### StoichiometryMass-Mass Problems#1 - 10

Problem #1: Given the following equation:

2C4H10 + 13O2 ---> 8CO2 + 10H2O

show what the following molar ratios should be.

(a) C4H10 / O2
(b) O2 / CO2
(c) O2 / H2O
(d) C4H10 / CO2
(e) C4H10 / H2O

Solution:

(a) 2 / 13
(b) 13 / 8
(c) 13 / 10
(d) 2 / 8 (or 1 / 4)
(e) 2 / 10 (or 1 / 5)

Problem #2: Given the following equation:

2KClO3 ---> 2KCl + 3O2

How many moles of O2 can be produced by letting 12.00 moles of KClO3 react?

Solution:

The KClO3 to O2 molar ratio is 2:3.

2 mol KClO3 / 3 mol O2 = 12.00 mol KClO3 / x = 18.00 mol.

x = 18.00 mol of O2

Problem #3: Given the following equation:

2K + Cl2 ---> 2KCl

(a) How many grams of KCl is produced from 2.50 g of K and excess Cl2.
(b) From 1.00 g of Cl2 and excess K?

Solution:

(a) (b) Problem #4: Given the following equation:

Na2O + H2O ---> 2NaOH

(a) How many grams of NaOH is produced from 1.20 x 102 grams of Na2O? (b) How many grams of Na2O are required to produce 1.60 x 102 grams of NaOH?

Solution:

(a) (b) Problem #5: Given the following equation:

8Fe + S8 ---> 8FeS

(a) What mass of iron is needed to react with 16.0 grams of sulfur?
(b) How many grams of FeS are produced?

Solution:

(a) (b) Problem #6: Given the following equation:

2NaClO3 ---> 2NaCl + 3O2

(a) 12.00 moles of NaClO3 will produce how many grams of O2?
(b) How many grams of NaCl are produced when 80.0 grams of O2 are produced?

Solution:

(a) (b) Problem #7: Given the following equation:

Cu + 2AgNO3 ---> Cu(NO3)2 + 2Ag

(a) How many moles of Cu are needed to react with 3.50 moles of AgNO3?
(b) If 89.5 grams of Ag were produced, how many grams of Cu reacted?

Solution:

(a) (b) Problem #8: Molten iron and carbon monoxide are produced in a blast furnace by the reaction of iron(III) oxide and coke (pure carbon). If 25.0 kilograms of pure Fe2O3 is used, how many kilograms of iron can be produced? The reaction is:

Fe2O3 + 3C ---> 2Fe + 3CO

Solution: Problem #9: The average human requires 120.0 grams of glucose (C6H12O6) per day. How many grams of CO2 (in the photosynthesis reaction) are required for this amount of glucose? The photosynthetic reaction is:

6CO2 + 6H2O ---> C6H12O6 + 6O2

Solution: Problem #10: Given the reaction:

4NH3(g) + 5O2(g) ---> 4NO(g) + 6H2O(ℓ)

When 1.20 mole of ammonia reacts, the total number of moles of products formed is:

(a) 1.20
(b) 1.50
(c) 1.80
(d) 3.00
(e) 12.0

Solution:

The correct answer is d.

The NH3 / (NO + H2O) molar ratio is 4:10

4 / 10 = 1.20 / x

x = 3.00 mol

Bonus Problem:Potassium superoxide, KO2, is used to produce O2 in space expeditions by using CO2, governed by the following equation:

4KO2 + 2CO2 ---> 2K2CO3 + 3O2

(a) How many grams of O2 are produced from 100. g KO2, given sufficient CO2?
(b) How many grams of CO2 can 100. g KO2 consume?

Solution to (a):

1) Determine moles of KO2:

100. g / 71.096 g/mol = 1.40655 mol

2) From the coefficients of the balanced equation, we determine that the KO2 to O2 molar ratio is 4 : 3.

3) Determine moles of O2 that are produced:

 4 1.40655 mol ––– = –––––––––– 3 x

x = 1.0549125 mol of O2

4) Determine grams of oxygen:

(1.0549125 mol) (31.9988 g/mol) = 33.8 g

Notice that carbon dioxide is also scrubbed out of the space vehicle's atmosphere by the KO2. A nice two'fer.

Solution to (b):

1) 100. g of KO2 is 1.40655 mol.

2) The molar ratio between KO2 and CO2 is 2 : 1.

3) Determine moles of CO2:

 2 1.40655 mol ––– = –––––––––– 1 x

x = 0.703275 mol of CO2 consumed

4) Determine mass of CO2:

(0.703275 mol) (44.009 g/mol) = 30.950 g

To three sig figs, this is 31.0 g (a common mistake would be to write 31 g, making the answer only 2 sig figs)