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Moles

Calculating the Empirical Formula from a Reaction.

Example 2.

20 g  of calcium was found to react with 35·5 g  of chlorine.
What is the empirical formula of calcium chloride?

Method.
1) Find how many moles of calcium
react with how many moles of chlorine.

RAM of Ca = 40,  RAM of Cl = 35·5.

moles = mass ÷ RAM

for calcium   moles = 20 ÷ 40
                                   = 0·5 moles.

for chlorine   moles = 35·5 ÷ 35·5
                              = 1·0 moles.


2) The proportion of moles of calcium to moles of chlorine
is reduced to the lowest whole number.

0·5 moles of Ca to 1·0 moles of Cl.
Multiply by 2.
1 mole of Ca to 2 moles of Cl.
The empirical formula is CaCl2.

Since calcium chloride is an ionic compound,
the actual formula is the same as the empirical formula.

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