c. **Example 2**. What is the concentration of a solution that was prepared by

adding 75 mL of alcohol to 175 mL of water?

Solution. Read the problem carefully and determine the unknown quantity.

Percent concentration.

Solving this type of problem involves expressing the volume of solute per unit

volume.

Determine the total volume of solution.

175 mL water

75 mL alcohol

250 mL total volume

Since the unit volume is deciliters, convert the given volume in milliliters to

deciliters.

1 dL

250.0 mL X -------- = 2.50 dL

100 mL

To conclude solving the problem express the volume of solute per unit

volume and evaluate the numerical data as follows.

75 mL

-------- = 30 mL/dL or equivalently, 30

2.50 dL

This type, weight/weight percent solutions (w/w), of percent solution is usually

expressed as (w/w), where "w" denotes weight (usually grams) in both cases. An

example of a correct designation for this type of solution is as follows: 10 g/100 g (w/w),

which indicates to the technician that there are 10 grams of solute for every 100 grams

total solution. The (w/w) denotes that the solution is a "weight in weight" percent

solution. This type of solution is rarely if ever prepared in the clinical laboratory since it

is easier to measure volumes of liquids rather than weigh the liquid on an analytical

balance. One must be familiar with w/w solutions because the common laboratory

acids and bases are manufactured on a weight-in-weight basis. For more discussion on

this, please refer to Lesson 8, Concentrated Acids and Bases.

MD0837

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