Question

What is the mass of `1.2*10^18` formula units of calcium chloride?

Answer 1

`2.2 * 10^(-4)` `"g"`

Explanation:

The thing to keep in mind about formula units is that you need `6.022 * 10^(23)` of them to have exactly `1` mole of an ionic compound `->` think Avogadro's constant here.

In this case, you know that `6.022 * 10^(23)` formula units of calcium chloride are needed in order to have exactly `1` mole of calcium chloride.

Moreover, you know that calcium chloride has a molar mass of `"110.98 g mol"^(-1)`, which means that `1` mole of calcium chloride has a mass of `"110.98 g"`.

So if `1` mole of calcium chloride contains `6.022 * 10^(23)` formula units and has a mass of `"110.98 g"`, you can say that `6.022 * 10^(23)` formula units of calcium chloride have a mass of `"110.98 g"`.

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.