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. |
| mass number/mæs ˈnʌmbə/ | The total number of protons and neutrons in an atomic nucleus. |
| relative formula mass/ˈrelətɪv ˈfɔːmjʊlə mæs/ | The sum of the relative atomic masses in a formula, written Mr. |
Why is chlorine's relative atomic mass 相对原子质量 not a whole number?
- An element can contain different isotopes. Their atoms have different masses, so the average need not be a whole number.
- Relative atomic mass compares that average with a standard; relative molecular mass 相对分子质量 adds the relative atomic masses in one molecule.
Separate an isotope from an average
- Relative atomic mass, $A_r$, is the average mass of an element's isotopes compared with one twelfth of the mass of a carbon-12 atom. It has no unit.
- Mass number 质量数 counts protons plus neutrons in one isotope. For example, chlorine-35 and chlorine-37 have mass numbers 35 and 37; a mixture of them can have $A_r=35.5$. Do not round a supplied $A_r$ to a mass number.
Which statement correctly describes relative atomic mass?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Mass number belongs to one isotope. Relative atomic mass is an average and can be non-integer.
Add the contribution of every atom
- For a molecule, $M_r$ is the sum of the $A_r$ values for all its atoms. For an ionic compound, use the term relative formula mass 相对式量: an ionic lattice does not contain separate molecules.
- Both relative masses have no unit. In carbon dioxide, $M_r=12+2(16)=44$. In sodium chloride, the relative formula mass is $23+35.5=58.5$.
For calcium hydroxide, Ca(OH)₂, use $A_r$: Ca = 40, O = 16, H = 1. The bracket means two O and two H atoms. Relative formula mass $=40+2(16+1)=74$. The 2 multiplies the whole OH group, not the calcium.
Using Ar(H)=1 and Ar(O)=16, what is the Mr of water (H₂O)?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
(2 × 1) + 16 = 18.
Using Ar(C)=12 and Ar(O)=16, what is the Mr of CO₂?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
12 + (2 × 16) = 44.
The relative molecular mass is found by adding up the ______ of every atom in the formula.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Mr = sum of all the Ar values.
Check brackets, subscripts and units
- Write the number of each atom before adding masses. In Al₂(SO₄)₃ there are 2 Al, 3 S and 12 O atoms.
- A coefficient in an equation counts whole particles or formula units. It does not change a substance's $M_r$: 2H₂O still has $M_r=18$ for water, not 36.
$A_r$ is an isotope average, not an electron count or a charge. $M_r$ is a relative mass, not a mass in grams. Molar mass has units and is introduced in the next lesson.
Using Ar values Ca=40, C=12, O=16, what is the Mr of CaCO₃?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
40 + 12 + (3 × 16) = 100.
Use Ar: Ca = 40, N = 14, O = 16. Find the relative formula mass of Ca(NO₃)₂.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
There are 1 Ca, 2 N and 6 O: 40 + 2×14 + 6×16 = 164.
Core reacting masses without moles
- A balanced equation also lets you scale known reacting masses by simple proportion. If 24 g of magnesium reacts with 16 g of oxygen to make 40 g of magnesium oxide, half those amounts are 12 g, 8 g and 20 g.
- This assumes the stated reaction and complete reaction with enough oxygen. Check that reactant masses add to the product mass in a closed system.
24 g magnesium forms 40 g magnesium oxide with sufficient oxygen. What mass of oxide forms from 6.0 g magnesium under the same conditions, in g?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
6.0 g is one quarter of 24 g, so oxide mass = 40÷4 = 10.0 g.
Build and check a new calculation
- For an unfamiliar formula, count each atom, multiply by its supplied $A_r$, then add. Keep decimal values such as 35.5 throughout.
- Check whether the question asks for a relative mass or an actual sample mass. A relative mass alone does not say how many grams are in the sample.
$A_r$ describes an isotope average. $M_r$ adds all atom contributions in a molecule or formula unit. Neither has a unit; brackets multiply every atom inside them.
Use Ar: H=1, S=32, O=16. Calculate Mr of H₂SO₄, showing each atom contribution.
All four oxygen atoms contribute 64 to the total 98.
Use Ar: H=1, S=32, O=16. Which is the Mr of H₂SO₄?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The four oxygen atoms contribute 64. Other options — 18 (the Mr of water); 100 (the Mr of calcium carbonate).
Use Ar: Al = 27, S = 32, O = 16. Find the relative formula mass of Al₂(SO₄)₃.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
2×27 + 3×32 + 12×16 = 342. The 3 multiplies all four oxygens in each sulfate group.
A learner writes: “NaCl has Mr 58.5 g because each NaCl molecule has mass 58.5 g.” Correct the terminology and units.
Compare the reasoning and each criterion; equivalent wording is valid.