# Hydrogen reacts with .771 g of carbon to form .90 g of a compound. What is the mass percent of hydrogen in the compound?

May 7, 2016

14.3%

#### Explanation:

In order to find the mass percent of hydrogen in this compound, you must determine how many grams of hydrogen you'd get in $\text{100 g}$ of compound.

In your case, you know that an unknown mass of hydrogen reacts with $\text{0.771 g}$ of carbon to form $\text{0.90 g}$ of hydrocarbon, which is a compound that contains only carbon and hydrogen.

Use the total mass of the hydrocarbon to determine how many grams of hydrogen reacted with the carbon

${m}_{\text{hydrocarbon}} = {m}_{C} + {m}_{H}$

${m}_{H} = \text{0.90 g" - "0.771 g" = "0.129 g}$

Now, if $\text{0.90 g}$ of this compound contain $\text{0.129 g}$ of hydrogen, it follows that $\text{100 g}$ of this compound will contain

100 color(red)(cancel(color(black)("g hydrocarbon"))) * "0.129 g H"/(0.90color(red)(cancel(color(black)("g hydrocarbon")))) = "14.33 g H"

So, if $\text{100 g}$ of this compound contain $\text{14.33 g}$ of hydrogen, it follows that the mass percent of hydrogen is

"mass percent H" = color(green)(|bar(ul(color(white)(a/a)"14.3%"color(white)(a/a)|)))

I'll leave the answer rounded to three sig figs.