Relative atomic mass compares an atom to carbon-12
Definition
Relative atomic mass
The weighted mean mass of the atoms of an element compared with one twelfth of the mass of a carbon-12 atom.
- The relative atomic mass (ArA_rAr) of an element tells you how heavy its atoms are compared with one twelfth of a carbon-12 atom.
- You read each ArA_rAr straight from the periodic table, where it is the larger of the two numbers by each symbol.
- Relative masses are ratios, so ArA_rAr has no units.
Relative formula mass adds up every atom in the formula
Definition
Relative formula mass
The sum of the relative atomic masses of all the atoms shown in the chemical formula of a compound.
- The relative formula mass (MrM_rMr) of a compound is the sum of the ArA_rAr values of all the atoms in its formula.
- Like ArA_rAr, the relative formula mass has no units.
Key Idea
To find an MrM_rMr, multiply each element's ArA_rAr by how many of its atoms appear in the formula, then add all the totals together.
Working out an MrM_rMr step by step
Definition
Compound
A substance containing atoms of two or more different elements chemically bonded together.
- List each element in the compound and how many atoms of it the formula shows.
- Multiply each ArA_rAr by the number of atoms, then add the results.
Example
- Water, H2O\text{H}_2\text{O}H2O, has Mr=(2×1)+16=18M_r = (2 \times 1) + 16 = 18Mr=(2×1)+16=18.
- Calcium carbonate, CaCO3\text{CaCO}_3CaCO3, has Mr=40+12+(3×16)=100M_r = 40 + 12 + (3 \times 16) = 100Mr=40+12+(3×16)=100.
Brackets multiply everything inside them
- A subscript outside a bracket multiplies every atom inside the bracket.
- In calcium hydroxide, Ca(OH)2\text{Ca(OH)}_2Ca(OH)2, the two applies to both the oxygen and the hydrogen inside the bracket.
Example
Calcium hydroxide, Ca(OH)2\text{Ca(OH)}_2Ca(OH)2, has Mr=40+2×(16+1)=40+34=74M_r = 40 + 2 \times (16 + 1) = 40 + 34 = 74Mr=40+2×(16+1)=40+34=74.
Exam technique
- Never write a unit after an ArA_rAr or MrM_rMr value, as marks are lost for adding one.
- Deal with any brackets before you add, because forgetting to multiply inside them is the most common slip.
The total formula mass is the same on both sides of a balanced equation
- Higher tier only: in a balanced equation, the total relative formula mass of the reactants equals the total relative formula mass of the products, once each formula is counted with its multiplier.
- For 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}2Mg+O2→2MgO, the reactants total (2×24)+32=80(2 \times 24) + 32 = 80(2×24)+32=80.
- The product totals 2×40=802 \times 40 = 802×40=80, which matches and shows mass is conserved.
Self review
- What is the relative formula mass of a compound?
- Work out the MrM_rMr of CO2\text{CO}_2CO2 using ArA_rAr values of 121212 and 161616.
- Work out the MrM_rMr of Mg(NO3)2\text{Mg(NO}_3\text{)}_2Mg(NO3)2 using ArA_rAr values of 242424, 141414 and 161616.
- Why does a relative formula mass have no units?
