Determine the mass of 5.25×10^24 atoms Cl

300 g

Explanation:

To solve this convert atoms to moles to grams.  To convert atoms to moles, you will need Avogadro's number (6.022*10²³ atoms/mol), and to convert moles to grams, you will need the atomic mass of Cl (34.45 g/mol).

Divide the given mass of Cl by Avogadro's number to find moles.  Get 8.718..

Multiply the moles by the atomic mass.  Get 300.3..

Your answer is 300 g because of significant figures.