You dissolve 5 g of calcium hydroxide in 2 liters of water. What is the molarity of the hydroxide ions in this solution? What is the molarity of the calcium ions in this solution?
How many moles Ca(OH)2 did you dissolve? That is 5.0g/molar mass Ca(OH)2.
M = moles/L soln. L = 2.
There is 1 Ca ion in a molecule of Ca(OH)2 and 2 OH ion in a molecule of Ca(OH)2 so the M of the Ca^2+ is the same as the M of the Ca(OH)2 but M OH^- is twice that of Ca(OH)2.