Chemistry Reference
In-Depth Information
1. Find the molar mass of the solute.
2. Divide the mass of our sample of solute by its molar mass, to
get the number of moles of solute.
3. Divide the number of moles of solute by the number of liters
of solution, in order to get the molarity.
For the sake of clarity, I will review each of these steps, before I sum-
marize the solution.
1.
Find the molar mass of the solute, CaCl
2
. Remember: The process of
finding the molar mass of a substance is the same as finding the molecu-
lar or formula mass of the substance. We look up the masses listed on
the Periodic Table of Elements for each of the elements involved and
multiply by the appropriate subscripts. The only difference is that you
use the unit symbol “g” for grams, instead of “u” for atomic mass units.
Calculating the Molar Mass of CaCl
2
Ca = 40.1 g
Cl
2
= +
71.0 g
Molar mass of CaCl
2
= 111.1 g
2.
Divide the mass of our sample of solute by its molar mass, to get the
number of moles of solute. The mass of our original sample of CaCl
2
given in our problem is 200.0 g. We need to convert that into moles by
dividing by the molar mass.
mass of sample
molar mass
200.0 g
111.1 g/mole
Number of moles (n) =
=
= 1.800 moles
3.
Divide the number of moles of solute by the number of liters of solu-
tion, in order to get the molarity.
moles of solute
liters of solution
=
1.800 moles
2.5 L/mole
Molarity (M) =
= 0.72 M
Given: liters of solution = 2.5 L; mass of solute = 200.0 g
Find:
A. moles of solute
B. molarity of solution
mass of sample
molar mass
200.0 g
111.1 g/mole
A.
Number of moles (n) =
=
= 1.800 moles
moles of solute
liters of solution
=
1.800 moles
2.5 L/mole
B.
Molarity (M) =
= 0.72 M
As with many of the calculations in chemistry, you should always be pre-
pared to work with a different unknown. For example, let's suppose that you
were given a problem where you knew the molarity and the volume of the
