of the mass of an atom of carbon-12.

112

NO UNITS!

Relative Molecular Massmany elements and compounds

exist as molecules

the mass of a molecule is found by

adding the relative atomic masses of all atoms in the molecule

this mass is known as the relative molecular mass (Mr)

Relative Molecular Mass

The relative molecular mass (Mr) of a molecule is

the average mass of one molecule of the substance when compared with of the mass of an atom of carbon-12.

112

NO UNITS!

Relative Formula Massionic compounds consist of ions, not molecules

instead of relative molecular mass, we use the term relative formula mass

the relative formula mass of an ionic compound is found by

adding the relative atomic masses of all the atoms in the formula

Let’s try! TB P137 Q2

Calculate the relative molecular/formula masses of the following substances.

a. H2O

b. NH3

c. O2

d. CaCO3

e. H2SO4

f. Cu(NO3)2

g. (NH4)2SO4

h. CuSO4.5H2O

Percentage Compositionthe percentage composition of an element in a

compound can be calculated from:the formula, and the relative atomic masses of its elements

percentage of an element in a compound =number of atoms of the element in the formula

x Ar of the element

Mr of the compoundx 100%

Calculating the mass of an element in a compound

mass of an element in a compound =number of atoms of the element in the formula

x Ar of the element

Mr of the compoundx mass of the sample

Calculating the mass of water in a compound

hydrated compounds are compounds that contain water (“water of crystallisation”)

crystals are dry because water is part of the crystal structure

mass of water in a compound =number of water

molecules in the formulax Mr of H2O

Mr of the compoundx mass of the sample

Let’s try! TB P140 Q2

Calculate

a. the percentage of calcium and oxygen in calcium carbonate, CaCO3, and

b. the percentage of oxygen in CuSO4.5H2O

Let’s try! TB P140 Q3

Calculate the mass of

a. calcium in 25 g of calcium carbonate, CaCO3, and

b. chlorine in 27 g of copper(II) chloride, CuCl2