Calculations in chemistry

• Relative formula masses can be calculated and used in conservation of mass calculations.
• Calculations can be carried out to find out concentrations of solution and uncertainties in measurements.
• Relative formula mass is the smallest part of an element that can exist.
• Mass is measured in kilograms (kg) or grams (g).
• The relative atomic mass of an element is the mean relative mass of the atoms of the different isotopes in an element.
• The relative atomic mass of an element is the number of times heavier an atom is than one-twelfth of a carbon-12 atom.
• The relative atomic mass of an element, A r, is the relative mass of its atoms compared to the mass of a carbon-12 atom.
• The A r values for elements are given in the periodic table.
• Since A r is a measure of relative mass, it has no units.
• The atomic structure study guide explains how to calculate relative atomic mass.
• The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
• Relative formula mass, M r , is represented by the symbol.
• To calculate the M r for a substance, work out how many atoms of each element there are in the chemical formula and add together the A r values for all the atoms of each element present.
• The formula for carbon dioxide is CO 2 and it consists of one carbon atom ( A r = 12) and two oxygen atoms ( A r = 16), resulting in a relative formula mass, M r , of 44.
• Like A r values, M r values are just numbers with no units because they are relative masses.
• An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron), such as sodium chloride, which do not exist as molecules.
• The relative formula mass of an ionic compound is calculated in the same way as for molecules, from the numbers shown in the formula.
• The relative formula mass of calcium hydroxide, Ca(OH) 2 , can be calculated as 40 + (2 × 16) + (2 × 1) = 40 + 32 + 2 = 74.
• The relative formula mass of magnesium nitrate, Mg(NO 3 ) 2 , can be calculated as 24 + (2 × 14) + (2 × 3 × 16) = 24 + 28 + 96 = 148.
• The law of conservation of mass states that matter cannot be created or destroyed.