8.18 Perchloroethylene, also known as perc is the solvent used in most dry clean

Question

8.18 Perchloroethylene, also known as perc is the solvent used in most dry cleaning, H consists of C and Cl. Determine the empirical formula of Perchloroethylene given that a 2.58-g sample yields 1.37 g CO2 in a combustion analysis experiment, 8.19 Lactic acid, which consists of C, H, and o, has long been thought to be responsible for muscle soreness following strenuous exercise. Determine the empirical formula of lactic acid given that combustion of a 10.0-g sample produces 14.7 g CO2 and 6.00 g H2O, 8.20 Ascorbic acid (vitamin C) contains C, H, and O, In one analysis, combustion of 5.24 g of ascorbic acid yields 7.86 g CO2 and 2.14 g H2O. Calculate the empirical formula and molecular formula of ascorbic acid given that its molar mass is about 176 g. 8.21 Paradichlorobenzene, a common ingredient in solid air fresheners, contains only C, H, and Cl and has a molar mass of about 147 g. Given that combustion of 1.68 g of this compound produces 3.02 g CO2 and 0.412 g H2O, determine its empirical and molecular formulas. 8.22 The diagram shows the products of a combustion analysis. Determine the empirical formula of the compound being analyzed if (a) it is a hydrocarbon and (b) it is a compound containing C, H, and O, and has a formula weight of approximately 102.

Explanation / Answer

I am going to try to back-engineer your question asked....
I feel that it was likely:
In one combustion analysis, 5.24 g of ascorbic acid yields 7.86 CO2 & 2.14 grams of water
find the empirical formula

convert grams of CO2 & H2O intoo gramms of C & H by using molar masses:
7.86 CO2 (12.01 g/mol C) / (44.01 g/mol CO2) = 2.145 grams of carbon
&
2.14 g H2O (2.016 g H2) / (18.01 g/mol H2O) = 0.240 grams of Hydrogen

find grams of oxygen
5.24 g of ascorbic acid - 2.145 g C - 0.240 g of H = 2.86 grams of Oxygen

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now find moles using molar masses
(2.145 grams of carbon) ( 1 mol C / 12.01 g/mol = 0.178 mol C
(0.240 grams of Hydrogen) ( 1 mole H / 1.008 g ) = 0.238 mol H
(2.86 grams of Oxygen) (1 mol O / 16 grams) = 0.178 mol O

dividing by the smallest, in order to ratio the moles
0.178 mol C / 0.178 = 1 mol C
0.238 mol H / 0.178 = 1.33 mol H
0.178 mol O / 0.178 = 1 mol O

triple the moles to get whole numbers
& your empirical formula would be
C3H4O3

(which has a fomula mass of 88)

==========================
these questions sometimes as for the molecular formula as well
if so ,
they will provide the molar mass
the molar mass of ascorbic acid is 176 g/mol
http://en.wikipedia.org/wiki/Ascorbic_ac...

so .... since we hav an empirical formula of C3H4O3 has a mass 88,
& the molar mass is double that at 176....
so is the molacular formula
which should be
C6H8O6

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