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Moles

Volumes of Gases.

Example 2.

What volume of carbon dioxide is produced by burning 90 g  of ethane in air?

Method.
1) Find how many moles of ethane are present in 90 g  of ethane.
RAM of  C = 12,  RAM of  H = 1.

RMM of ethane (C2H6)  =  (2 x 12) + (6 x 1)
                = 30.

moles = mass ÷ RAM

moles = 90 ÷ 30
                               = 3·0 moles of ethane.


2) Write the equation for the reaction.

ethane + oxygen  arrow  carbon dioxide + water.
 2C2H6(g)  + 7O2(g)   arrow      4CO2(g)   +   6H2O(l)

Use the big numbers to find the proportion of reactant to product.
2C2H6 makes 4CO2, the proportion is 2 to 4,  or 1 to 2,
so 3·0 moles of ethane will make 6·0 moles of carbon dioxide.

3) Convert moles into volume.

volume  =  moles x 24,000 cm3

volume  =  6·0 x 24,000 cm3
                                   = 144,000 cm3 of carbon dioxide.

So, burning 90 g  of ethane in air will produce 144,000 cm3 of carbon dioxide.

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