b. **Example 2**. How many milligrams of Na2CO3 are needed to prepare 100 mL

of a 10.0 mg/dL sodium standard?

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

Milligrams of Na2CO3.

Calculate the amount of the sodium needed.

1 dL

10.0 mg

100 mL X --------X --------= 10.0 mg

100 mL

1 dL

Calculate the gram molecular weight of each substance.

Na 23.0 X 2 = 46.0

Na 23.0 X 2 = 46.0

C

12.0 X 1 = 12.0

46.0 g/mol

O

16.0 X 3 = + 48.0

106.0 g/mol

Use ratio and proportion to determine the amount of available form needed.

46.0 g/mol Na

10.0 mg Na

------------------------ = --------------

106.0 g/mol Na2CO3

x mg Na2CO3

(106.0 g/mol NaCl) (10.0 mg Na)

x mg Na2CO3 = --------------------------------

46.0 g/mol Na

x mg Na2CO3 = 23.0 mg

The main advantage of percent solutions is the ease in calculations. There are,

however, two distinct disadvantages associated with percent solutions: (1) the

technician does not know the number of molecules or ions that are present in the

solution, and (2) the technician has no idea of the reacting strength of the solution.

MD0837

2-18

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