Standard level
The nature, or properties, of a pure substance is determined by the type of particles that it contains. These may be atoms, molecules or ions.
Ionic solids contain a giant lattice (network) of ions, simple covalent solids contain discrete molecules held together by loose forces and giant covalent structures have a lattice (network) of atoms all joined together by strong covalent bonds. Metals may be considered to be a regular arrangement of atoms for the purpose of mass calculations.
Syllabus ref: S1.4.2Structure 1.4.2 - Masses of atoms are compared on a scale relative to 12C and are expressed as relative atomic mass Ar and relative formula mass Mr .
- Determine relative formula masses Mr from relative atomic masses Ar .
Guidance
- Relative atomic mass and relative formula mass have no units.
- The values of relative atomic masses given to two decimal places in the data booklet should be used in calculations.
Tools and links
- Structure 3.1 - Atoms increase in mass as their position descends in the periodic table. What properties might be related to this trend?
Amedeo Avogadro
Relative mass terminology
The relative mass scale measures atoms, molecules, formula units and giant structures in slightly different ways, but always using the simplest formula unit and comparing the masses of the particles to the mass of one 12C atom = 12 units exactly.
The terminology used depends on the type of structure under consideration. There are three ways to describe the relative mass of solids:
- Relative atomic mass (abbreviated Ar)
- Relative mass (abbreviated Mr)
- Relative formula mass (abbreviated Mr)
Where each of these applies is considered below.
1. Relative atomic mass
Relative atomic mass is used to define the relative mass of one atom of a metallic element, or non-metallic element, which may be considered to consist of unique atoms for the purposes of calculations.
| Inert gases | Relative atomic mass |
|---|---|
| Neon - Ne | 20 |
| Argon - Ar | 40 |
| Giant covalent elements | Relative atomic mass |
| Boron - B | 10.8 |
| Silicon - Si | 28 |
| Carbon - C | 12 |
| Other non-metallic elements | Relative atomic mass |
| Phosphorus - P | 31 |
| Sulfur - S | 32 |
| Metals | Relative atomic mass |
| Sodium - Na | 23 |
| Magnesium - Mg | 24 |
| Iron - Fe | 56 |
| Uranium - U | 238 |
Note Although sulfur and phosphorus contain discrete molecules (S8 and P4, respectively), they are usually treated in calculations as monatomic.
2. Relative mass
Relative mass is used to define the relative mass of one molecule of a covalent substance, whether element or compound.
| Molecular elements | Relative mass |
|---|---|
| Hydrogen - H2 | 2 |
| Nitrogen - N2 | 28 |
| Oxygen - O2 | 32 |
| Fluorine - F2 | 38 |
| Chlorine - Cl2 | 71 |
| Bromine - Br2 | 160 |
| Iodine - I2 | 254 |
| simple covalent compounds | Relative mass |
| Ammonia - NH3 | 17 |
| Methane - CH4 | 16 |
| Water - H2O | 18 |
| Hydrogen chloride - HCl | 36.5 |
| Glucose - C6H12O6 | 180 |
| Carbon dioxide- CO2 | 44 |
Note The halogens bromine and iodine are liquid and solid respectively, at room temperature and pressure.
3. Relative formula mass
Relative formula mass is used to define the mass of the simplest ratio of particles in ionic and giant covalent compounds.
| Ionic compounds | Relative formula mass |
|---|---|
| sodium chloride - NaCl | 58.5 |
| magnesium oxide - MgO | 40 |
| potassium nitrate - KNO3 | 101 |
| calcium carbonate - CaCO3 | 100 |
| Giant covalent compounds | Relative formula mass |
| Silicon dioxide - SiO2 | 60 |
The Mole
The amount of substance that contains an Avogadro number of particles (6.02 x 1023) is called 1 mole of that substance. Where the word amount is used it always refers to the number of moles. The unit is correctly written as "mol".
1 mol of any substance contains 6.02 x 1023 simplest formula units of that substance.
