e. Examining the solution reveals that the given volume in milliters is first
converted to deciliters, using the appropriate conversion factor. The volume of solution
expressed in deciliters is then multiplied by the weight per volume concentration to yield
the desired quantity, mass of solute expressed in grams per given volume. Notice that
if the correct factor is used the unwanted units cancel out.
2-28. SOLVING WEIGHT/VOLUME PROBLEMS
The first step in solving any solution problem is to carefully read the problem.
Remember that a unit of concentration consists of two components-- a weight (or
volume) and a volume, thus w/v or v/v. Also notice that in reading the problem, the
wording will suggest that a solution is to be prepared. Words such as "prepare" or
"make" will be used in this type of problem. When making a percent solution, three
things are necessary, that is, mass of solute needed, concentration of the solution to be
prepared, and total volume of solution. Consider and see that all three of these
requirements are met. Mass of solute is usually expressed in grams or milligrams.
Concentration of the solution in g/dL, mg/dL, or (mL/dL). Total volume of solution is
expressed in deciliters. Use the information given in the problem or calculated from
information given in the problem as appropriate factors.
a. Example 1. How much NaCl is needed to make 2.0 liters of a 3.0 g/dL NaCl
Solution. The first step of any problem is to read the problem carefully.
Determine what the problem is asking for.
Grams of solute (NaCl).
Express the volume in deciliters.
2.0 L X -------- = 20 dL
Multiply the volume expressed in deciliters times the percent concentration to
determine the grams of NaCl contained in 2.0 liters of a 3.0 g/dL solution.
20 dL X -------- = 60 g