Chem question for polystyrene?

Question

Polysterene is a network solid. Its carbon and hydrogen atoms are connected by huge networks of covalent bonds. It isn't made up of identical small molecules, so it doesn't really have a molar mass. But like any compound, polystyrene's atoms are combined in definite proportions, so it does have an empirical formula. To learn more about the composition of polystyrene, combustion analysis was performed on a 2.57 g sample. In the combustion analysis, 8.67 g of carbon dioxide and 1.77 g of water were produced.

A) How many moles of carbon were present in the original sample of polystyrene?

B) How many moles of hydrogen were present in the original sample of polystyrene?

C) What is the empirical formula of polystyrene?

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I have no idea how to find A and B,

but my answer for C was CH

Answer 1

The molar mass of carbon dioxide is 12 + (16)*2 = 44g

27% of that weight is carbon (12/44)

So if 8.67g of carbon dioxide was produced in this experiment, then 2.36g of that was carbon.

One mole of carbon weighs 12g.  

So 0.2 moles of carbon was produced (2.36/12=0.2)

Using the same logic on the hydrogen:

The molar mass of water is 1*2+16=18

12.5% of that is hydrogen  (2/18=0.125)

So 1.77g of water is 0.22 g of hydrogen

One mole of hydrogen atoms is 1 g,  (hydrogen gas weighs twice as much because the it is diatomic though)

So 0.22 moles of hydrogen atoms (H) were present, but that's 0.11 moles of hydrogen gas (H2).

Your estimate for C was close, but there was a little more hydrogen than carbon.  So the empirical formula is:

C(0.2)H(0.22) = CH(1.1) = C10H11