Stoichiometry - Chemistry - page 206

the lab. How would you figure out how many grams of CuCl 2 to start with?
First, you would determine the molar mass of CuCl 2 , as shown here:
Copper (Cu) = 63.5 g/mole × 1 mole = 63.5 g

Chlorine (Cl 2 ) = 35.5 g/mole × 2 mole = + 71.0 g

Molar mass of CuCl 2 = 134.5 g

Next, take the formula that we used for Example 1 and algebraically
isolate the unknown for this problem, which is the number of grams.
Starting with:

total mass of the sample
molar mass of substance

# of moles of a substance =

Multiply both sides by “molar mass of substance” to get:
Total mass of the sample = # of moles of a substance × molar mass of substance

You now have everything you need to solve the problem.

Example 3
Convert 0.750 moles of CuCl 2 to mass in grams.

Total mass of the sample = # of moles of a substance × molar mass of substance
Total mass of CuCl 2 = 0.750 mole × 134.5 g/mole = 101 g of CuCl 2

Note that 0.750 mole is less than 1 mole, so it makes sense that the
mass that we need is less than the molar mass of CuCl 2 .

Changing the Number of Moles to the Number of Particles
Let's say your instructor wants you to calculate the number of mol-
ecules found in 3.0 moles of carbon dioxide. If you get confused, remember
to go back to thinking about the “dozen.” If you were asked to figure out
how many eggs there are in 3.0 dozen, you would instantly know enough to
multiply 3.0 × 12, because 1 dozen equals 12 items. The mole, as is the
dozen, is simply a set or a group of items. All you would need to do is
multiply the number of moles times Avogadro's number. Look how similar
these two calculations actually are.

Example 4
How many eggs does 3.0 dozen eggs represent?

12
1 doz

# of eggs = 3.0 doz × = 36 eggs