1. Relative Formula Mass
Noadswood Science, 2016

2. Relative Formula Mass To know how to calculate relative formula mass
Monday, June 18, 2018
Relative Formula Mass
To know how to calculate relative formula mass

3. Precise Learning
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

4. Mass Number
Each atom consists of a nucleus containing protons and neutrons, with electrons arranged around it.
Protons and neutrons have a relative mass unit of 1
Electrons have a very small mass compared to protons and neutrons with a mass unit of 1/2000th
Most of the mass of an atom is found in its nucleus: -
The mass number of an atom is the total number of protons and neutrons it contains
The atomic number (also called the proton number) is the number of protons it contains

5. Chemical Symbol
The full chemical symbol for an element shows its mass number at the top, and atomic number at the bottom, e.g. carbon
It tells us that a carbon atom has six protons (it will also have six electrons, because the number of protons and electrons in an atom is the same)
The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12
*You can work out the number of neutrons from the mass number and atomic number – in this example, it is = 6 neutrons

6. Relative Atomic Mass (Ar)
The atoms of each element have a different mass – the relative atomic mass is a way of saying how heavy atoms are in comparison to others
Carbon is given a relative atomic mass (RAM) of 12
The RAM of other atoms compares them with carbon
Element
Symbol
x Heavier Than Carbon (C)
RAM
Helium He
1/3 4
Beryllium Be
3/4 9
Molybdenum Mo 8 96
Krypton Kr 7 84
Oxygen O
1 & 1/3 16
Silver Ag
108
Calcium Ca
3 & 1/3 40

7. Formula
Formulas tell you how many atoms are used but not the mass of each of the elements
For example in the reaction: -
Mg + 2HCl → MgCl2 +H2
This shows us we need twice as many hydrogen and chlorine atoms, but not the masses needed (because different atoms have different masses)
Even though we are using more hydrogen atoms the mass of 2 atoms will be less than the mass of 1 magnesium

8. Relative Formula Mass (Mr)
Relative formula mass is the sum of all the relative atomic masses added together
For example, MgCl2 has a relative formula mass of 95: -
MgCl2
*Relative atomic mass for any element can be found using the periodic table – relative formula mass is also ‘molar mass’…
Mg = 24
Cl = 35.5 x 2

9. Relative Formula Mass (Mr)
Relative formula mass is the sum of all the relative atomic masses added together
For example, Fe2O3 has a relative formula mass of 160: -
Fe2O3
*Relative atomic mass for any element can be found using the periodic table – relative formula mass is also ‘molar mass’…
Fe = 56 x 2
O = 16 x 3

10. Relative Formula Mass
Work out the following relative formula masses using the atomic masses given (N = 14; H = 1; Na = 23; O = 16; Mg = 24; Ca = 40): -
Ammonia NH3
Sodium oxide Na2O
Magnesium hydroxide Mg(OH)2
Calcium nitrate Ca(NO3)2
Ammonia = 14 + (3 x 1) = 17
Sodium oxide = (2 x 23) + 16 = 62
Magnesium hydroxide = x (16 + 1) = 58
Calcium nitrate = x (14 + (3 x 16)) = 164

11. Practice Questions – Application
Describe how you would work out the relative formula mass (Mr) of a compound
What is the relative formula mass of
Oxygen (O2)
Nitric Acid (HNO3)
Potassium hydroxide (KOH)
Calcium carbonate (CaCO3)

12. Answers
Add together the relative atomic masses of all the atoms in the compound
What is the relative formula mass of
Oxygen (O2) (2 x 16) = 32
Nitric Acid (HNO3) (1 x 1) + (1 x 14) + (3 x 16) = 63
Potassium hydroxide (KOH) (1 x 39) + (1 x 16) + (1 x 1) = 56
Calcium carbonate (CaCO3) (1 x 40) + (1 x 12) + (3 x 16) = 100