Question

caffeine contains 49.5% si 28.9% h 16.5% o and 5.20 % calculate the molecular formula given that 0.2 moles of caffeine weight 38.0 4 gram​

Answer 1

Explanation:

The way tackle these problems is using a process that goes like this:

Percent to mass, mass to mole, divide by small, multiply 'till whole

So first we wants to take the percents and make them masses, We assume we have a 100g sample of caffeine, and so 49.48% carbon is equal 49.48g of carbon in sample. 5.15% hydrogen is equal to 5.15g of hydrogen in sample. and so on

Now we take those numbers and convert them to moles of the elements we are dealing with, so;

49.48g / 12.01 (g/mol) = 4.12 mol of carbon

5.15g / 1.01 (g/mol) = 5.1 mol of hydrogen

28.82g / 14.01 (g/mol) = 2.06 of nitrogen

16.48g / 6 (g/mol) = 1.03 mol of oxygen

Now we divide all the moles by the smallest number of moles, in this case the 1.03 from Oxygen,so;

4.12/1.03 = 4 for Carbon

5.1/ 1.03 = 5 for Hydrogen

2.06/ 1.03 = 2 for nitrogen

1.03/1.03 = 1 for Oxygen

Since all the numbers are already whole we can skip the last part and go to an empirical

Now we have to check the molar mass of empirical formula to make sure it matches up with the molar mass given.

(12.4) + (1.5) + (14.2) + (16.1) = 97 which is not what we were given, so we take that we were given and divide it by this number

194.2 ÷ 97 which is approximately 2, so we have to multiply the number in empirical formula by 2 to get our molecular formula

C8H10N4O2