1. Calculations and the Chemical Equation
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Chapter 5
Calculations and the Chemical Equation
Denniston Topping Caret
4th Edition

2. 5.1 The Mole Concept and Atoms
The Mole and Avogadro’s Number
atomic mass unit (amu) - unit of measure for the mass of atoms.
carbon-12 assigned the mass of exactly 12 amu
1 amu = 1.66 x g
periodic table gives atomic weights in amu.
What is the atomic weight of one atom of Fluorine? 1

3. 5.1 The Mole Concept and Atoms
Answer: amu
What would be the mass of this one atom in grams?
5.1 The Mole Concept and Atoms
Chemists usually work with much larger quantities.
It is more convenient to work with grams than amu.

4. 5.1 The Mole Concept and Atoms
To make the connection we must define the mole.
The mole is abbreviated mol
1 mol of atoms = x 1023 atoms of an element
5.1 The Mole Concept and Atoms
Avegadro’s number
A mole is simply a unit that defines an amount of something
Just as a dozen defines 12
Just as a gross defines 144

5. 5.1 The Mole Concept and Atoms
The average mass of one atom of an element in amu is numerically equivalent to the mass of one mole of an element expressed in grams.
That is, 1 atom F is amu and mole of F is g. Or,
19.00 amu/atom F
19.00 g/mole F 2
5.1 The Mole Concept and Atoms
=19.00 g F/mol F or g/mol F

6. 5.1 The Mole Concept and Atoms
Molar mass - The mass in grams of mole of atoms.
What is the molar mass of carbon?
12.01 g/mol C
This means if you counted out a mole of Carbon atoms (i.e, x 1023 of them) they would have a mass of g.
5.1 The Mole Concept and Atoms

7. 5.1 The Mole Concept and Atoms
Calculating Atoms, Moles, and Mass 2
We now know the following conversion factors:
Conversion Tool
g  ml density
amu  g 1 amu = x g
6.022 x 1023 amu = 1g
moleactual number Avegadro’s No.
g  mole Molar Mass
5.1 The Mole Concept and Atoms

8. 5.1 The Mole Concept and Atoms
Let's Practice Using these Tools
1. What is the mass in grams of 2.5 mol Na.
2. Calculate the number of atoms in 1.7 mol B.
3. Calculate the number of atoms in 5.0 g Al.
4. Calculate the mass of 5,000,000 atoms of Au.
5.1 The Mole Concept and Atoms

9. Figure 5.2

10. 5.2 Compounds 3 The Chemical Formula
Chemical Formula - a combination of symbols of the various elements that make up the compound.
Formula unit - the smallest collection of atoms that provide two important pieces of information
the identity of the atoms and
the relative number of each type of atom

11. 5.2 Compounds Let’s look closely at the following formulas: H2O NaCl
Fe(CN)3
(NH4)3PO4
CuSO4.5H2O
This is an example of a hydrate - compounds containing one or more water molecules as an integral part of their structure.
5.2 Compounds

12. 5.3 The Mole Concept Applied to Compounds4
Formula weight - the sum of the atomic weights of all atoms in the compound, as represented by its formula.
expressed in amu
Molar mass applies to compounds also.
What is the formula weight of H2O?
16.00 amu + 2(1.008 amu) = amu
What is the molar mass of H2O?
18.02 g/mol H2O

13. 5.3 The Mole Concept Applied to Compounds
When calculating the formula weight (or molar mass of an ionic compound, use the smallest unit of the crystal
5.3 The Mole Concept Applied to Compounds
What is the molar mass of (NH4)3PO4?
g/mol (NH4)3PO4

14. 5.4 The Chemical Equation and the Information It Conveys
A Recipe For Chemical Change
Chemical Equation - shorthand notation of a chemical reaction.
Reactants - (starting materials) - the substances that undergo change in the reaction.
Products - substances produced by the reaction.

15. Law of Conservation of Mass - matter cannot be either gained or lost in the process of a chemical reaction.
The total mass of products must equal the total mass of the reactants.
5.4 The Chemical Equation

16. 5.4 The Chemical Equation Features of a Chemical Equation
 - means heat is needed.
Products and reactants must be specified using chemical symbols
5.4 The Chemical Equation
Reactants- written on the left of arrow
Products - written on the right
Physical states are shown in parentheses

17. Coefficient - how many of the substance are in the reaction
The equation must be balanced.
All the atoms of every reactant must also appear in the products.
How many Hg’s on left?
on right?
How many O’s on left?
5.4 The Chemical Equation

18. The Experimental Basis of a Chemical Equation
We know that a chemical equation represents a chemical change. The following is evidence for a reaction:
Release of a gas.
CO2 is released when acid is placed in a solution containing CO32- ions.
H2 is released when Na is placed in water.
Formation of a solid (precipitate.)
A solution containing Ag+ ions is mixed with a solution containing Cl- ions.
5.4 The Chemical Equation

19. Heat is produced or absorbed (temperature changes)
Acid and base are mixed together
The color changes
Light is absorbed or emitted
Changes in the way the substances behave in an electrical or magnetic field
Changes in electrical properties.
5.4 The Chemical Equation

20. 5.5 Balancing Chemical Equations6
Consider the following reaction:
hydrogen reacts with oxygen to produce water
Write the above reaction as a chemical equation.
You probably wrote the following:
H2 + O2  H2O
Don’t forget the diatomic elements.

