a. **Example 1**. How many moles are contained in 117.0 grams of NaCl?

Solution. Determine the mass of one gram molecular weight of the

substance.

Na

23.0

Cl

+ 35.5

58.5 g/mol

Now use ratio and proportion to determine the unknown quantity.

58.5 g

117.0 g

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

1 mol

x mol

(58.5 g)(x mol) = (117.0 g)(1 mol)

(117.0 g)(1 mol)

x mol = ------------------

58.5 g

x mol = 2.00 mol There are 2.00 moles contained in 117.0 g of NaCl.

Equivalently:

the number of grams given

moles = --------------------------

GMW

b. **Example 2**. How many moles are contained in 71.0 grams of Na2SO4?

Solution. Calculate the GMW of the compound.

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

Use ratio and proportion to determine the unknown quantity.

142.1 g

71.0 g

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

1 mol

x mol

(142.1 g)(x mol) = (71.0 g)(1 mol)

(71.0 g)(1 mol)

x mol = ----------------

142.1 g

x mol = 0.500 mol. There are 0.500 moles contained in 71.0 grams of

Na2SO4

MD0837

3-4

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