What is the mass of #1.2*10^18# formula units of calcium chloride?
The thing to keep in mind about formula units is that you need
In this case, you know that
Moreover, you know that calcium chloride has a molar mass of
This means that your sample has a mass of
#1.2 * 10^8 color(red)(cancel(color(black)("f. units CaCl"_2))) * "110.98 g"/(6.022 * 10^(23)color(red)(cancel(color(black)("f. units CaCl"_2)))) = color(darkgreen)(ul(color(black)(2.2 * 10^(-4) quad "g")))#
The answer is rounded to two sig figs, the number of sig figs you have for the number of formula units.