4- Stoichiometry - Chemical calculations - IGCSE Chemistry

4.1 Relative atomic mass

-A relative atomic mass (also called atomic weight; symbol: Ar) is a measure of how heavy atoms are.

-It is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.

-Relative atomic mass is the same as atomic weight, which is the older term.

-The molecular formula of a compound is defined as the number and type of diff. atoms in one molecule

-Relative molecular mass, Mr, is the sum of the relative atomic masse. -Relative formula mass, Mr, is used for ionic compounds.

-The mole, symbol mol, is the unit of amount of substance. 1 mole contains 6,02x10^23 particles, e.g. atoms, ions, molecules. This number is called the Avogadro constant.

->Mass (in grams) = number of moles x molar mass of the element

->Number of moles= mass of the element/molar mass

->Number of moles of a gas = volume of the gas (in dm^3 at r.t.p.)/24dm^3

or Volume of a gas (in dm^3 at r.t.p.) =number of moles of gas x 24dm^3

-> Concentration can be measured in g/dm^3 or mol/dm^3

-> Concentration (in mol/dm^3) = number of moles/volume (in dm^3)

-> Number of moles = concentration (in mol.dm^3) x volume in solution (in dm^3)

-The empirical formula of a compound is the simplest whole number ratio of the different atoms or ions in a compound

->Mr of compound/Mr of empirical formula unit

->The molecular formula of a compound is the number and type of different atoms in one molecule

->Percentage yield = actual yield/theoretical yield x100

->Percentage purity = mass of the pure product/ mass of the impure product obtained x 100%