Caffeine a stimulant found in coffee has the mass composition of C, 49.48%, H, 5.19%, N, 28.85%, O 16.48%. The molar mass of caffeine is 194.19g/mol. Find the molecular formula for caffeine.
C8H10N4O2 is the molecular formula.
I got that as well but then I took the molar mass and divided by the empirical formula and got C8H10N4O2
Take 100 g sample which gives you
48.48g C
5.19 g H
28.85g N
16.48g O.
Now convert grams to moles with moles = grams/molar mass
Then find the ratio of the elements to the smallest one with it being no less than 1.00. The easy way to do that is to divide the smallest number by itself (thereby assuring a 1.00 for that element), then divide all of the other numbers by the same small number, round to whole numbers, and that should be the empirical formula. Check the mass of the empirical formula to see if the molar mass is 1x, 2x, 3x, etc of the empirical mass. I worked it and found the empirical formula of C4H5N2O.
To determine the molecular formula for caffeine, we need to find the empirical formula first. The empirical formula represents the simplest, whole number ratio of atoms in a compound.
Given the mass composition of caffeine (%C = 49.48%, %H = 5.19%, %N = 28.85%, %O = 16.48%), we can assume we have 100 grams of caffeine. This allows us to convert the mass to grams for each element.
Mass of C in 100g caffeine = (49.48 g) / 100 × 100g = 49.48 g
Mass of H in 100g caffeine = (5.19 g) / 100 × 100g = 5.19 g
Mass of N in 100g caffeine = (28.85 g) / 100 × 100g = 28.85 g
Mass of O in 100g caffeine = (16.48 g) / 100 × 100g = 16.48 g
Next, we need to find the moles of each element by dividing their mass by their molar mass. The molar mass of C is 12.01 g/mol, H is 1.008 g/mol, N is 14.01 g/mol, and O is 16.00 g/mol.
Number of moles of C = 49.48 g / 12.01 g/mol = 4.120 moles
Number of moles of H = 5.19 g / 1.008 g/mol = 5.151 moles
Number of moles of N = 28.85 g / 14.01 g/mol = 2.062 moles
Number of moles of O = 16.48 g / 16.00 g/mol = 1.030 moles
Now we need to divide each mole value by the smallest mole value to obtain the mole ratio of each element.
Dividing all the mole values by 1.030 gives:
C: 4.120 / 1.030 = 4
H: 5.151 / 1.030 = 5
N: 2.062 / 1.030 = 2
O: 1.030 / 1.030 = 1
So the empirical formula for caffeine is C4H5N2O.
To find the molecular formula, we need to determine the ratio between the empirical molar mass and the actual molar mass of caffeine.
The empirical molar mass of C4H5N2O can be calculated by summing the molar mass of each element:
(4 × molar mass of C) + (5 × molar mass of H) + (2 × molar mass of N) + (1 × molar mass of O)
(4 × 12.01 g/mol) + (5 × 1.008 g/mol) + (2 × 14.01 g/mol) + (1 × 16.00 g/mol) = 194.17 g/mol
Comparing this to the given molar mass of caffeine (194.19 g/mol), we see that they are quite close.
Thus, the molecular formula for caffeine is also C4H5N2O.