So, if a sample of a compound decomposed to its constituent elements and produced 25.6 g of o, and 9.60 g of carbon, and another sample produced 21.6 g of o, how much carbon was produced in the second sample?

Method 1:

First find out the formula of the sample.

no. of moles of O = 25.6/16 = 1.6

no. of moles of C = 9.6/12 = 0.8

ratio of C:O = 1:2

formula is CO2

In the second sample,

no. of moles of O = 21.6/16 = 1.35

no. of moles of C = 1.35/2 = 0.675

mass of C = 0.675 x12 = 8.1g

method 2:

Actually it's quite simple. The two samples are the same compound so the elements produced should be in the same ratio.

C/9.6 = 21.6/25.6

C = 8.1g