C2(i) - Bonding and Reactions 13.

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Calculating Masses in Reactions.

Three Important Steps.

1) Write out the balanced equation.

2) Work Out Mr - Just for the two bits you want.

3) Apply the rule: Divide to get one, then multiply to get all (but you have to apply this first to the substance they give information about, then the other.

Example: What mass of magnesium oxide is produced when 60g of magnesium is burned in the air.

Ar for Mg = 24.


1) Write our the balanced equation:       2Mg + O2 ----->   2MgO

2) Work out the relative formula masses:    2 x 24 ------>   2 x (24 + 16) 

(Don't do oxygen, you don't need it)              48    ------>  


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