How many formula units make up 37.4 g of magnesium chloride (`MgCl_2`)?

The first thing to do here is to convert the mass of magnesium chloride to moles by using the compound's molar mass.

Magnesium chloride has a molar mass of `"95.211 g mol"^(-1)`, which means that every mole of magnesium chloride has a mass of `"95.211 g"`.

This means that your sample will contain

`37.4 color(red)(cancel(color(black)("g"))) * "1 mole MgCl"_2/(95.211 color(red)(cancel(color(black)("g")))) = "0.3928 moles MgCl"_2`

Now, in order to find the number of formula units of magnesium chloride present in the sample, you must use Avogadro's constant to go from moles to formula units.

`color(purple)(bar(ul(|color(white)(a/a)color(black)("1 mole" = 6.022 * 10^(23)"f. units")color(white)(a/a)|))) ->` Avogadro's constant

In your case, the sample will contain

`0.3928 color(red)(cancel(color(black)("moles MgCl"_2))) * (6.022 * 10^(23)"f. units MgCl"_2)/(1color(red)(cancel(color(black)("mole MgCl"_2))))`

`= color(green)(bar(ul(|color(white)(a/a)color(black)(2.37 * 10^(23)"f units MgCl"_2)color(white)(a/a)|)))`

The answer is rounded to three sig figs.