Determine the mass of 6.44 * 1023 atoms Cl.

For 6.44 x 10^23 atoms:

That amount is 1.07 moles of Cl atoms

multiply the number of moles by the molar mass of Cl which is 35.45

That equals 37.9 grams of Cl. You only use 1 decimal place because there are only 3 significant digits.

Now if it is a text book problem that says chlorine gas, it may be referring to atmospheric Cl gas which is Cl2

In that case, its just 1.07 moles x 70.9 (molar mass of Cl2)

That turns out to be 75.9 g Cl2