1. This document discusses writing chemical formulae and balancing chemical equations. It explains how relative atomic mass is used to compare the masses of different atoms by using carbon-12 as the standard. 2. Isotopes are discussed, which are atoms of the same element that have different numbers of neutrons and masses. The average atomic mass of an element takes into account the different isotopes and their abundances. 3. It also explains how the mole concept relates grams of a substance to numbers of atoms or molecules using Avogadro's number. Empirical formulae are derived from determining the ratios of elements that combine in a compound.

1. WritingFormulae and Balancing Equations
The massesof atoms
Relative AtomicMass
The mass of an atom is tiny. A single hydrogenatomisonlyabout 0.000 000 000 000 000 000 000 002
grams!
Dealingwithsuchsmall numbersisdifficult.Therefore, scientistsfoundasimplerwayof comparingthe
mass of differentatoms.
Theychose the carbon atom and comparedall the otheratomswithit. Since a carbonatom consists
of 6 protonsand 6 neutrons,theygave ita massof 12 units(theyignoredthe electrons).
The mass of an atomrelative tothat of carbon-12 is calledthe Relative AtomicMass.
If we compare the mass of a hydrogenatomwiththat of the carbon-12, we finditsmassis a 1/12th of
the carbon-12 atom. Therefore,a hydrogenatom isassignedthe mass unit 1.
Whena magnesiumatomwascompared,itsmass wasfoundto be twice that of carbon-12, therefore
magnesiumwasassignedthe massunit 24.
Massand Isotopes
Many elementspossessatomswith differingmassesdue to them having differentnumbers of
neutrons.
The atoms shownabove belongtothe isotopesof chlorine.Theyhave differentmassesbecauseone has
two more neutrons than the other. Also itwas found that chlorine-35 is more abundant than chlorine-37.
For everyfourchlorine atoms,one will be achlorine-37the otherthree will be chlorine-35.Therefore,
the average massof a chlorine atomis35.5.
The presence of isotopesandtheirabundancesmustbe takenintoaccountwhencalculatingRelative
AtomicMass (RAM).
The RAM of an elementisthe average mass of itsisotopesrelative to an atom of carbon-12.
Avogadro'sNumber
The MagicNumber!
If you calculate the RAMof a substance,andthenweighoutthatnumberof grams of the substance you
can calculate the numberof atoms or moleculesthat itcontains.

2. Carbonhas a RAMof 12, if you were toweighoutexactly12grams of carbon it wouldcontain 602 000
000 000 000 000 000 000 carbon atoms.
Thisis calledamole of atoms.The numberiscalled Avogadro'snumber, usuallywrittenas 6.02 x 1023
Examples:
1. 24 grams of magnesiumwouldcontain6.02 x 1023
magnesiumatoms.
2. 56 grams of ironwouldcontain6.02 x 1023
ironatoms.
3. 18 grams of waterwouldcontain6.02 x 1023
watermolecules.
One mole ofa substance is6.02 x 1023
particlesof that substance. It is obtainedby weighingout the
RAM or the formula mass in grams.
What informationcan we get from a chemical formula?
The formula ofwater isH2O. Thisisbecause 1 atom of oxygencombineswith 2atomsof hydrogen.
Alternatively,we cansaythat 1 mole of oxygenatomscombineswith 2 molesof hydrogenatomsto
form1 mole of watermolecules.
Molescan be changedtograms; therefore we cansay 16 grams of oxygencombine with 2 grams of
hydrogento form 18 grams of water.
The formula of carbon dioxide is CO2. Thisis because 1 atom of carbon combines with 2 atoms of oxygen.
Therefore,12grams of carbon combine with32 gramsof oxygentoform44 grams of carbon dioxide.
Alternatively, 1mole of carbon atoms combine with 2molesof oxygenatoms to produce 1 mole of
carbon dioxide.
To findthe formulaofa compound
1. Start withthe numberof grams that combine
2. Change the grams to moles
3. Thisgivesyouthe ratio whichtheycombine
4. So nowyouknowthe formula
A formulaobtainedinthiswayiscalledthe Empirical Formula.
Example:
1. 32 grams of sulphurreactwith32 grams of oxygen
2. 1 mole of sulphurreactswith2 molesof oxygen

3. 3. Ratioof 1:2
4. FormulaSO2 - sulphurdioxide
Findingmass by experiment
The Reactionbetween MagnesiumandOxygen
The apparatus belowis usedtocalculate the massof magnesiumoxide,butfirstlyyoumustknowthe
massesof the elementsthatcombine.
Method:
1. Weighthe massof the crucible andlid
2. Adda coil of magnesiumribbonandreweigh.
3. Heat crucible strongly,liftinglidoccasionallytoallow oxygenin.
4. Whenburningiscomplete allowthe apparatustocool.
5. Withthe lidonreweighthe crucible anditscontents.
Results:
mass of magnesiumoxide- massof magnesium=mass of oxygen
4.0g - 2.4g = 1.6g
2.4g of magnesium= 0.1 moles
1.6g of oxygen= 0.1 moles
Therefore the ratioof magnesiumtooxygenusedis1:1
Conclusion:
The formulaof magnesiumoxide isMgO.
How to write an equation
Four stepsto writingequations:
1. Write the equationinwords
2. Write the equationinsymbols.Checkthatyou are usingthe correct formulae.

4. 3. Checkthat the equationisbalanced.Balancingmeansthatyouhave the same numberof atom
on one side asyou do onthe other.The reasonforbalancingisbecause atomsare notlostor
createdduringa reaction.Rememberwhenyoubalance youmultiplythe wholeformulae
whetheritsanelementormolecule- youdonot change its formulae.
4. Addstate symbols.
Example1:
Magnesiumburns in oxygento produce magnesiumoxide.
Because oxygenhastwoatomson the left,we multiply by2the MgO sothat we now have 2 oxygen
atomson the right.
Howeverwe nowhave twomagnesiumatomsonthe right,so we needtomultiplyMgby 2 on the left
to balance.
Example2:
Hydrogen gas reacts with oxygengas to form water when a spark isplaced in the mixture.
There are twooxygenatomsonthe leftbutonlyone onthe right.Hence we needtomultiplythe H2Oon
the right by2.
Thisgivesustwo oxygenatomsonbothsidesbutwe are now leftwithonlytwohydrogens onthe left
and fourhydrogenatomsonthe right.Therefore,we multiplythe H2 by2.