Chemistry Reference
In-Depth Information
solution that you needed to make, and you need to calculate the number of
grams of solute that you need to use. Example 3 represents this type of
problem.
Example 3
How many grams of NaCl would be required to produce a 3.0 M
solution with a volume of 500. cm
3
?
The first thing that I would do, in order to avoid confusion, is change
the volume of the solution into liters. Dividing 500. cm
3
by 1000 cm
3
/liter,
you find that you have 0.500 L of solution. I would make the change right in
the problem, so that I don't forget to do it later. I will also add the given
information and the unknowns. You want to find the mass of NaCl, but to
do that you will first need to find out how many moles of NaCl would be
required to produce a 3.0 molar solution with a volume of 0.5 L.
Given: volume = 0.500 L; molarity = 3.0 M
Find:
A. Number of moles (n) of NaCl
B. Mass of NaCl
Now, to solve this type of problem, you'll need to do the following:
1.
Take the original formula for molarity and isolate the unknown, which
in this case is the number of moles. Use this formula to calculate the
number of moles of NaCl you will need.
Original formula:
Isolating the number of moles of solute by multiplying both sides of the
equation by liters of solution, you get:
moles of solute
liters of solution
Molarity (M) =
Number of moles of solute = molarity × liters of solution
Solving, you get:
Number of moles of solute = 3.0 M × 0.500 L = 1.5 moles of NaCl
2.
Change the number of moles of NaCl to grams by multiplying by the
molar mass of NaCl, which is 58.5 g/mole.
Mass of sample = number of moles × molar mass
Mass of sample = 1.5 moles of NaCl × 58.5 g/mole = 87.75 g
which you round to 88 g of NaCl
Given: volume = 0.500 L; molarity = 3.0 M
Find:
A. Number of moles (n) of NaCl
B. Mass of NaCl
