Converting a Compound from grams to number of atoms

Question

I'm a little confuse on how to convert from grams to number of atoms.

The question is: convert the amount of chalk CaCO3 used from grams to number of atoms

The amount used is 0.20g and this figure needs to be converted to # of atoms.

I found two examples from different books:

Example #1. Convert 2.12 mol C3H8 to # of atoms. (it consider 11 atoms in a molecule)

2.12 mol C3H8 x 6.02 x 10^23 molecules C3H8 / 1 mol C3H8 x 11 atoms /1 molecules C3H8

ans: 1.40 x 10^23 atoms

(it says desired conversion is moles -->molecules-->atoms)

Example # 2 Convert 7.52g CH4 to # of atoms

Molar mass of CH4 is 16.040g CH4/mol

7.52g CH4 x 1mol CH4/ 16.040g CH4 x 6.02 x 10^23 atoms/ 1 mol CH4

ans: 2.82 x 10^23 atoms of CH4

This example convert grams ---> moles ---> atoms

So back to my original question: convert 0.20 grams of CaCO3 to # of atoms.

Should I follow example # 1: (molar mass is 100.09 g CaCO3)

0.20 g CaCO3 x 1 mol CaCO3 / 100.09 g CaCO3 x

6.02 x 10^23 molecules CaCO3 / 1 mol CaCO3 x

5 atoms / 1 molecules CaCO3

Or Example 2:

0.20 g CaCO3 x 1 mol CaCO3 / 100.09 g CaCO3 x

6.02 x 10^23 atoms CaCO3 / 1mol CaCO3

I'm not sure what's the difference between the two examples I found on the book and if I need to convert a COMPOUND from grams --> moles --> molecules --> atoms? (or I only convert to molecules if moles is given)

Or just grams ---> moles---> atoms

Please help! Thank you!

Answer 1

Hi Liz B.,

No problem.

Since your question did specify number of atoms as the requested answer, your Example 1 is correct.

Example 2 only gives you the number of moles of CaCO3, which is only 1/5 of the true answer.

You'll have lots of problems requiring conversion from mass to and from number of moles, for elements, compounds, etc.

The only thing that you can't convert well is mixtures, since 1) mixtures are varyingly homogeneous, and 2) analysis or preparation data for mixtures have inherent imprecision (for weighing, testing, etc.) so you can't know exactly what you're converting from!

If you are far enough advanced to appreciate the difference of masses of isotopes of various elements, that can (in rare instances) contribute to differences in numbers of atoms or molecules in a substance, also.

-- Cheers, --Mr. d.