Download - Basic Stoichiometry
Basic Stoichiometry
Pisgah High SchoolM. Jones
Revision history: 5/16/03, 02/04/12, 04/27/12
With the Gas Laws
The word stoichiometry comes from the Greek words stoicheion which means “element” and metron which means “measure”.
Stoichiometry deals with the amounts of reactants and products in a chemical reaction.
Stoichiometry deals with moles.
Recall that …
1 mole = 22.4 L of any gas at STP
1 mole = the molar mass
molecules1 mole = 6.022 x 1023 atoms or
The word mole is one that represents a very large number.
… “mole” means 6.022 x 1023
Much like “dozen” means 12,
The key to doing stoichiometry is the balanced chemical equation.
2 H2 + O2 2 H2O22
The coefficients give the relative number of atoms or molecules of each reactant or product …as well as the number
of moles.
2 H2 + O2 2 H2O
2 H2 + O2 2 H2O2 moleculesof hydrogen
1 molecule of oxygen
2 moleculesof water
Two molecules of hydrogen combine with one molecule of oxygen to make two molecules of water.
2 H2 + O2 2 H2O
The balanced chemical equation also gives the smallest integer number of moles.
2 moleculesof hydrogen
1 molecule of oxygen
2 moleculesof water
2 H2 + O2 2 H2O
The balanced chemical equation also gives the smallest integer number of moles.
2 molesof hydrogen
1 moleof oxygen
2 molesof water
2 H2 + O2 2 H2O2 molesof hydrogen
1 moleof oxygen
2 molesof water
Two moles of hydrogen combine with
one mole of oxygen to make two moles of water.
Applications of Gay-Lussac’s Law
Simply put, Gay-Lussac’s Law says this:
The volumes of the gases are in the same ratio as the coefficients in the balanced equation.
2 H2 + O2 2 H2O2 moles 1 mole 2 moles2 L 1 L 2 L
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)
1 mol 5 mols 3 mols 4 mols
Suppose 3.5 L of propane gas at STP is burned in oxygen, how many L at STP of oxygen will be required?
17.5 L O2
1 L 5 L 3 L 4 L
HC L 1
O L 5 HC L .53
83
283
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)
1 mol 5 mols 3 mols 4 mols
Suppose 3.5 L of propane gas at STP is burned in oxygen, how many L at STP of oxygen will be required? 17.5 L O2
How many liters of carbon dioxide gas and water vapor at STP would be produced?
10.5 L CO2 and 14.0 L H2O
1 L 5 L 3 L 4 L
CH4(g) + 4 Cl2(g) CCl4(g) + 4 HCl(g)
When methane gas is allowed to react with an excess of chlorine gas, tetrachloromethane and hydrogen chloride gas will be produced.
How many L of methane will react with 0.800 L of chlorine gas at STP?
Cl L 4
CH L 1 Cl L 800.0
2
42 0.200 L Cl2
Stoichiometry problems involving gases
Find the mass of HOCl that can be produced when 2.80 L of chlorine gas at STP reacts with excess hydrogen peroxide.
Cl2(g) + H2O2 (l) 2 HOCl (l)2.80 L ??? g
Convert 2.80 L of Cl2 gas at STP to moles
Cl2(g) + H2O2 (l) 2 HOCl (l)2.80 L ??? g
Cl L 22.4
Cl mol 1 Cl L .802
2
22 0.125 mol Cl2
0.125 mol.125 x 2
0.250 mol
HOCl mol 1
HOCl g 52.5 HOCl mol 250.0 13.1g
HOCl
The reaction between copper and nitric acid
Will copper dissolve in acids?
Cu + 2HCl CuCl2 + 2H2 (g) No Reaction
Most metals react with HCl to produce a metal chloride solution and H2 gas.
Not copper
Consider hydrochloric acid
Will copper dissolve in acids?
Cu + 2HCl CuCl2 + 2H2 (g) No Reaction
Consider hydrochloric acid
Copper is below hydrogen in the activity series. Copper metal will only replace elements that are below it in the activity series.
What about other acids?
The same is true for all acids except nitric acid
Cu + HBr NRCu + HI NRCu + HF NRCu + H2SO4 NRCu + HC2H3O2 NR
Nitric acid is the only acid that will dissolve copper.
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
A beaker contains a penny and some nitric acid is added.
Nitric acid is the only acid that will dissolve copper.
The penny begins to disappear and the solution turns blue-green and a brown gas is given off.
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
Nitric acid is the only acid that will dissolve copper.
The gas produced in the reaction is NO, which is colorless. Reddish brown NO2 forms when NO reacts with the oxygen in the air.
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
Nitric acid is the only acid that will dissolve copper.
The penny is gone and the solution turns a dark blue. The brown NO2 gas escapes from the open beaker.
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
Nitric acid is the only acid that will dissolve copper.
