Green Chemistry and the Ten Commandments

Note that in this calculation units cancel above and below the line, starting with units of
g C 2 H 6.
Now that the mass of O 2 reacting has been calculated, it is possible using the
appropriate mole ratios and molar masses to calculate the masses of CO 2 and of H 2 O
produced as follows:

Mass of CO 2 = 18.0 g C 2 H 6 ×

1 mol C 2 H 6

×

4 mol CO 2

×

44.0 g CO 2

30.0 g C 2 H 6 2 mol C 2 H 6 1 mol CO 2

Mass of H 2 O = 18.0 g C 2 H 6 ×

1 mol C 2 H 6

×

6 mol H 2 O

×

18.0 g H 2 O

30.0 g C 2 H 6 2 mol C 2 H 6 1 mol H 2 O

= 52.8 g CO 2

= 32.4 g H 2 O
Are the masses calculated above correct? A good check is to compare the total
mass of reactants, 18.0 g C 2 H 6 + 67.2 g O 2 = 85.2 g of reactants, with the total mass of
products, 52.8 g CO 2 + 32.4 g H 2 O = 85.2 g of products. The fact that the total mass of
reactants is equal to the total mass of products gives confidence that the calculations are
correct.
As one more example consider the reaction of 15.0 g of Al with Cl 2 to give AlCl 3 :

2Al + 3Cl 2 → 2AlCl 3

What mass of Cl 2 reacts and what is the mass of AlCl 3 produced? The atomic mass of
Al is 27.0 and that of Cl is 35.5. Therefore, the molar mass of Cl 2 is 71.0 g/mol and the
molar mass of AlCl 3 is 133.5 g/mole. The mass of Cl 2 reacting is

Mass of Cl 2 = 15.0 g Al ×

1 mol Al

×

3 mol Cl 2

×

71.0 g Cl 2

27.0 g Al 2 mol Al 1 mol Cl 2

Mass of AlCl 3 = 15.0 g Al ×

1 mol Al

×

2 mol AlCl 3

×

133.5 g AlCl 3

27.0 g Al 2 mol Al 1 mol AlCl 3

(^) = 59.2 g Cl
2
= 74.2 g AlCl 3
As a check, 15.0 g Al + 59.2 g Cl 2 reactant gives a total of 74.2 g of reactants equal to
the mass of the AlCl 3 product.
Exercise: Calculate the mass of CH 4 that reacts and the masses of the products
when 25.0 g of Fe 2 O 3 undergo the reaction below. The atomic masses involved
are H 1.0, C 12.0, O 16.0, Fe 55.8.
4Fe 2 O 3 + 3CH 4 → 8Fe + 3CO 2 + 6H 2 O
Answer: 1.88 g CH 4 , 17.5 g Fe, 5.2 g CO 2 , 4.2 g H 2 O
96 Green Chemistry, 2nd ed