7.00 g of a certain Compound X, known to be made of carbon, hydrogen and perhaps oxygen, and to have a molar mass of 70. g/mol, is burned completely in excess oxygen, and the mass of the products carefully measured. 21.96 g of carbon dioxide and 8.99 g of water are formed as products. a) How much carbon, hydrogen and oxygen is in 7.00 g of Compound X

5.993 g C

1.01 g H

0 g O

Explanation:

Let's consider the combustion of the unknown compound X.

X + O₂ = CO₂ + H₂O

Due to the law of conservation of mass, all the C in CO₂ and all the H in H₂O come from X.

In CO₂, there are 12.01 g of C per 44.01 g of CO₂. The mass of C in 21.96 g of CO₂ is:

21.96 g CO₂ * (12.01 g C/44.01 g CO₂) = 5.993 g C

In H₂O, there are 2.02 g of H per 18.02 g of H₂O. The mass of H in 8.99 g of H₂O is:

8.99 g H₂O * (2.02 g H / 18.02 g H₂O) = 1.01 g H

If we sum the mass of H and the mass of C, we get

5.993 g + 1.01 g = 7.00 g

which is the mass of X. Then, X has no oxygen