Calculate the volume of NO gas at STP when 20.0 grams of copper dissolves.
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
Nitric acid is the only acid that will dissolve copper.
20.0 g ??? L
Cu g 63.5
Cu mol 1Cu g 0.20 0.315 mol Cu
0.315 mol 0.210 mol x 0.667
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
Nitric acid is the only acid that will dissolve copper.
20.0 g
NO mol 1
NO L 22.4 NO mol 0.210 4.70 L NO
0.315 mol
??? L
0.210 mol
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
x 0.667
Using the Combined Gas Law
What if the conditions are not at STP?
What will be the volume of the NO gas at room temperature
in the mountains?
The temperature is 25 C, and the pressure is 691 torr.
What if the conditions are not at STP?
There are two possible solutions.
1. Use the Combined Gas Law to compute the new volume at the new temperature and pressure.
2
22
1
11
T
VP
T
VP
Nitric acid is the only acid that will dissolve copper.
20.0 g
NO mol 1
NO L 22.4 NO mol 0.210 4.70 L NO
0.315 mol
??? L
0.210 mol
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
x 0.667
What is the volume at a temperature of 25 C, and a pressure of 691 torr?
20.0 g 4.70 L (STP)
Using the Combined Gas Law
2
22
1
11
T
VP
T
VP
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
2
22
1
11
T
VP
T
VPSolve for V2
21
2112 PT
TVPV
torr691K 273
K 298L 70.4torr607
V2 = 5.65 L
V2 is the new volume at 25 C and 691 torr
Using the Ideal Gas Equation
What if the conditions are not at STP?
There are two possible solutions.
2. Use the Ideal Gas Equation to compute the new volume using the new temperature and pressure.
nRTPV
nRTPV Moles of gas
Pressure
Volume in Liters
The gas constant
Temperature in K
The value of R depends on pressure units.
20.0 g
NO mol 1
NO L 22.4 NO mol 0.210 4.70 L NO
0.315 mol
??? L
0.210 mol
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
x 0.667
We know that 0.210 mol of NO gas at STP was produced from 20.0g Cu.
What is the volume at a temperature of 25 C, and a pressure of 691 torr?
20.0 g 0.210 mol
Using the Ideal Gas Eauation
PV = nRT
3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O
Solve PV=nRT for V
P
nRT V
torr 691
K 298 62.4 mol 0.210 V K mol
torrL
Then plug in the new temperature and pressure.
V = 5.65 L
Either method works fine in this case, since you have a gas that you know both the pressure, and the number of moles.
Use the Combined Gas Law when you are looking for a new pressure, volume or temperature for a confined gas. (moles are constant)
Use the Ideal Gas Equation when you are looking for a pressure, volume or temperature when the number of moles is known.
Of course the Ideal Gas Equation can be used whenever you are dealing with pressure, volume, temperature or the number of moles, and any one of the variables could be the unknown.
Practice solving PV=nRT for each variable.
Remember, when using PV = nRT …
Pressure is usually in atmospheres, torr, mm Hg, or kilopascals.
Volume is usually in liters.… and n is in moles.
The value of R, the gas constant, depends on the units of V and P.
Temperature must be in Kelvin…
Simple ideal gas problem:
How many moles are in a 2.00 L container of oxygen gas at a pressure of 0.950 atm and a temperature of 20.0 C?
This could actually be part of a larger problem:
Find the mass of iron(III) oxide formed when excess iron reacts with 2.00 L of oxygen gas at 0.950 atm and a temperature of 20.0 C.
Start with the balanced equation:
4Fe(s) + 3O2(g) 2Fe2O3(s)Excess 0.950 atm, 2.00 L, 20.C
Use the ideal gas law to find the moles of O2.
PV = nRT
K2930821.0
L00.2atm950.0
RT
PVn
molK
Latm
= 0.0790 mol
Start with the balanced equation:
4Fe(s) + 3O2(g) 2Fe2O3(s)Excess 0.950 atm,
2.00 L, 20.C 0.0790 mol 0.0527 mol
8.41 g
x 2/3
32
3232 OFemol1
OFeg6.159OFemol0527.0 = 8.41g Fe2O3
There is a useful variation on the Ideal Gas Equation when dealing with molar mass. Since …
Molar mass
mass n
M
m
Then PV=nRT becomes M
mRT PV
An example of using
M
mRT PV
Find the volume of 0.325 grams of nitrogen gas at a pressure of 1.20 atm and a temperature of 300. K.
M
mRT PV
Use this equation because you are given the mass of a gas for which you can easily calculate the molar mass.
Find the volume of 0.325 grams of nitrogen gas at a pressure of 1.20 atm and a temperature of 300. K.
P
mRT V
M
atm 1.20 0.28
300.K0821.0 g 0.325
mol
gK mol
atm L
V = 0.238 L
Solve for V
Look forward to more stoichiometry problems using
the Ideal Gas Equation.