Hi Hina,
Please see the work below:
The objective of this problem is to get from a number of atoms, to a mass in grams. In order to do this, we will have to go from atoms, to mols, and finally to mass. A given number of Arsenic atoms was 2.11e21 (or 2.11x10^21, e and x10^# mean the same thing). You want to first convert the number of atoms to mols of atoms. As a reminder, 1 mol = Avogadro's number = 6.02e23.
(2.11e21 arsenic atoms) x (1 mol/6.02e23 atoms) = .0035 mols of arsenic, this conversion changes the individual number of atoms, into the units of mols.
Next you want to multiply by the molar mass that was given to us (74.92g/mol of Arsenic).
(.0035 mols of arsenic) x (74.92g/1 mol of arsenic) = .2626g of arsenic
Hope this helps!
