How many moles are in #9.461 * 10^24# formula units of manganese(III) dichromate?
As you know, ionic compounds form giant lattice structures in which the positively charge ions, or cations, and the negatively charged ions, or anions, are held together by electrostatic attraction.
This of course implies that you cannot speak of molecules of an ionic compound. Instead, the formula unit is used to express the smallest ratio that exists between the cations and anions in the ionic compound.
Simply put, the formula unit is the empirical formula of an ionic compound.
This means that one mole of manganese(II) dichromate,
So, you know that you're dealing with
In fact, looking at the exponents, you can say that you'll have more than ten moles of the compound.
Confirm this by doing
#9.461 * 10^(24)color(red)(cancel(color(black)("form. units"))) * ("1 mole MnCr"_2"O"_7)/(6.022 * 10^(23)color(red)(cancel(color(black)("form. units")))) = color(green)("15.71 moles MnCr"_2"O"_7)#
The answer is rounded to four sig figs, the number of sig figs you have for the number of formula units.