Question

The relative atomic mass of hydrogen is 1 and of carbon is 12. 12g of an unknown hydrocarbon (containing hydrogen and carbon only) was found to contain 9g of carbon. This is shown below. What is the value of x?

Answer 1

Answer:

CH4

Explanation:

I'll assume the x is the subscript for the hydrogen, as in CH$x$

We have 9g of C in the 12 gram sample.  The remainder is hydrogen, H, we're promised.  That means 3 grams of H.

Let's convert these two masses into moles of C and H.  With moles, we are able to see the count relationship between the two elements.  

Moles C = 9 g/12 g/mole = 0.70 moles C

Moles H = 3g/1 g/mole = 3.0 moles H

See the attached table.

We want whole numbers so let's take (1/0.70) = 1.33455 and then multiply this factor by both mole values:

Moles C = 1

Moles H = 4.0037, which we'll round to 4

There are 4 H for every 1 C

CH$4$