1. Chapter 6 Chemical Reactions and Quantities
Mole Relationships in Chemical Equations

2. Law of Conservation of Mass
The Law of Conservation of Mass indicates that in an
ordinary chemical reaction,
matter cannot be created or destroyed
no change in total mass occurs in a reaction
mass of products is equal to mass of reactants

3. Conservation of Mass Reactants Products
2 moles of Ag + 1 mole of S = mole Ag2S
2 (107.9 g) + 1(32.1 g) = (247.9 g)
= g

4. Reading Equations with Moles
Consider the following equation:
4Fe(s) + 3O2(g) Fe2O3(s)
An equation can be read in “moles” by placing the
words “moles of” between each coefficient and formula.
4 moles of Fe + 3 moles of O moles of Fe2O3

5. Writing Mole–Mole Factors
A mole–mole factor is a ratio of the moles (from the
coefficients) for any two substances in an equation.:
4Fe(s) + 3O2(g) Fe2O3(s)
Fe and O2: moles Fe and 3 moles O2
3 moles O moles Fe
Fe and Fe2O3: moles Fe and 2 moles Fe2O3
2 moles Fe2O3 4 moles Fe
O2 and Fe2O3: 3 moles O and 2 moles Fe2O3
2 moles Fe2O3 3 moles O2

6. Learning Check Consider the following equation: 3H2(g) + N2(g) 2NH3(g)
A. A mole–mole factor for H2 and N2 is
1) 3 moles N ) 1 mole N ) 1 mole N2
1 mole H moles H moles H2
B. A mole–mole factor for NH3 and H2 is
1) 1 mole H ) 2 moles NH ) 3 moles N2
2 moles NH moles H moles NH3

7. Solution Consider the following equation: 3H2(g) + N2(g) 2NH3(g)
A. A mole–mole factor for H2 and N2 is
2) 1 mole N2
3 moles H2
B. A mole–mole factor for NH3 and H2 is
2) 2 moles NH3

8. Guide to Using Mole–Mole Factors

9. Learning Check
How many moles of Fe are needed for the reaction of 12.0 moles of O2?
4Fe(s) + 3O2(g) Fe2O3(s)
1) moles of Fe
2) moles of Fe
3) moles of Fe

10. Solution 3) 16.0 moles of Fe STEP 1 Given: 12 moles of O2
4Fe(s) + 3O2(g) Fe2O3(s)
Need: moles of Fe
STEP 2 Plan: moles of O2 moles of Fe
STEP 3 Write mole–mole factors from coefficients:
4 moles of Fe = 3 moles of O2
4 moles Fe and 3 moles O2
3 moles O moles Fe

11. Solution (continued) STEP 4 Set up problem to cancel moles of O2:
12.0 moles O2 x 4 moles Fe = moles of Fe
3 moles O2