Question

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

Answer 1

`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 `"100 g"` of compound.

In your case, you know that an unknown mass of hydrogen reacts with `"0.771 g"` of carbon to form `"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_"hydrocarbon" = m_(C) + m_(H)`

`m_(H) = "0.90 g" - "0.771 g" = "0.129 g"`

Now, if `"0.90 g"` of this compound contain `"0.129 g"` of hydrogen, it follows that `"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 `"100 g"` of this compound contain `"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.