finding the formula of a 2-element compound e.g. potassium bromide step 1 find potassium (k) in the...
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![Page 1: Finding the formula of a 2-element compound e.g. potassium bromide Step 1 Find potassium (K) in the periodic table and identify its group number. Do the](https://reader035.vdocument.in/reader035/viewer/2022081514/56649d705503460f94a5255f/html5/thumbnails/1.jpg)
Finding the formula of a 2-element compound e.g.potassium bromide
Step 1 Find potassium (K) in the periodic table and
identify its group number. Do the same for bromine (Br)
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Finding the formula of a 2-element compound e.g.potassium bromide
Step 2 Use the group number to give the valency of
the elements involved For groups 1 to 5 valency = group number For groups 5 to 8 valency = 8 – group number
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Finding the formula of a 2-element compound e.g.potassium bromide
So the valency of potassium is 1 The valency of bromine is 1
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Finding the formula of a 2-element compound e.g.potassium bromide
Step 3 Put the element formulae next to each other,
metal first and the valency underneath K Br 1 1
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Finding the formula of a 2-element compound e.g.potassium bromide
Cross over the valencies to give the formula of the compound
K Br so: formula is KBr (number ones are not shown)
1 1
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Finding the formula of sodium oxide
Na in group 1 so valency = 1 (group number) Oxygen in group 6 so valency = 2 (8-gp No)
Na O
1 2
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Finding the formula of sodium oxide
Formula = Na2O
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Finding the formula of aluminium chloride
AlCl3
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Finding the formula of calcium oxide
Ca gp 2 so valency = 2 O gp 6 so valency =2 Ca2O2
2 of each = 1 of each So formula = CaO
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Try these
Lithium fluoride Magnesium chloride Aluminium oxide Potassium sulphide Carbon chloride Silicon oxide Boron nitride
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Answers
LiF MgCl2 Al2O3
K2S
CCl4 SiO2
BN
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What about group 5??
Nitrogen always has a valency of 3 Phosphorus can be 3 or 5 so how do we
know which? It is given in the name (using Roman
numerals) E.g Phosphorus(V) chloride PCl5
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What is the formula of phosphorus (III) chloride
PCl3
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This is also used with transition metal compounds
What is the formula of: iron(II) bromide, cobalt(III) fluoride, titanium(IV) oxide
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Answers
FeBr2
CoF3
TiO2
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What about compounds with more than 3 elements?
Calcium carbonate – limestone Sodium chlorate – an explosive weed killer Sodium hydrogencarbonate – baking powder To work out these formula we have to
LEARN the following list of valencies.
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Ion Formula Valency
Hydroxide OH- 1
Nitrate NO3- 1
Hydrogen Carbonate
HCO3- 1
Carbonate CO32- 2
Sulphate SO42- 2
Sulphite SO32- 2
Phosphate PO43- 3
Ammonium NH4+ 1
Manganate(VII) MnO4- 1
Dichromate(VI) Cr2O72- 2
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These are then used in exactly the same way as before
E.g. Calcium nitrate Nitrate has the formula and valency of NO3
-
Valency of 1 Calcium (gp II) has a valency of 2 Ca(NO3)2
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Additional notes
Metals-Use Symbols only (also true for Carbon Sulphur Phosphorus)
Commmon Gases - Molecules containing 2 atoms (–diatomic)
E.g O2 N2 Cl2 H2 F2 [Br2(l), I2(s)]
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Common Acids HCl Hydrochloric H2SO4
Sulphuric
HNO3
Nitric H3PO4 Phosphoric CH3COOH Ethanoic
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Watch out for ammo-nia/nium
NOTE: Ammonia NH3
Ammonium NH4+ Valency=1