STOICHIOMETRY: QUANTITATIVE INFORMATION ABOUT CHEMICAL REACTIONS S

Question

An unknown compound has the formula C_xH_yO.You burn 0.1322 g of the compound and isolate 0.3228g of CO₂ and 0.1321g of H₂O . What is the empirical formula of the compound? If the molar mass is 72.1 g/mol, what is the molecular formula? 

(Enter the elements in the order: C,H,O.)

Empirical formula _______

Molecular Formula______

Answer 1

First, find the moles of C and H:

0.3228 g CO2 x 1 mol CO2/44 g x 1 mol C/mol CO2 = 0.007336 moles C

0.1321 g H2O x 1 mol H2O/18 g x 2 mol H/mol H2O = 0.01468 moles H

Next, we find the moles of O. We do this by finding mass of C and H, subtracting that from original mass of sample, and then converting that to moles.

mass C = 0.007336 mol C x 12 g/mol = 0.0880 g C

mass H = 0.01468 mol H x 1 g/mol = 0.01468 g H

mass of O = 0.1322 g - 0.0880 g - 0.01468 g = 0.02952 g O

moles O = 0.02952 g O x 1 mol/16 g = 0.001845 moles O

Now we divide all moles by the lowest number in an attempt to get all whole numbers.

moles O = 0.001845/0.001845 = 1.00 moles O

moles C = 0.007336/0.001845 = 3.98 ~ 4 moles C

moles H = 0.01468/0.001845 = 7.96 ~ 8 moles H

Finally, our empirical formula = C₄H₈O

For the molecular formula, we find how many empiricals it takes to make 1 molecular. The molar mass of the empirical is 48+8+16 = 72 g/mole

molecular is 72g/mol so it takes only 1 empirical to make the molecular.

Molecular formula = C₄H₈O