Keep in mind that moles are not an expression of concentration. The unit

mole is an expression of mass indicating the number of gram molecular

weights. Take caution not to confuse mole with molarity (mol/L). When

reporting moles, write the word moles, DO NOT abbreviate by using an M!!

Molarity (mol/L) is defined as the number of moles of a substance per liter of

solution.

a. **Example 1**. How much Na 4 is needed to make 300 mL of a 2.00 mol/L

2SO

Na2SO4 solution?

Solution. Notice in this problem that the wording suggests that a solution is to

be prepared. If you examine the problem, you should find that the type of solution to be

prepared is a molar solution. Read the problem carefully and determine the unknown

quantity.

Grams of Na2SO4.

Regardless of the type, most molarity problems require the gram molecular

weight, so calculating the gram molecular weight is a good starting point.

Na2SO4

Na

23.0 X 2 = 46.0

S

32.1 X 1 = 32.1

O

16.0 X 4 = + 64.0

142.1 g/mol

Express the desired volume in liters.

1L

300 mL X ---------- = 0.300 L

1000 mL

Multiply the volume expressed in liters times the molar concentration times

the gram molecular weight to determine the amount of salt, in grams, needed to prepare

the solution.

2.00 mol

142.1 g

0.300 L X --------X ----------= 85.3 g

L

mol

MD0837

3-5

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