Solve for the unknown quantity.
(3.0%) (100 mL
V1 = --------------= 10 mL
30.0
10 mL of 30.0 % alcohol must be added to a 100 mL volumetric flask and q.s.
to the mark with water in order to attain a concentration of 3.0 %.
Solution via dimensional analysis:
Calculate the amount of solute in the final (diluted) solution.
1 dL
3.0 mL
100 mL X ------X -------- = 3.0 mL alcohol
100 mL dL
Multiply the amount of solute needed in the final solution times the percent
concentration of the original solution, used appropriately as a factor.
dL
3.0 mL alcohol X -------------- = 0.10 dL
30.0 mL alcohol
Expressing the volume of original solution in milliliters yields the same results
as before.
100 mL
0.10 dL X -------- = 10 mL
dL
NOTE:
A key for this particular type of problem is that in most instances the problem
refers to the preparation of a dilute solution from a concentrated stock.
Another key is in the fact that this type of problem will have either two
concentrations and one volume or two volumes and one concentration.
c. Example 2. How much 0.100 Eq/L Ca(OH)2 can be prepared from 10.0 mL
of 6.00 mol/L Ca(OH)2?
Solution. Read the problem carefully and determine the relationship that will
allow you to solve the problem for the unknown quantity.
MD0837
6-7