21. 5.5 Balancing Chemical Equations
H2 + O2  H2O
Is the law of conservation of mass obeyed as written? NO
Balancing chemical equations uses coefficients to ensure that the law of conservation of mass is obeyed.
You may not change subscripts!
WRONG: H2 + O2  H2O2
5.5 Balancing Chemical Equations

22. 5.5 Balancing Chemical Equations
H2 + O2  H2O
Using the above equation, let’s see the steps to balancing:
Step 1. Count the number of moles of atoms of each element on both product and reactant side.
5.5 Balancing Chemical Equations
Reactants Products
2 mol H 2 mol H
2 mol O 1 mol O

23. 5.5 Balancing Chemical Equations
H2 + O2  H2O
Step 2. Determine which elements are not balanced.
Oxygen is not balanced.
Step 3. Balance one element at a time using coefficients.
5.5 Balancing Chemical Equations
H2 + O2  2H2O
This balances oxygen, but is Hydrogen still balanced?

24. 5.5 Balancing Chemical Equations
H2 + O2  2H2O
How will we balance hydrogen?
2H2 + O2  2H2O
5.5 Balancing Chemical Equations
Step 4. Check! Make sure the Law of conservation of mass is obeyed.
Reactants Products
4 mol H 4 mol H
2 mol O 2 mol O

25. 5.5 Balancing Chemical Equations
Let's Practice Balancing
Balance the following equations:
1. C2H2 + O2  CO2 + H2O
2. AgNO3 + FeCl3  Fe(NO3)3 + AgCl
3. C2H6 + O2  CO2 + H2O
4. N2 + H2  NH3
5.5 Balancing Chemical Equations

26. 5.6 Calculations Using the Chemical Equation7
We will learn in this section to calculate quantities of reactants and products in a chemical reaction.
Need a balanced chemical equation for the reaction of interest.
Keep in mind that the coefficients represent the number of moles of each substance in the equation.

27. 5.6 Calculations Using the Chemical Equation
Let’s examine the reaction:
2H2 + O2  2H2O
What do the coefficients tell us?
2 mol H2 reacts with 1 mol O2 to produce 2 mol H2O.
What if 4 moles of H2 reacts with 2 moles of O2?
It yields 4 moles of H2O
5.6 Calculations Using the Chemical Equation

28. 5.6 Calculations Using the Chemical Equation
2H2 + O2  2H2O
The coefficients of the balanced equation are used to convert between moles of substances.
Let’s see how:
How many moles of O2 are needed to react with 4.26 moles of H2?
You may be able to do this in your head, but let’s see how to use the factor label method.
5.6 Calculations Using the Chemical Equation

29. 5.6 Calculations Using the Chemical Equation
2H2 + O2  2H2O 1
2.13 mol O2
5.6 Calculations Using the Chemical Equation 2
Comes from the balanced equation.

30. 5.6 Calculations Using the Chemical Equation
Conversion Tool
g  ml density
amu  g 1 amu = x g
moleactual number Avegadro’s No.
g  mole Molar Mass
mol A mol B Coef. of balanced eq.
5.6 Calculations Using the Chemical Equation

31. 5.6 Calculations Using the Chemical Equation
Plan your route before calculating.

32. 5.6 Calculations Using the Chemical Equation
Let's do some examples.
Na + Cl2  NaCl
1. Balance the equation.
2. Calculate the moles of Cl2 which will react with 5.00 mol Na.
3. Calculate the number of grams of NaCl which will be produced when 5.00 mol Na reacts with an excess of Cl2.
4. Calculate the grams of Na which will react with 5.00 g Cl2.
5.6 Calculations Using the Chemical Equation

33. 5.6 Calculations Using the Chemical Equation
Theoretical and Percent Yield 7
Theoretical yield - the maximum amount of product that can be produced
Pencil and paper yield
Actual yield - the amount produced when the reaction is performed
Laboratory yield
Percent yield:
5.6 Calculations Using the Chemical Equation

34. 5.6 Calculations Using the Chemical Equation
Another example
Given the reaction:
CaH2(s) + 2H2O(l)  Ca(OH)2(aq) + 2H2(g)
Calculate the theoretical yield (in grams) of H2 given that 5.00 g CaH2 reacted with 6.00 g H2O.
Calculate the % yield if 0.20 grams was collected in the laboratory when the above amounts were reacted.
5.6 Calculations Using the Chemical Equation

35. The End
Chapter 5