moles

Moles and molar mass

The mole is the unit for amount of substance. The molar mass is the relative formula mass of a substance in grams (measured in g/mol). Mass is conserved in chemical reactions, allowing the mass of a reactant or product to be calculated if the masses of the other substances in the reaction are known.

Molar mass

Relative formula mass, M_r

To find the relative formula mass (or M_r) of a substance, you add together the relative atomic mass [relative atomic mass: The relative atomic mass is the number of times heavier an atom is compared to one twelth of a carbon-12 atom.]  for all the atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons.]  shown in its chemical formula.

Remember that the values for relative atomic masses (A_r) are given in the periodic table. So, although it helps to learn some of the common values, you do not have to do this.

Example 1

What is the relative formula mass of water, H₂O?

(A_r of H = 1, A_r of O = 16)

M_r of H₂O = 1 + 1 + 16 = 18

Example 2

What is the relative formula mass of calcium hydroxide, Ca(OH)₂?

(A_r of Ca = 40, A_r of O = 16, A_r of H = 1)

M_r of Ca(OH)₂ = 40 + 16 + 1 + 16 + 1 = 74

The mole

The unit for amount of substance is called the mole, shown as mol. One mole of atoms, ions [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity.]  or molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance.]  is around 6 × 10²³ (6 followed by 23 zeroes). This is the same number as the number of carbon atoms in 12 g of carbon.

Molar mass

The molar mass of a substance is its relative formula mass in grams. The unit for molar mass is g/mol. For example:

• the M_r of water is 18, so its molar mass is 18 g/mol

• the M_r of calcium hydroxide is 74, so its molar mass is 74 g/mol

Remember: work out the relative formula mass of the substance in the question first, then give this number as g/mol for its molar mass.

Conservation of mass

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