Question

How can I calculate the mass percent of carbon, nitrogen and oxygen in caffeine, C8H10N4O2.

Answer 1

You must first calculate the total weight of 1 mole of the substance. You do this by finding the molar weights on the periodic table of each of the elements in the compound:

$8(12.01g)+10(1.008g)+4(14.007g)+2(15.999g)=96.08g+10.08g+56.028g+31.998g=194.186g$

So now we take the amount of each element that is given in the compound and divide it by the total mass of the compound:

Carbon: 96.08g

$\frac{96.08g}{194.186g}= 0.49478=49.48$

Carbon = 49.48%

Nitrogen: 56.028g

$\frac{56.028g}{194.186g}=0.28853=28.85$

Nitrogen = 28.85%

Oxygen: 31.998g

$\frac{31.998g}{194.186g} =0.16478=16.48$

Oxygen = 16.48%

Answer 2

Explanation:

Molecular mass of carbon  caffeine($C_8H_{10}N_4O_2$), = 194 g/mol

Atomic mass of carbon = 12 g/mol

Atomic mass of hydrogen atom = 1 g/mol

Atomic mass of nitrogen atom = 1 g/mol

Atomic mass of oxygen  atom = 1 g/mol

Percentage of an element in a compound:

$\frac{\text{Number of atoms of element}\times \text{Atomic mass of element}}{\text{molecular mass of compound}}\times 100$

Percentage of carbon:

$\frac{8\times 12 g/mol}{194 g/mol}\times 100=49.48\%$

Percentage of hydrogen:

$\frac{10\times 1g/mol}{194 g/mol}\times 100=5.15\%$

Percentage of nitrogen:

$\frac{4\times 14 g/mol}{194 g/mol}\times 100=28.86\%$

Percentage of oxygen :

$\frac{2\times 16g/mol}{194 g/mol}\times 100=16.49\%$