The mass of 1 mol of any substance has the totalled mass of all of the atoms in 1 simplest formula unit. Hence 1 mol of sodium sulfate, Na2SO4 has a relative mass equal to (2 x 23) + (1 x 32) + (4 x 16) = 142.
| metals and other elements Moles = mass / relative atomic mass Simple covalent substances Moles = mass / relative mass Ionic and giant covalent compounds Moles = mass / relative formula mass |
Example 1: Calculate the number of moles in 12 g of magnesium
Magnesium has a relative atomic mass = 24
Moles of magnesium in 12 g = 12/24 = 0.5 moles
Example 2: Calculate the amount represented by 15g of silicon dioxide
Silicon dioxide, SiO2, is a macromolecular structure with a relative formula mass = 60
Moles of silicon dioxide in 15 g = 15/60 = 0.25 moles
Calculate the number of moles present for each of the substances described.
Q331-01 56g of ironAnswer
|
The smallest particle of iron can be considered to be an iron atom. 1 mole of iron atoms have a mass = relative atomic mass of iron expressed in grams = 56g Moles = Mass/Ar = 56/56 Therefore there is 1 mole of iron atoms |
Q331-02 3.65g of hydrogen chloride
Answer
|
Hydrogen chloride is a covalent substance with a formula HCl. The Mr of HCl = 1 + 35.5 = 36.5 Therefore the mass of 1 mole of HCl = 36.5 g Moles = mass/Mr = 3.65/36.5 = 0.1 moles |
Q331-03 5.85 g of sodium chloride
Answer
|
Sodium chloride is an ionic substance NaCl It is a giant structure of sodium and chloride ions with the simplest formula unit given by NaCl Its relative formula mass = 23 + 35.5 = 58.5 Moles NaCl = mass/relative formula mass = 5.85/58.5 = 0.01 moles |
Q331-04 1.27 g of iodine
Answer
|
Iodine is a molecular non-metallic element. It exists as I2 molecules held together by weak dispersion forces. The relative formula mass of I2 = 2 x 127 g = 254 Moles of iodine = mass /relative mass = 1.27/254 = 0.005 moles Note: If the question was to ask for the number of moles of iodine ATOMS then the calculation would have to use the relative ATOMIC mass of iodine. |
Q331-05 4.6 g of sodium
Answer
|
Sodium is a metal and may be considered to be separate atoms. It has a relative atomic mass of 23. 1 mole of sodium has a mass of 23 g Moles of sodium = mass/Ar = 4.6/23 = 0.2 moles |
Q331-06 0.9 g of glucose C6H12O6
Answer
|
Glucose is a covalent substance made of individual molecules held together by London dispersion forces. Relative mass = [(6 x 12) + (12 x 1) + (6 x 16)] = 180 Mass of 1 mole of glucose = 180 g Moles of glucose = mass /Mr = 0.9/180 = 0.005 moles |
Q331-07 3.19 g of anhydrous copper sulfate CuSO4
Answer
|
Copper sulfate is an ionic substance made up a giant lattice of copper ions and sulfate ions. The simplest formula unit is given by the formula CuSO4. The relative formula mass of copper sulfate = 63.5 + 32 + (4 x 16) = 159.5 1 mole of copper sulfate = 159.5 g Moles of copper sulfate = 3.19/159.5 = 0.02 moles |
Q331-08 18 g of magnesium
Answer
|
Magnesium is a metal and can be treated as individual atoms. The relative atomic mass of magnesium = 24 1 mole of magnesium = 24 g Moles of magnesium = mass/Ar = 18/24 = 0.75 moles |
Q331-09 2 g of sulfur(VI) oxide SO3
Answer
|
Sulfur(VI) oxide SO3 is a covalent substance consisting of individual molecules held together by weak London dispersion forces. It has a relative mass = 32 + (3 x 16) = 80 Mass of 1 mole of sulfur(VI) oxide = 80 g Moles of sulfur(VI) oxide = mass /Mr = 2/80 = 0.025 moles |
Q331-10 2.22 g of calcium chloride CaCl2
Answer
|
Calcium chloride is an ionic subsance consisting of a giant structure of calcium ions and chloride ions in a simplest ratio of one Calcium particle (in this case an ion) to two chloride ions, i.e. 2 Cl- ions, making the formula unit CaCl2 The relative formula mass = 40 + (2 x 35.5) = 111 1 mole of calcium chloride = 111 g Moles of calcium chloride = mass/ relative formula mass = 2.22/111 = 0.02 moles |