Relative formula mass
atomThe smallest part of an element that can exist. have very little massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g).. So instead of using their actual masses in kilograms, their relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom. are used.
The relative atomic mass of an elementA substance made of one type of atom only., symbol Ar, is the mean mass of its atoms compared to the mass of an atom of the carbon-12 isotopeAtoms of an element with the same number of protons and electrons but different numbers of neutrons.. The Ar values for elements are often stated in the periodic tableA tabular representation of all known elements in order based on atomic number, eg all the noble gases are found on the right of the periodic table.. As Ar is a measure of the relative masses of atoms, it has no units.
The relative atomic masses of elements are proportional measures. For example, the Ar for helium is 4.0, and the Ar for carbon is 12.0. This means that carbon atoms have three times the mass of helium atoms.
Calculating relative formula mass
The relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula., symbol Mr, is used for a substance made up of moleculeA collection of two or more atoms held together by chemical bonds.. This is calculated by adding up the relative atomic masses of all the atoms in the formulaA combination of symbols that indicates the chemical composition of a substance. for the substance.
To calculate the Mr for a substance:
- work out how many atoms of each element are in the chemical formula
- add together the Ar values for all the atoms of each element
For example, the formula for carbon dioxide is CO2. It has one carbon atom (relative atomic mass = 12.0) and two oxygen atoms (relative atomic mass = 16.0):
Mr of CO2 = 12.0 + 16.0 + 16.0 = 44.0
It could also be calculated this way:
Mr of CO2 = (1 × 12.0) + (2 × 16.0) = 12.0 + 32.0 = 44.0
Like Ar values, Mr values also have no units.
Relative formula masses of ionic compounds
ionic compoundAn ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). such as sodium chloride do not exist as molecules. However, their relative formula masses are calculated in the same way. The formulae used are their empirical formulaThe simplest whole number ratio of the atoms of each element in a compound..
Question
Calculate the relative formula mass, Mr, of calcium hydroxide, Ca(OH)2.
(Relative atomic masses: Ca = 40.1, O = 16.0, H = 1.0)
Mr = 40.1 + (2 × 16.0) + (2 × 1.0)
= 40.1 + 32.0 + 2.0
= 74.1
It could also be calculated this way:
Mr = 40.1 + 2 × (16.0 + 1.0)
= 40.1 + 34.0 = 74.1
Masses in balanced chemical equations
Relative atomic masses and relative formula masses may be used to show that mass is conserved during a reaction.
1. Write down the balanced chemical equation
Zn + 2HCl \(\rightarrow\) ZnCl2 + H2
2. Write down the relative atomic masses and relative formula masses of the reactantA substance that reacts together with another substance to form products during a chemical reaction. and productA substance formed in a chemical reaction.. Relative atomic masses may be found in the periodic table.
Relative atomic mass of Zn = 65.4
Relative formula masses:
HCl = 1.0 + 35.5 = 36.5
ZnCl2 = 65.4 + (2 × 35.5) = 136.4
H2 = 2 × 1.0 = 2.0
3. Calculate the relative mass of the reactants and of the products. The large numbers in the equation show the ratio in which the substances react and are produced. Masses should be multiplied by these numbers.
Relative mass of reactants
= (Ar of Zn) + (2 × Mr of HCl)
= 65.4 + (2 × 36.5)
= 138.4
Relative mass of products
= (Mr of ZnCl2) + (Mr of H2)
= 136.4 + 2.0
= 138.4
4. Compare the relative masses of the reactants and products.
The relative masses of the reactants and products is the same. Mass is conserved.