If a solution of barium hydroxide contains 9.1x10^25 hydroxide ions, how many grams of barium hydroxide were originally dissolved?

The formula for barium hydroxide is Ba(OH)2. We know this because the charge of the barium ion is +2 and the hydroxide ion is OH-.

The steps to solving this problem are:

1. Calculate the number of moles of Ba(OH)2 that contain 9.1 x 10^23 OH- ions. One mole = 6.02x10^23, and we can see from the formula that there are two OH- ions in each formula unit of Ba(OH)2.

2. Calculate the molar mass of Ba(OH)2.

3. Calculate the mass of the number of moles obtained in step one. 

Moles Ba(OH)2 =

(9.1x10^25 OH- ions)(1 mol Ba(OH)2/2 OH- ions)(6.02x10^23 particles/1mol) = 75.6 moles Ba(OH)2

molar mass = 137.3g + 2(16.0 g/mol) + 2(1.01 g/mol) = 171.3 g/mol

Mass of Ba(OH)2 - (75.6 mol)(171.3 g/1mol) = 1295 g Ba(OH)2