1. Quantitative Chemistry Calculations in Chemistry

2. Chemical analysis and formula It is important in chemistry to not only know what is in a chemical product but how much of each substance there is. EU regulations mean that all foodstuffs must show the amount of the various chemicals present. Chemists need to know the concentration of an acid, i.e. how much HCl is in a solution? Chemists also need to be able to predict how much of a substance in going to be produced in a reaction.

3. Relative Atomic Mass The mass of a single hydrogen atom is very small; -24 Mass of H atom = 1.7 x 10 -24 = 0.0000000000000000000000017g = 0.0000000000000000000000017g It is much more useful to measure the masses of atoms relative to one another. A standard atom was chosen. All others are compared to this. Carbon 12 isotope – the relative mass is given the value of exactly 12.

4. Relative Atomic Mass AtomMass in gramsWhole number ratio H -24 1.7 x 10 -24 1 C -23 2.0 x 10 -23 12 Fl -23 3.2 x 10 -23 19 A mass spectrometer was used to measure the mass of each atom. They then used the following process to work out the relative atomic mass of each atom. Relative atomic mass = mass of element / mass of carbon x 12 -24 / -23 X 12 Example: Hydrogen Relative atomic mass = 1.7 x 10 -24 / 2.0 x 10 -23 X 12 = 1 = 1

5. Relative Atomic Mass Definition: The relative atomic mass (Ar ) of an element is the average mass of the naturally occuring atoms of the element, using a scale where an atom of carbon-12 has a mass of exactly 12.

6. Relative Formula Mass, M r Atoms combine together to make molecules or groups of ions. The formula of a compound is the base unit. The relative formula mass (Mr) is calculated by adding together the masses of the atoms in the formula. H 2 O Ar H = 1 O = 16 Mr = ( 2 x H ) + O = 2 + 16 = 18

7. Activity Complete Worksheet 6.1 – Q1 and 2 (10 mins)

8. Percentage by Mass of an Element You can use M r to determine how much % of an element there is in a compound. H 2 O Ar H = 1 O = 16 M r = ( 2 x H ) + O = 2 + 16 = 18 Calculate the % of H in H 2 O? % of H = Mass of H x 100 Mass of H 2 0

9. Example Work out the % by mass of water in magnesium sulfate crystals. Formula MgSO 4.7H 2 O M r = 24 + 32 + (4x16) + (7x18) = 246 Mass of water in formula = 126 Mass of water as a % = 126 / 246 x 100 = 51.2%

10. Activity Worksheet 6.1 – Q3 (5 mins)