# 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?

Feb 4, 2017

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

#### Explanation:

And, of course for a binary compound,

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

Feb 7, 2017

The compound contains $\text{81.72 % C}$ and $\text{18.28 % H}$.

#### Explanation:

We know

$\textcolor{w h i t e}{m m m m m} \text{carbon + hydrogen → compound}$
$\text{Mass/g:} \textcolor{w h i t e}{m l} 3.907 \textcolor{w h i t e}{m m m} 0.874 \textcolor{w h i t e}{m m m m} 4.781$

The mass of the compound must be the sum of the masses of $\text{C}$ and $\text{H}$.

$\text{Mass of compound" = "(3.907 + 0.874) g" = "4.781 g}$

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 $\text{ % H}$ by subtracting the $\text{% C}$ from $\text{100 %}$.

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

If you make a mistake in calculating $\text{% C}$, the error carries over into your $\text{% H}$ and makes it wrong.

The first method does not depend on the $\text{% C}$.