# Question bfdfb

Sep 25, 2017

$\text{3.95 g}$

#### Explanation:

For starters, you know that $1$ molecule of pentane contains

• five atoms of carbon, $5 \times \text{C}$
• twelve atoms of hydrogen, $12 \times \text{C}$

This implies that carbon and hydrogen have a $5 : 12$ atom ratio in pentane, i.e. for every $5$ atoms of carbon present in your sample, you get $12$ atoms of hydrogen.

You can use this atom ratio to figure out the number of atoms of carbon that are present in the sample.

4.75 * 10^(23)color(red)(cancel(color(black)("atoms H"))) * "5 atoms C"/(12color(red)(cancel(color(black)("atoms H")))) = 1.979 * 10^(23)color(white)(.)"atoms C"

Now, in order to convert the number of atoms of carbon to moles, use Avogadro's constant.

1.979 * color(blue)(cancel(color(black)(10^(23)))) color(red)(cancel(color(black)("atoms C"))) * "1 mole C"/(6.022 * color(blue)(cancel(color(black)(10^(23))))color(red)(cancel(color(black)("atoms C")))) = "0.3286 moles C"#

Finally, to convert the number of moles to grams, use the molar mass of carbon.

$0.3286 \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{moles C"))) * "12.011 g"/(1color(red)(cancel(color(black)("mole C")))) = color(darkgreen)(ul(color(black)("3.95 g}}}}$

The answer is rounded to three sig figs, the number of sig figs you have for the number of atoms of hydrogen present in the sample.