Stoichiometry Module: Solutions

Preparing Solutions

There are two ways solutions can be prepared: from a pure solute and from another solution.

The first method involves measuring an amount of a pure solute. An example of this would be the sucrose solution described earlier in this module. If you recall, 21.7 grams of pure sucrose was weighed and then water was added until the total volume of the solution was 500 mL. The concentration of this solution was 0.127 M.

A similiar solution could be prepared using the second method. Suppose a 1.0 M sucrose solution was available to you instead of pure, solid sucrose. In using this method, it is important to realize that adding water to a solution does not change the number of moles of solute that is present. Since it increases the total volume of the solution, the concentration will decrease (since concentration is expressed in moles of solute per liters of solution). For example, if you took one liter of the 1.0 M sucrose solution and added water until the total volume was equal to two liters, the resulting solution would still contain one mole of sucrose. However, since the volume of solution has increased to two liters, the concentration would now be 0.50 M. This can be expressed mathematically:

Move mouse over parts of equation, starting at the left.

This example shows that the moles taken from the original solution must equal the moles in the new solution. The moles of a solute contained in any solution can be calculated by multiplying the molarity by the volume (in liters):

Since the moles taken from the original solution have to equal the moles in the new solution,

If the original solution is called the concentrated solution (or conc) and the new solution is called the dilution (or dil), the above expression can be written

If you wanted to prepare 500 mL of a sucrose solution similiar to the solution prepared previously (concentration = 0.127 M), what volume of a 1.0 M sucrose solution would you use?

63.5 mL	127 mL	3.94 L