What mass of sulfur would you need in order to have 2.5x1021 atoms of sulfur?

6.19⋅109ng

Explanation:

The first thing to do here is to determine the mass in grams of 2.33⋅1020 atoms of oxygen.

To do that, use Avogadro's number to convert the number of atoms to moles, the molar mass of oxygen to convert the moles to grams.

So, Avogadro's number, which acts as the definition of a mole, tells you that in order to have one mole of oxygen, you need to have 6.022⋅1023 atoms of oxygen.

In your case, the sample will be equivalent to

2.33⋅1020 atoms of O ⋅1mole O

                                    6.022⋅1023 atoms of O   =3.869⋅10−4 moles O

Now, oxygen has a molar mass of 15.9994 g mol−1 as listed in the Periodic Table of Elements

This means that your sample will have a mass of

3.869⋅10−4 moles O   ⋅15.9994 g

                                       1 mole O           =6.190⋅10−3g

Finally, you can convert the mass from grams to nanograms by using the conversion factor  

1 g=109ng

You will end up with

6.190⋅10−3g ⋅109ng

                        1g=6.19⋅106ng

meaning The answer is rounded to three sig figs.