Question #bfdfb

1 Answer
Sep 25, 2017

Answer:

#"3.95 g"#

Explanation:

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

  • five atoms of carbon, #5 xx "C"#
  • twelve atoms of hydrogen, #12 xx "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 color(red)(cancel(color(black)("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.