Question

If 75.8 grams of Ca(OH)2 were dissolved in enough water to make a solution with a total volume of 1.20 liters, what would the molarity of the solution?

Answer 1

The molarity of the solution is calculated by dividing the number of moles of the solute, in moles, by the total volume of the solution, in liters. 

The molar mass of calcium hydroxide, Ca(OH)2 is calculated below.

     m = molar mass Ca + 2(molar mass O) + 2(molar mass H)

Substituting,
     m = 40 g/mol + 2(16 g/mol) + 2(1 g/mol)
      m = 74 g/mol

To get the number of moles Ca(OH)2 in 75.8 grams,
     n = (75.8 grams)(1 / 74 g/mol) = 1.024 moles

The molarity, M, is therefore,
           M = (1.024 moles Ca(OH)2) / (1.2 L)
               M = 0.854 M

Answer: 0.854 M

Answer 2

m = molar mass Ca + 2*(molar mass O) + 2(molar mass H)     = 40 g/mol + 2(16 g/mol) + 2(1 g/mol)     = 74 g/mol
n = (75.8 grams)(74 g/mol)^-1
   = 1.024 moles

 M = (1.024 moles Ca(OH)2) / (1.2 L)
 M = 0.854 M