Question

What is the concentration in molarity of a solution that has 54.21g of calcium hydroxide dissolved in 560mL of water?

Answer 1

We know that Molarity = # mol/L

In the question, we are given grams - so we need to calculate the number of moles that we are dealing with. We can do this by first finding out what the molecular formula of calcium hydroxide is: $Ca(OH)_{2$

This means that we have 1 calcium and 2 hydroxides.

We can find the number of moles by taking the grams given divided by the molar mass of the molecule. We can find the molar mass of the molecule by taking each atomic mass of each atom (found on the periodic table) and adding them together:

Ca = 40.08g
O = 15.999g
H = 1.008g

We have 1 calcium, 2 oxygens, and 2 hydrogens. So let's add the atomic masses together:

40.08 + 2(15.999) + 2(1.008) = 74.094g

Now that we have the molar mass, we can find the number of moles due to already knowing the given amount in the question:

54.21g/74.09g = 0.73 moles of $Ca(OH)_{2}$

Now that we know the number of moles, we can solve for the molarity; The other part of the question gives us 560 mL of water - but this can quickly be converted to liters by moving the decimal to the left 3 places => 0.560 L

Knowing all of this information, we can plug it into the molarity equation:

Molarity = 0.73 mol/0.56 L
Molarity = 1.30

The molarity of the solution is 1.3 M.