Jennifer H. answered • 11/17/21
Certified chemistry teacher with 16+ years of tutoring experience
To answer this question, you must convert each option into atoms so that you can compare them.
To convert 5.0 L of Ar into atoms, you will need to convert L into moles first (using 22.4 L/mol, because you are at STP) and then moles into atoms (using Avogadro's number: 6.02 x 10^23 atoms/mol).
The second answer choice, 6.02 x 10^23 atoms of Ar, is already in atoms, so you don't need to do any conversions here.
To convert 0.75 moles of Ar into atoms, you will need to use Avogadro's number (6.02 x 10^23 atoms/mol).
To convert 20.0 g of Ar into atoms, you will need to convert grams into moles first (using argon's molar mass from the periodic table) and then moles into atoms (using Avogadro's number: 6.02 x 10^23).
Once you have completed all three calculations, just find the answer choice with the largest number of atoms and that will be the correct one.
Side note: because your second answer choice (6.02 x 10^23 atoms of Ar) is equal to 1 mole, you can also use moles as your standard of comparison. For example, 5.0 L of Ar at STP is definitely less than 1 mole, since 1 mole of any gas is equal to 22.4 L at STP. Therefore, the first option is not correct. The third option, 0.75 moles of Ar, is also definitely less than 1 mole, and therefore is also not correct. The fourth option, 20.0 g of Ar, is also less than 1 mole, because the molar mass of argon is approximately 40 grams in 1 mole. Therefore, the fourth option is also not correct.
