How many grams of hydrogen are in 85.0 g of Ca(OH)2

2.30 grams

Explanation: First find how many moles you have:

The molar masses are taken from the periodic table: Ca is 40, O is 16, H is 1. There are two O and 2 H from (OH)2 40+32+2=74 g/mol. Grams (given) x mol/grams (molar mass) = mols, so 85 grams x 1 mol / 74 grams = 1.149 mol. For every mol of Ca(OH)2, there is 2 mol of H so x2 = 2.297 mol H. For every mol of H, there is one gram so 2.297 grams. Rounded to three significant figures you get 2.30 grams.