Homework Help: Chemistry

Posted by sean P. on Sunday, April 25, 2010 at 10:19am.

Terephthalic acid is an important chemical used in the manufacture of polyesters and plasticizers. It contains only C, H, and O. Combustion of 19.81 mg terephthalic acid produces 41.98 mg CO2 and 6.45 mg H2O. If 0.262 mol of terephthalic acid has a mass of 43.5 g, determine the molecular formula for terphthalic acid.

would i convert these to grams, making it look like this?
.01981g--> .04198g CO2 + .00645g H2O

um the percent of carbon in CO2 is 27.29%
and Hydrogen is 11.19% then i multiply it to get the grams... but idk what to do after that

Chemistry - DrBob222, Sunday, April 25, 2010 at 12:55pm
Right. You will then have grams C and grams H. Note that the problem did not tell you how much O was in the sample. You find that by
0.01981 g - g C - g H = g Oxygen.

Now find moles.
moles C = g C/12.01 = ??
moles H = g H/1.008 = ??
moles O = g O/16 = ??

Now you want to fine the ratio of the moles to each other in small whole numbers. The easy way to do that is to divide the smallest number by itself thereby making sure that is 1.000. Then divide the other numbers by the same small number and round to whole numbers (don't round too much; i.e., if you end up with x.5 multiply everything by 2; if by x.25 multiply everything by 4 etc). That will give you the empirical formula. Using the empirical formula mass you can determine the molecular formula. I'll leave that for you but you can check back if you need additional assistance.

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