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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, Mr

To find the relative formula mass (or Mr) 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 (Ar) 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?

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

Mr of H2O = 1 + 1 + 16 = 18

Example 2

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

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

Mr 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 Mr of water is 18, so its molar mass is 18 g/mol

  • the Mr 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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