1. Moles and Formula Mass

2. The Mole 1 dozen = 12 1 gross = 144 1 ream = 500 1 mole = 6.022 x 1023
There are exactly 12 grams of carbon-12 in one mole of carbon-12.

3. I didn’t discover it. Its just named after me!
Avogadro’s Number
6.022 x 1023 is called “Avogadro’s Number” in honor of the Italian chemist Amadeo Avogadro ( ).
I didn’t discover it. Its just named after me!
Amadeo Avogadro

4. Calculations with Moles: Converting moles to grams
How many grams of lithium are in 3.50 moles of lithium?
3.50 mol Li
6.94 g Li
= g Li
45.1
1 mol Li

5. Calculations with Moles: Converting grams to moles
How many moles of lithium are in 18.2 grams of lithium?
18.2 g Li
1 mol Li
= mol Li
2.62
6.94 g Li

6. Calculations with Moles: Using Avogadro’s Number
How many atoms of lithium are in 3.50 moles of lithium?
3.50 mol
6.02 x 1023 atoms
= atoms
2.07 x 1024
1 mol

7. Calculations with Moles: Using Avogadro’s Number
How many atoms of lithium are in 18.2 g of
lithium?
18.2 g Li
1 mol Li
6.022 x 1023 atoms Li
6.94 g Li
1 mol Li
(18.2)(6.022 x 1023)/6.94
= atoms Li
1.58 x 1024

8. Calculating Formula Mass
Calculate the formula mass of magnesium carbonate, MgCO3.
24.31 g g + 3(16.00 g) =
84.32 g

9. Calculating Percentage Composition
Calculate the percentage composition of magnesium carbonate, MgCO3.
From previous slide:
24.31 g g + 3(16.00 g) = g
100.00

10. Formulas
Empirical formula: the lowest whole number ratio of atoms in a compound.
Molecular formula: the true number of atoms of each element in the formula of a compound.
molecular formula = (empirical formula)n [n = integer]
molecular formula = C6H6 = (CH)6
empirical formula = CH

11. Formulas (continued)
Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio).
Examples:
NaCl
MgCl2
Al2(SO4)3
K2CO3

12. Formulas (continued)
Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio).
Molecular:
H2O
C6H12O6
C12H22O11
Empirical:
H2O
CH2O
C12H22O11

13. Empirical Formula Determination
Base calculation on 100 grams of compound.
Determine moles of each element in 100 grams of compound.
Divide each value of moles by the smallest of the values.
Multiply each number by an integer to obtain all whole numbers.

14. Empirical Formula Determination
Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?

15. Empirical Formula Determination (part 2)
Divide each value of moles by the smallest of the values.
Carbon:
Hydrogen:
Oxygen:

16. Empirical Formula Determination (part 3)
Multiply each number by an integer to obtain all whole numbers.
Carbon: 1.50
Hydrogen: 2.50
Oxygen: 1.00
x 2
x 2
x 2 3 5 2
Empirical formula:
C3H5O2

17. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
1. Find the formula mass of C3H5O2
3(12.01 g) + 5(1.01) + 2(16.00) = g

18. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
2. Divide the molecular mass by the mass given by the emipirical formula.
3(12.01 g) + 5(1.01) + 2(16.00) = g

19. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
3. Multiply the empirical formula by this number to get the molecular formula.
3(12.01 g) + 5(1.01) + 2(16.00) = g
(C3H5O2) x 2 =
C6H10O4

20. Conservation of Mass Law
In a closed system, the total mass of the
system must remain constant. The total mass of
the reactants must be equal to the total mass
of the products.
LAB CONNECTION: This law allowed us to calculate the amount of hydrate in the mixture because the amount of water lost would be uncertain if the starting mass was not the same as the ending mass.

21. More Conservation Laws
This law implies that matter cannot be created or destroyed. [Law of conservation of matter]
This is also connected to the Law of Conservation of Atoms, which states that the number of atoms of each type of element on both sides of the equations must be the same.

22. What happens when a given equation does not conserve mass?
# atoms for reactants ≠ # atoms of products
Mass is NOT conserved
Must balance the equation by adding coefficients to the elements/compounds in the reaction.

23. How do we balance equations?
For example: Think about a grilled cheese sandwich… 2 6

24. Chemical Equations Chemical change involves a reorganization of
the atoms in one or more substances.
C2H5OH + 3O2 ® 2CO H2O
reactants
products
When the equation is balanced it has quantitative significance:
1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water

25. How do we balance equations?
Example
Steps
__ Zn +  _ HCl →  __ H2 + __ ZnCl2
1. Draw a T chart under the equation. Label the left side reactants and the right side products. Write the symbol and number of atoms for each element present in the reaction on its own line on both sides.
No, mass is not conserved as the equation is written.
2. Does each element start and end with the same number of atoms?
 Yes → Mass is conserved and the equation is balanced
 No → Mass is not conserved and you need to balance the equation. (move on to step 3)

26. Example
Steps
__ Zn +  2 HCl →  __ H2 + __ ZnCl2
3. If mass is not conserved, you will balance the equation by adding coefficients before molecules. *Do not change any subscripts.*
Start with an element that only appears once on each side. On the side that has fewer atoms of that element, add a coefficient before the molecule containing that element, so that the number of atoms of that element on both sides are equal.
In the T chart, multiply the coefficient by the subscript on each element in the molecule to get the new numbers of atoms.

27. Example
Steps
Final answer:
Zn + 2 HCl → H2 + ZnCl2
Mass is now conserved so the equation is balanced properly.
4. Again, does each element start and end with the same number of atoms?
Yes → Mass is conserved and the equation is balanced
 No → Mass is not conserved and you need to change another coefficient. Repeat step 3

28. Calculating Masses of Reactants and Products
Balance the equation.
Convert mass or volume to moles, if necessary.
Set up mole ratios.
Use mole ratios to calculate moles of desired substituent.
Convert moles to mass or volume, if necessary.

29. Working a Stoichiometry Problem
6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.
1. Identify reactants and products and write the balanced equation. 4 Al + 3 O2 2
Al2O3
a. Every reaction needs a yield sign!
b. What are the reactants?
c. What are the products?
d. What are the balanced coefficients?

30. Working a Stoichiometry Problem
6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?
4 Al O2  2Al2O3
6.50 x 2 x ÷ ÷ 4 =
12.3 g Al2O3
6.50 g Al
1 mol Al
2 mol Al2O3
g Al2O3 =
? g Al2O3
26.98 g Al
4 mol Al
1 mol Al2O3

31. Limiting Reactant
The limiting reactant is the reactant
that is consumed first, limiting the amounts of products formed.