1.2.1 Relative Atomic Mass & Relative Molecular Mass

Understanding relative masses is essential for calculating the quantities involved in chemical reactions, particularly for determining the mass of atoms, molecules, or compounds.

Relative Atomic Mass $A_r$

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The relative atomic mass ( $A_r$ ) of an element is the average mass of an atom of that element compared to 1/12th the mass of a carbon-12 atom. This value is dimensionless, meaning it has no unit.

Key points:

Carbon-12 is chosen as the reference because it is non-toxic, solid at room temperature, and easily separable.
$A_r$ values account for the natural isotopic distribution of the element.

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Example: If an element has an $A_r$ of 24, this means an average atom of that element is 24 times heavier than 1/12th the mass of a carbon-12 atom.

Relative Molecular Mass $M_r$

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The relative molecular mass ( $M_r$ ) is the average mass of a molecule compared to 1/12th the mass of a carbon-12 atom. Like $A_r$ , $M_r$ is dimensionless.

For covalent compounds, $M_r$ is used to describe the sum of the relative atomic masses of the atoms within a molecule.

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Example: For water $H_2O$

M_r = 2 \times A_r(\text{H}) + A_r(\text{O})

= 2 \times 1 + 16 = :highlight[18]

Relative Formula Mass (RFM)

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The relative formula mass (RFM) is used for ionic compounds, but $M_r$ is often accepted for both ionic and covalent compounds. It is calculated by summing the relative atomic masses of all the ions in the formula unit.

lightbulbExample

Example: For sodium chloride (NaCl):

RFM = A_r(\text{Na}) + A_r(\text{Cl})

= 23 + 35.5 = :highlight[58.5]

Molar Mass

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The molar mass is the mass of one mole of a substance, typically measured in grams per mole (g/mol). While $A_r$ and $M_r$ are dimensionless, molar mass links these relative values to the physical mass of a substance.

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Example: The molar mass of water $H_2O$ is 18 g/mol, meaning one mole of water molecules weighs 18 grams.

Key Equations

$A_r$ or $M_r$ = Relative Mass
Molar Mass (g/mol) = Relative Mass ( $A_r$ or $M_r$ ) in grams per mole

Relative Atomic Mass & Relative Molecular Mass Simplified Revision Notes for A-Level AQA Chemistry

1.2.1 Relative Atomic Mass & Relative Molecular Mass

Relative Atomic Mass $A_r$

Relative Molecular Mass $M_r$

Relative Formula Mass (RFM)

Molar Mass

Key Equations

500K+ Students Use These Powerful Tools to Master Relative Atomic Mass & Relative Molecular Mass For their A-Level Exams.

Other Revision Notes related to Relative Atomic Mass & Relative Molecular Mass you should explore

Relative Atomic Mass & Relative Molecular Mass Simplified Revision Notes for A-Level AQA Chemistry

1.2.1 Relative Atomic Mass & Relative Molecular Mass

Relative Atomic Mass ArA_rAr​

Relative Molecular Mass MrM_rMr​

Relative Formula Mass (RFM)

Molar Mass

Key Equations

500K+ Students Use These Powerful Tools to Master Relative Atomic Mass & Relative Molecular Mass For their A-Level Exams.

Other Revision Notes related to Relative Atomic Mass & Relative Molecular Mass you should explore

Relative Atomic Mass $A_r$

Relative Molecular Mass $M_r$