Question

ibuprofen an aspirin substitute has the following percent composition C, 75.69%; H,8.80%; O,15.51%. determine the empirical formula

Answer 1

Empirical formulas...


1. Start with the number of grams of each element, given in the problem.
If percentages are given, assume that the total mass is 100 grams so that
the mass of each element = the percent given.

2. Convert the mass of each element to moles using the molar mass from the periodic table.

3. Divide each mole value by the smallest number of moles calculated.

4. Round to the nearest whole number. This is the mole ratio of the elements and is
represented by subscripts in the empirical formula.
If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same
factor to get the lowest whole number multiple.
e.g. If one solution is 1.5, then multiply each solution in the problem by 2 to get 3.
e.g. If one solution is 1.25, then multiply each solution in the problem by 4 to get 5.


Once the empirical formula is found, the molecular formula for a compound can be determined if the molar mass of the compound is known. Simply calculate the mass of the empirical formula and divide the molar mass of the compound by the mass of the empirical formula to find the ratio between the molecular formula and the empirical formula. Multiply all the atoms (subscripts) by this ratio to find the molecular formula.



1) C = 75.69% = 75.69 g
H = 8.80% = 8.80g

O = 100 - %C - %H
O = 100 - 75.69 - 8.80 = 15.51% = 15.51g

2) n = m / M
n = moles
m = mass (g)
M = molar mass (gmol^-1)

n(C) = 75.69g / 12gmol^-1 = 6.3075 mol

n(H) = 8.80g / 1gmol^-1 = 8.80 mol

n(O) = 15.51g / 16gmol^-1 = 0.969375 mol


3) Divide each mole value by the smallest number of moles calculated. in this case, n(O) = 0.969375 mol

C = 6.3075 mol / 0.969375 mol
C = 6.506769826


H = 8.80 mol / 0.969375 mol
H = 9.078014184


O = 0.969375 mol / 0.969375 mol
O = 1

4) multiply each value by 2 to get whole numbers;

C = 6.506769826 x 2 = 13.01353965

H = 9.078014184 x 2 = 18.15602837

O = 1 x 2 = 2

thus, empirical formula = C13H18O2


Molecular formula.......

first, find the molar mass of the empirical formula

M(C13H18O2) = (13 X 12) + (18 X 1) + (2 X 16)
M(C13H18O2) = 206 gmol^-1

then find the ratio between the molar mass of the empirical formula and that of the molecular mass that you are given;

206/206 = 1

thus, empirical formula = molecular formula = C13H18O2