Name: ________KEY_______________
Class: ________
Sample Problem (mole-mass)
Potassium chlorate is sometimes decomposed in the laboratory to generate oxygen. What
mass of KClO3 do you need to produce 0.50 moles of O2?
__2__KClO3 (s) → __2__KCl(s) + __3__O2(g).
What do you know? (What are the facts?)
Amount of O2 in moles: ______0.5 mol O2________
Where do you want to go? (What is the unknown?)
Mass of potassium chlorate: ____mass of KClO3____
How do you get there? (conversion factor set-up)
What steps are needed to calculate the mass of KClO3 needed to produce 0.50 moles O2?
0.5 mol O2 x 2 mol KClO3 x
1
3 mol O2
122 g KClO3
1 mol KClO3
=
40.7 g KClO3
Does it make sense?
Are the units correct? ________yes___________
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Practice Problems (Mole-Mass or Mass-Mole)
1. Phosphorus burns in air to produce a phosphorus oxide in the following reaction:
__4__P (s) + __5_O2 → ____P4O10 (s).
a. What mass of phosphorus will be needed to produce 3.25 moles of P4O10?
3.25 mol P4O10 x _4 mol P___ x _31 g P_ = 403 g P
1
1 mol P4O10
1 mol P
b. If 0.489 moles of phosphorus burn, what mass of oxygen is used? What mass of
P4O10 is produced?
(1)
0.489 mol P X 5 mol O2 x 32 g O2 = 19.56 g O2
1
4 mol P
1 mol O2
(2) 0.489 mol P x 1 mol P4O10 x 284 g P4O10 = 34.72 g P4O10
1
4 mol P
1 mol P4O10
2. Hydrogen peroxide breaks down, releasing oxygen, in the following reaction:
__2__H2O2(aq) → __2__H2O (l) + ____O2(g)
a. What mass of oxygen is produced when 1.840 moles of H2O2 decompose?
1.84 mol O2 x _1 mol O2_ x _32 g O2 = 29.44 g O2
1
2 mol H2O2
1 mol O2
b. What mass of water is produced when 5.0 moles O2 are produced by this reaction?
5 mol O2 x 2 mol H2O x 18 g H2O = 180 g H2O
1
2 mol H2O2
1 mol H2O
3. Sodium carbonate reacts with nitric acid according to the following equation:
___Na2CO3(s) + _2_HNO3(aq) → _2_NaNO3(aq) +___ CO2 (g) +___ H2O(l)
a. How many moles of Na2CO3 are required to produce 100.0 g of NaNO3?
100 g NaNO3 x 1 mol NaNO3 x 1 mol Na2CO3 = 0.588 mol Na2CO3
1
85 g NaNO3
2 mol NaNO3
b. If 7.50 g of Na2CO3 reacts, how many moles of CO2 are produced?
7.5 g Na2CO3 x 1 mol Na2CO3 x 1 mol CO2__ = 0.071 mol CO2
1
106 g Na2CO3
1 mol Na2CO3
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Sample Problem (mass-mass)
Methane burns in air by the following reaction:
___CH4 (g) + _2_O2→ ___CO2(g) + _2_H2O (g)
What do you know? (What are the facts?)
The mass of methane in grams__500 g CH4__________ (Molar mass = ___16 g____)
Where are you going? (What is the unknown?)
The mass of water produced _______unknown_______ (Molar Mass = ___18 g ____)
How do you get there? (conversion factor set-up)
What steps are needed to calculate the mass of H2O produced from the burning of 500g
of CH4?
500 g CH4 x 1 mol CH4 x 2 mol H2O x 18 g H2O = 1125 g H2O
1
16 g CH4
1 mol CH4
1 mol H2O
Does it make sense? yes
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Practice Problems (Mixed Stoichiometry)
To tell which equation process to use, look to the units on your fact and unknown. If your
fact is in grams (mass) and the unknown is to be moles, then it is a mass to mole problem.
If your fact is in mole, and the unknown is to be mole, then it is a mole to mole problem.
Again, your units will let you know which process to use.
1. Ammonium hydrogen phosphate, (NH4)2HPO4, a common fertilizer, is made from reacting
phosphoric acid, H3PO4, with ammonia, NH3.
____H3PO4 +___2__ NH3→_____ (NH4)2HPO4
a. If 10.0 g of ammonia react, how many moles of fertilizer will be produced?
(mass-mol)
10 g NH3 x 1 mol NH3 x 1 mol (NH4)2HPO4 = 0.29 mol (NH4)2HPO4
1
17 g NH3
2 mol NH3
b. What mass of ammonia will react with 2800 kg of H3PO4? (hint: convert kg to g)
(mass-mass) 2800 kg H3PO4 x 1000 g x 1 mol H3PO4 x 2 mol NH3 x 17 g NH3 = 9.71x105 g NH3
1
1 kg
98 g H 3PO4
1 mol H3PO4
1 mol NH3
2. The following reaction shows the synthesis of zinc citrate, and ingredient in toothpaste,
from zinc carbonate and citric acid.
_3_ZnCO3 + _2_C6H8O7(aq) →___Zn3(C6H5O7)2(aq)+ _3_H2O(l)+ _3_CO2(g)
a. How many moles of ZnCO3 and C6H8O7 are required to produce 30.0 mol of
Zn3(C6H5O7)2? (mol-mol)
(1) 30 mol Zn3(C6H5O7)2
1
(2) 30 mol Zn3(C6H5O7)2
1
x
x
3 mol ZnCO3____
1 mol Zn3(C6H5O7)2
= 90 mol ZnCO3
_2 mol C6H8O7____
1 mol Zn3(C6H5O7)2
= 60 mol C6H8O7
(mol-mass)b.
What quantities, in kilograms, of H2O and CO2 are produced by the reaction of
500 mol of citric acid? (hint: answer calculates to grams, then convert to kg.)
(1) 500 mol C6H8O7 x 3 mol H2O
1
2 mol C6H8O7
x 18 g H2O x 1 kg
= 13.5 kg H2O
1 mol H2O
1000 g
(2) 500 mol C6H8O7 x 3 mol CO2___ x
1
2 mol C6H8O7
44 g CO2
1 mol CO2
x
1 kg
1000 g
=
33 kg CO2
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Source: Team 3, ICLCS Project, UIUC, 2007-08; IL State Stds: 11.A.4d; 11.5d; 12.C.5a
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