[momo]

Please help me understanding formulate percentage compositions.

9.03 gMG COMBINED COMPLETELY WITH 3.48 G N TO FORM A COMPOUND.  WHAT IS THE PERCENT CONPOSITION OF THIS COMPOUND?  

[Donaldson Tan]

I think u meant "9.03g of Magnesium combined completely with 3.48g of Nitrogen to form a compound. what is the percentage composition of this compond?"

We work by percentage composition by mass.

Using Law of Mass Combination, and that we know
9.03g of Mg reacts completely with 3.48g N₂,
the mass of the substance formed = 9.03 + 3.48 = 12.51g

percentage composition of Mg would be
= 9.03g / 12.51g X 100%
= 72.2%

U can obtain the percentage composition of mass for nitrogen using the same method. The idea behind is that u need to know: 100g of the substance formed would contain how many grams of each elemental constituent.

[jdurg]

Actually Geo, you can't assume that all the ingredients were in a proper molar proportion.  I can almost guarantee that one of the reactants was in excess.  So the first thing that needs to be done is a balanced equation needs to be written.

3Mg(s) + N₂(g) -> Mg₃N₂(s)

With the product calculated, you can figure out the percent composition of Magnesium and Nitrogen from the molecular formula as the percent composition will be the same whether twenty grams or twenty kilograms of product were made.