## Friday, November 27, 2009

### formulae and equations

Formulae can be worked out from valency.
Valency
atom or ion with that valency
valency 1
hydrogen, group 1 e.g. sodium and potassium, group 7e.g. fluorine and chlorine, ammonium NH4+, hydroxide OH-, nitrate NO3-
valency 2
group 2 e.g. magnesium and calcium, group 6, sulphate SO42-, carbonate CO32-, copper , II Cu2+ iron II Fe2+
valency 3
group 3 e.g. aluminium, group 5, phosphate PO43-, iron III Fe3+
valency 4
group 4 e.g. carbon
naming of ionic compounds
aluminium oxide
valency 3 2
symbols Al O
formula Al2O3

Use valency to work out the formulae of the following compounds: sodium chloride, potassium bromide, magnesium oxide, calcium sulphide, aluminium nitride, calcium iodide, lithium oxide, aluminium chloride, aluminium sulphide, magnesium nitride.

Calculating relative formula massAdd up the relative atomic mass (found in periodic table) of each atom in the compounsds

e.g. Al203 relative atomic masses of Al = 27, O = 16 (found in periodic table). The formula shows 2 atoms of aluminium and 3 atoms of oxygen so:formula mass of = (2*27) + (3*16) =54 + 48 = 102

Work out the relative formula masses of the following: MgO, FeS, O2, H2O, CaBr2, Na2S, CaCO3, NaOH, HCl, (NH4)2SO4. Relative atomic masses Mg=24, O=16, Fe=56, S=32, Ca=40, Br=80, C=12, Na=23, H=1, Cl=35.5.
Simple balanced equationsIt is possible to write balanced equations for reactions. For example substances such as hydrogen and magnesium combine with oxygen. One method to write them is:

Write a word equation first.

Magnesium + oxygen --> magnesium oxide

Write in the formulae of the substances used.

Mg + O2 --> MgO

Balance the equation so that each element has the same number of atoms on each side.

2Mg + O2 --> 2MgO

sodium + oxygen --> sodium oxide

4Na{s} + O2{g} --> 2Na2O{s}(word equation)

hydrogen + oxygen ---> water(formulae)

H2 + O2 -----> H2O(balance) 2H2 + O2 -----> 2H2O