Question

Question #75781

Answer 1

There are 2.61 × 10²² formula units of Ca(OH)₂ in the sample.

You must convert grams of Ca(OH)₂ → moles of Ca(OH)₂ → formula units of Ca(OH)₂.

The molar mass of Ca(OH)₂ is 74.09 g/mol. You use this to convert grams to moles.

3.21 g Ca(OH)₂ × `("1 mol Ca(OH)"_2)/("74.09 g Ca(OH)"_2)` = 0.043 33 mol Ca(OH)₂ (3 significant figures + 1 guard digit)

You know that 1 mol of anything contains Avogadro's number (6.022 × 10²³) of those things. So

0.043 33 mol Ca(OH)₂ × `(6.022 × 10^23 "formula units Ca(OH)"_2)/("1 mol Ca(OH)"_2)` =
2.61 × 10²² formula units Ca(OH)₂