What is the mass of 4.91 x10^21 platinum atoms?

To calculate the mass, go from atoms to moles and moles to grams:

Atoms ... Moles ... Grams

From atoms to moles, use the Avogadro’s no. which is 6.022 x 10^23 and from moles to grams use atomic mass of Pt that is 195.08g.

So, 4.91 x 10^21 Pt atoms x (1 mole Pt / 6.022x10^23 Pt atoms) x (195.08g Pt / 1 mole Pt) = 1.59g Pt

Mass of 4.91 x 10^21 Pt atoms = 1.59g Pt