Download relative formula mass - chemistry

Have you ever tried counting the number of
rice particles in your bowl?
Relative Masses of Atoms
and Molecules
Masses of Particles in Chemistry
 the 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 Masses
 scientists 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 1 of the mass of an atom of carbon-12.
12

Relative Molecular Mass
 many 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 1 of the mass of an atom of carbon-12.
12

Relative Formula Mass
 ionic 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 Composition
 the 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 compound
x 100%
Calculating the mass of an element
in a compound
 mass of an element in a compound =
number of atoms of the x Ar of the
element in the formula
element
Mr of the compound
x 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
x Mr of H2O
molecules in the formula
Mr of the compound
x 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