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?

2 Answers
Feb 4, 2017

Answer:

#%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

Answer:

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"#.