Question

The molar mass of copper(II) chloride, `"CuCl"_2`, is `"134 g/mol"`. How many formula units of `"CuCl"_2` are present in `"17.6 g"` of `"CuCl"_2`?

Answer 1

`7.91 * 10^(22)color(white)(a)"formula units CuCl"_2`

Explanation:

The problem provides you with the molar mass of copper(II) chloride, `"CuCl"_2`, and asks you to calculate how many formula units are present in a `"17.6 g"` sample, so right from the start you know that you must also use Avogadro's number.

According to Avogadro's number, which is essentially the definition of the mole, a mole of an ionic compound contains `6.022 * 10^(23)` formula units of that compound.

This means that your strategy here will be to use the molar mass to convert the sample to moles, then use Avogadro's number to convert the moles to formula units.

Your sample contains

`17.6 color(red)(cancel(color(black)("g"))) * "1 mole CuCl"_2/(134color(red)(cancel(color(black)("g")))) = "0.13134 moles CuCl"2`

Now use Avogadro's number to figure out how many formula units would be present in that many moles

`0.13134 color(red)(cancel(color(black)("moles CuCl"_2))) * (6.022 * 10^(23)"f. units")/(1color(red)(cancel(color(black)("mole CuCl"_2)))) = color(green)(|bar(ul(color(white)(a/a)color(black)(7.91 * 10^(22)"f. units")color(white)(a/a)|)))`

The answer is rounded to three sig figs, the number of sig figs you have for the mass of the sample.