Question

Solve of for The percentage of carbon hydrogen and oxygen in vitamin C were determine by burning a sample weighing 2.00mg the masses Co2 and water found were 3.00mg and 0.81mg respectively,Given that the molar mass of vitamin C is 180g, determine the molecular mass

Answer 1

CxHyOz ==> CO2 + H2O
....2 mg...........3 mg....0.81 mg
mm = molar mass: am = atomic mass and
mass C = mass CO2 x (am C/mm CO2)
mass C = 3 mg x (12/44) = 0.818 mg
mass H = mass H2O x (2*am H/mm H2O)
mass H = 0.81 mg x (2*1/18) = 0.09 mg
mass O = mass CxHyOz -mass H - mass C or
mass O = 2 mg - 0.818 mg - 0.09 mg = 1.092 mg
%C = (mass C/total mass)*100 = (0.818/2)100 = 40.9%
%H = (mass H/total mass)*100 = (0.09/2)100 = 4.50%
%O = 100% - %C - %H = 100 - 40.9 - 4.50 = 54.6%
Next, determine the empirical formula for vitamin C.
Take a 100 g sample which gives
C = 40.9 grams
H = 4.50 grams
O = 54.6 grams.
Convert the grams to mols this way. mols = g/atomic mass
mols C = 40.9/12 = 3.41
mols H = 4.50/1 = 4.50
mols O = 54.6/16 = 3.41
You want to find the ratio of these elements to each other with the smallest being no less than 1 AND you want small whole numbers; i.e., the easiest way is to divide the smallest number by itself and divide the others numbers by the same small number.
mols C = 3.41/3.41 = 1.00
mols H = 4.50/3.41 = 1.32
mols O = 3.41/3.41 = 1.00
These aren't small whole numbers to you multiply everything by whole numbers to see if you can find one with whole numbers. If we try 2 here is what you get:
mols C = 1.00 x 2 = 2.00
mols H = 1.32 x 2 = 2.64
mols O = 1.00 x 2 = 2.00
Nope. That is it because 2.64 is not a small whole number. Try multiplying by 3 to get:
mols C = 1.00 x 3 = 3.00
mols H = 1.32 x 3 = 3.94 which rounds to 4
mols O = 2.00 x 3 = 3.00 so the empirical formula (that CxHyOz) is C3H4O3 so the empirical mass is 3*12 + 4*H + 3*16 = 88
The problem tells you that the molar mass is 180 (which isn't quite correct) but 88 x some whole number = 180 and
some whole number = 180/88 = 2.04 which rounds to 2.00 so the molecular formula is (C3H4O3)2 or C6H8O6. The molar mass is
6*12 + 8*1 + 6*16 = 176.
You're welcome.