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Calculating the Empirical Formula from a Reaction.

Example 1.

33·6 g  of iron was found to react with 14·4 g  of oxygen.
What is the empirical formula of iron oxide?

1) Find how many moles of iron
react with how many moles of oxygen.

RAM of Fe = 56RAM of O = 16.

moles = mass ÷ RAM

for iron   moles = 33·6 ÷ 56
                     = 0·6 moles.

for oxygen   moles = 14·4 ÷ 16
                     = 0·9 moles.

2) The proportion of moles of iron to moles of oxygen
is reduced to the lowest whole number.

0·6 moles of Fe to 0·9 moles of O.
Divide by 3, multiply by 10.
2 moles of Fe to 3 moles of O.
The empirical formula is Fe2O3.

Since iron oxide is an ionic compound,
the actual formula is the same as the empirical formula.

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