Question

If 3.907 g carbon combines completely with 0.874 g of hydrogen to form a compound, what is the percent composition of this compound?

Answer 1

`%C=(3.907*g)/(3.907*g+0.874*g)xx100%~=82%`

Explanation:

And, of course for a binary compound,

`%H=(100-82)%=??%`

Answer 2

The compound contains `"81.72 % C"` and `"18.28 % H"`.

Explanation:

We know

`color(white)(mmmmm)"carbon + hydrogen → compound"`
`"Mass/g:"color(white)(ml)3.907color(white)(mmm)0.874color(white)(mmmm)4.781`

The mass of the compound must be the sum of the masses of `"C"` and `"H"`.

`"Mass of compound" = "(3.907 + 0.874) g" = "4.781 g"`

The formula for percent by mass is

`color(blue)(bar(ul(|color(white)(a/a)"% by mass" = "mass of element"/"mass of compound" × 100 %color(white)(a/a)|)))" "`

`% "C" = (3.907 color(red)(cancel(color(black)("g"))))/(4.781 color(red)(cancel(color(black)("g")))) xx 100% = 81.72 %`

`% "H" = (0.874 color(red)(cancel(color(black)("g"))))/(4.781 color(red)(cancel(color(black)("g")))) xx 100% = 18.28 %`

NOTE: You could have gotten the `" % H"` by subtracting the `"% C"` from `"100 %"`.

That method is simpler, but I prefer using the first method.

If you make a mistake in calculating `"% C"`, the error carries over into your `"% H"` and makes it wrong.

The first method does not depend on the `"% C"`.