Are you taking chemistry at the same time? If not, this is a pretty hard question.
Facts
1. As a formula unit, Magnesium Hydroxide has a chemical formula of Mg(OH)2
2. That means for every Magnesium, there are 2 oxygens and 2 hydrogens
3. The weights of these components are found on a periodic table. Since every periodic table is different, I'm going to approximate the results.
Mg = 24
O = 16
H = 1
4. One mole of anything has 6.02 * 10^23 (in this case) formula units associated with it.
Step One
Find the number of grams in 1 formula unit using the numbers from the periodic table.
1 Mg = 1 * 24 = 24
2 O = 2 * 16 = 32
2 H = 2 * 1 = 2
Total = 58 grams
So
1 mole of Mg(OH)2 = 58 grams
1 mole / 6.02 * 10 ^23 formula units = x mole / 2.4 * 10 ^ 21 formula units
Cross multiply
1 mole * 2.4 * 10^21 formula units = x mole * 6.02 * 10^23 formula units.
Divide both sides by 6.02 * 10^23
1 * 2.4 * 10^21 / 6.02 * 10 ^ 23 = x
x = 0.003987 moles
Now do this again for grams
1 mole / 58 grams = 0.003987 mole / x grams Cross multiply
1 mole * x = 0.003987 * 58 grams
x = 0.2232 grams <<<<< Answer