1. CHEMICAL FORMULAE & REACTION QUANTITIES
CHEMISTRY N5
CHEMISTRY
CHEMICAL FORMULAE & REACTION QUANTITIES
FORMULA MASS
GRAM FORMULA MASS
& THE MOLE

2. CHEMICAL FORMULAE & REACTION QUANTITIES
CHEMISTRY N5
CHEMISTRY
CHEMICAL FORMULAE & REACTION QUANTITIES
FORMULA MASS
After completing this topic you should be able to :
Know the mass average of an atom is called the relative atomic mass.
Use the chemistry data booklet to find the relative atomic mass of an atom.
Know the total mass of all the atoms in a formula is called the formula mass.
Calculate the formula mass of an element or compound.

4. THE SUB ATOMIC PARTICLES+
The proton has a positive charge.
Neutron has zero charge. It is NEUTRAL.
The proton and neutron have the same mass. Their mass is very small at 1.67  10‒27 kg.
This number is called 1 atomic mass unit (amu for short).
The electron has a negative charge. Its mass is about 1850 times smaller than a proton / neutron.
The mass of an electron is so small it is taken to be 0 amu. +
This means the mass of an atom comes from the protons and neutrons in the nucleus of the atom.
Helium nucleus has a mass of 4 amu.

5. FORMULA MASS RELATIVE ATOMIC MASS
The average mass of an atom is called the RELATIVE ATOMIC MASS.
The relative atomic mass of selected elements is listed in the chemistry data booklet.
ATOM
RELATIVE ATOMIC MASS H 1
hydrogen O 16
oxygen Cl
35.5
chlorine

6. MASS OF A FORMULA carbon chloride (CCl4) oxygen gas (O2) water (H2O)
The total mass of all the atoms in a formula is called the FORMULA MASS.
carbon chloride
(CCl4)
oxygen gas
(O2)
water
(H2O) 16
R A M 16 16
35.5 12 Cl C 1 O H 1 O
35.5
35.5
FORMULA MASS 32 18
35.5
TO CALCULATE A FORMULA MASS:
154
Write the formula of the substance.
Make a listing of all the atoms in the formula.
Look up the R.A.M. for each element.
Calculate the mass of each element in the formula.
Calculate total mass of the formula.
Formula mass has no units, it is just a number.

7. N2 Example: Calculate the formula mass of nitrogen, N2.
2 × nitrogen atoms
= × N
= x 14
= 28
List of atoms in the formula.
List of atoms in the formula using each element’s symbol.
R.A.M. of nitrogen.
The formula mass of nitrogen is 28.

8. C6H12O6 Example: Calculate the formula mass of glucose, C6H12O6. 6 × O
List of atoms in the formula using each element’s symbol.
C6H12O6
R.A.M. of each element.
6 × O
= x 16
= 96
12 × H
= x 1
= 12
6 × C
= x 12
= 72
180
The formula mass of glucose is 180.

9. Example: Calculate the formula mass of ammonium sulphate, (NH4)2SO4.
When a formula has a group in brackets remember to multiply the atoms inside the brackets by the number outside.
(NH4)2SO4
4 × O
= x 16 =
1 × S
= x 32 =
8 × H
= x 1 =
2 × N
= x 14 =
132
The formula mass of ammonium sulphate is 132.

10. GRAM FORMULA MASS O2 H2O CCl4 N2 C6H12O6 (NH4)2SO4 Formula Mass
The gram formula mass of a substance is simply the formula mass written in grams.
The gram formula mass will be written as “GFM” for short in further calculations.
Formula Mass
Gram Formula Mass
Substance
Formula O2
oxygen 32
32 g
H2O
water 18
18 g
CCl4
carbon chloride
154
154 g N2
nitrogen 28
28 g
C6H12O6
glucose
180
180 g
ammonium sulphate
(NH4)2SO4
132
132 g

11. THE MOLE COPPER(II) SULPHATE (CuSO4) CARBON (C) WATER (H2O) GFM = 12 g
In chemistry the gram formula mass (GFM) of a substance is called THE MOLE.
The MOLE will be written as the letter “n” for short in further calculations.
COPPER(II) SULPHATE
(CuSO4)
CARBON
(C)
WATER
(H2O)
GFM = 12 g
GFM = 18 g
GFM = g
1 mole
= 12 g
1 mole
= 18 g
1 mole
= g

12. m n GFM MOLE and MASS TRIANGLE mass of substance
number of moles
gram formula mass of substance
MOLE and MASS TRIANGLE

13. 1 mole = gram formula mass (GFM)
CARBON
(C)
WATER
(H2O)
2 mole = 2 × GFM
0.5 mole = × GFM
TO CALCULATE THE MASS OF A GIVEN NUMBER OF MOLES
GFM = 12 g
GFM = 18 g
2 mole
= 2 × 12 g
= g
0.5 mole
= 0.5 ×18 g
= g
Calculate the gram formula mass (GFM) of the substance.
Multiply the number of moles by the GFM.

14. Example: Calculate the mass of 3 mole of sodium carbonate, Na2CO3.=
1 × C
= x 12 =
2 × Na
= x 23 =
Gram formula mass of sodium carbonate =
106 g
1 mole of Na2CO = g
Therefore, 3 mole of Na2CO = × g = g

15. Example: Calculate the mass of 0.25 mole of carbon dioxide, CO2.=
1 × C
= x 12 =
Gram formula mass of carbon dioxide =
44 g
1 mole of CO = g
Therefore, 0.25 mole of CO = × 44 g = 10 g
SUMMARY
m = n × GFM
Mass of a substance.
Number of moles.
Gram formula mass.

16. To calculate the mass (m) of a given number of moles (n) of a substance:
m = n × GFM
To calculate the number of moles (n) of a substance in a given mass (m) the equation has to be rearranged. m n =
GFM
To calculate the number of moles (n) of a substance in a given mass (m), divide the mass of substance (m) by the gram formula mass (GFM).

17. Example: Calculate the number of moles of methane, CH4, in 4 g.=
1 × C
= x 12 =
Gram formula mass of methane =
16 g
16 g of CH = mole m
GFM n =
Therefore the number of moles in, 4 g of CH4 : 4 = 16
0.25 mol =

18. Example: Calculate the number of moles of magnesium sulphate, MgSO4, in 180.75 g.=
1 × S
= x 32 =
1 × Mg
= x 24.5 =
Gram formula mass of magnesium sulphate =
120.5 g
120.5 g of MgSO = mole m
GFM n =
Therefore the number of moles in, g of MgSO4 :
180.75 =
120.5
1.5 mol =

19. SUMMARY OF MOLE CALCULATIONS
The two calculations can be summed up using the following triangle.
mass of substance m
gram formula mass of substance
number of moles n
GFM
MOLE and MASS TRIANGLE
To calculate the mass (m) of a given number of moles (n) cover the m.
To calculate the number of moles (n) of a given mass (m) cover the n. m
GFM n m
GFM n m
GFM n = × m
GFM n =