1. Balancing Chemical Equations
I’m coming back to haunt you!
Balancing Chemical Equations
Section 5.3

2. But why? “We have been practicing balancing.”
Yes, for a reason.
“So we know mass is conserved.”
Yes, but there’s more.
“Uh oh…”

3. Other reasons to balance…
Finding how many moles of reactants will be needed to make a certain number of moles of products
Mole Ratios
Law of Definite Proportions
Calculating relative masses of reactants and products from a chemical equation!
In other words: “putting it all together!”

4. Let’s Balance an Equation!
Mg + O2  MgO
Left: 1 Mg, 2 O
Right: 1 Mg, 1 O
Mg + O2  2MgO
Left: 1 Mg, 2 O
Right: 2 Mg, 2 O
2Mg + O2  2MgO
Left: 2 Mg, 2 O

5. = + 2Mg + O2  2MgO What can we learn from this?
Two Magnesium atoms and one oxygen molecule are required to produce Two MgO molecules.
Two dozen magnesium atoms and one dozen oxygen molecules are required to produce two dozen MgO molecules.
2 moles magnesium atoms and 1 mole oxygen molecules are required to produce 2 moles of MgO molecules.

6. A math problem: 2a + b = 2c 4a + 2b = 4c
What do you get if you multiply both sides of the equation by 2?
4a + 2b = 4c
One more time, using chemical equations:
2Mg + O2  2MgO
Multiply both sides of the chemical equation by 2
4Mg + 2O2  4MgO
4 moles of Mg and 2 moles of O2 will yield 4 moles of MgO

7. Graphically: + =
4Mg + 2O2  4MgO

8. Sample problem:
How many moles of oxygen molecules are required to react with (unlimited) hydrogen to produce only 6 moles of water?
Step 1: Write the equation
H2 + O2  H2O
Step 2: Balance it
2H2 + O2  2H2O
Step 3: Find the mole ratio of the reactants and products
H2 to O2 to H2O = 2:1:2
Step 4: What do I have to multiply the ratio by so that water has a 6 in front of it?
2:1:2 multiplied by 3: 6:3:6
Step 5: Pick off the correct number for oxygen:
3 moles of oxygen molecules are required to produce 6 moles of water

9. One more:
How many moles of oxygen molecules are required to react with magnesium to produce 24 moles of Magnesium Oxide?
2Mg + O2  2MgO
Mole ratio of Mg to O2 to MgO is 2:1:2
How do I find the correct ratio???
2:1:2 times 12 = 24:12:24
12 moles of oxygen molecules are required.

10. One last piece of the puzzle:
What mass of MgO can be produced with only 1 mole of O2 in the reaction we’ve been studying?
Mole ratio of Mg to O2 to MgO is 2:1:2
Only 2 moles MgO can be produced.
Periodic Table to the Rescue:
Mg: g/mol O: g/mol
MgO: g/mol = 40.3 g/mol.
40.3 g/mol * 2 mol MgO = 80.6 g MgO