First, find the molar mass of CaCO3 (100.0869 grams, but you can usually look on a periodic table for a less precise number and still be fine).
Convert grams to moles, and moles to atoms
20.0 grams * [tex]\frac{1mol}{100.0869g} * \frac{6.02E23 atoms}{1 mol}[/tex] = 1.20 * [tex]10^{23}[/tex] .
Check if the answer makes sense.
If you have 20 grams of chalk, you have ~20% of a mole of chalk. This means you have ~20% of the atoms a normal mole of chalk would have.
1.2 / 6.02 = 0.199, which is very close to 0.2 a.k.a. 20%
