A 20.0 gram sample of an element contains 4.95 x 1023 atoms. what is the element?

You will want to find how many grams are in a whole mole so you know which element it is. To do this, find out how much of a mole you have.

4.95 x 10^23 atoms / 6.022 x 10^23 atoms (one whole mole of any element) =.8219860511 or ~82% of 1 mole

Now we know that, find what to multiply 20 g by to get the rest of the mole.

1 mole /.8219860511 mole = 1.216565657

20 g x 1.216565657 = ~24.33 g / mol

Now that you have grams per mole, you can look at the periodic table and the molar masses to see which this number is closely aligned.

Your answer is Magnesium (Mg), which has a molar mass of 24.305 g