Relative masses of atoms and molecules
Introduced| English | Meaning |
|---|---|
| relative atomic mass/ˈrelətɪv əˈtɒmɪk mæs/ | The weighted mean mass of an element's atoms relative to one twelfth of the mass of a carbon-12 atom. |
| relative molecular mass/ˈrelətɪv məˈlekjʊlə mæs/ | The mass of a molecule relative to one twelfth of the mass of a carbon-12 atom, found by adding its atoms' relative atomic masses. |
Weighing atoms and molecules
- Atoms are far too tiny to weigh one at a time, so chemists compare their masses.
- This gives the relative atomic mass 相对原子质量 and relative molecular mass 相对分子质量.
Formula mass lab
mass = moles x Mr
Change number of formula units and see total mass scale.
The relative atomic mass of an element is approximately equal to its:
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Ar is roughly the mass number.
Relative atomic mass (Ar)
- The relative atomic mass (Ar) compares an atom's mass to a standard.
- It is roughly the mass number — e.g. carbon Ar = 12, oxygen Ar = 16.
A laboratory balance measures mass, the basis of mole calculations
Using Ar(H)=1 and Ar(O)=16, what is the Mr of water (H₂O)?
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(2 × 1) + 16 = 18.
Using Ar(C)=12 and Ar(O)=16, what is the Mr of CO₂?
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12 + (2 × 16) = 44.
The relative molecular mass is found by adding up the ______ of every atom in the formula.
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Mr = sum of all the Ar values.
Relative molecular mass (Mr)
- The relative molecular mass (Mr) is the sum of the Ar values of all atoms in a molecule.
- Just add up the Ar of each atom in the formula.
Worked example. Mr of water (H₂O) = (2 × 1) + 16 = 18. Mr of carbon dioxide (CO₂) = 12 + (2 × 16) = 44.
Using Ar values Ca=40, C=12, O=16, what is the Mr of CaCO₃?
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40 + 12 + (3 × 16) = 100.
Using brackets
- Multiply through brackets: Mr of Ca(OH)₂ = 40 + 2 × (16 + 1) = 40 + 34 = 74.
- Always count every atom the formula contains.
Add the Ar of every atom — including repeats. A common mistake is forgetting the subscript. In H₂O there are two hydrogens, so use 2 × 1, not just 1.
You've got it
- relative atomic mass (Ar) ≈ the mass number (C = 12, O = 16, H = 1)
- relative molecular mass (Mr) = sum of the Ar of every atom in the formula
- multiply through subscripts and brackets (e.g. H₂O = 18, CO₂ = 44)