percentage composition?

Question

a 1.500g sample of a mixture containg only Cu2O and CuO was treated with hydrogen to produce 1.252g pure copper metal. calculate the percentage composition of the mixture

Answer 1

First, determine the % Cu for the two extremes of composition (pure Cu2O and pure CuO):

Cu2O: 2 x 63.55 + 16 = 143.1 (88.82% Cu)
CuO 63.55 + 16 = 79.55 (79.89 %Cu)

Determine % Cu for unknown:

1.252/1.500 x 100% = 84.47%

You can easily see that the unknown is somewhere near a 50/50 mixture by just looking at the percentages.

Determine difference in Cu % between pure Cu2O and pure CuO:

88.82 - 79.89 = 8.93%

Now find the difference in Cu% between the unknown and CuO:

[83.47 - 79.89] x 100% = 3.58%

Now determine what percentage of the difference between Pure Cu2O and pure CuO we are at:

[3.58/8.93] x 100% = 40.01%

Since we are 40.01% of the way from CuO towards Cu2O we have a sample of composition: 40.01% Cu2O and 59.99% CuO.

Note: there might be other, more elegant ways of getting to this number.

Check your answer (calculating g of Cu formed from 1.5g of mixed oxides of the composition we calculated above)

[0.4001 x 0.8882 x 1.5] + [0.5999 x 0.7989 x 1.5] = 1.252

Matches original Cu weight for sample.

Answer 2

Percent composition can have two meanings: By volume and by weight. It so happens that people usually give the percent composition of the atmosphere by volume, which is 78% nitrogen, 21% oxygen, and 1% other gases, such as carbon dioxide and argon.