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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