1. Compounds and Molecules
CHAPTER 8
Compounds and Molecules
8.4 Formula Masses

2. A scientist working for the National Water Quality Assessment Program
By measuring the amount of different elements in a particular substance, we can - determine its formula
- identify the specific substance
A scientist working for the National Water Quality Assessment Program

3. An important tool

4. Atomic mass is the molar mass for that element

5. Molar mass H2SO4 Calculating molar mass
Subscripts apply only to the element or group they follow
H2SO4
For H: 2 x = g/mole
For S: 1 x = g/mole
For O: 4 x = g/mole
For H2SO4: g/mole

6. Molar mass H2SO4 Calculating molar mass
Subscripts apply only to the element or group they follow
H2SO4
For H: 2 x g/mole = g/mole
For S: 1 x g/mole = g/mole
For O: 4 x g/mole = g/mole

7. Molar mass H2SO4 Calculating molar mass
Subscripts apply only to the element or group they follow
H2SO4
For H: 2 x g/mole = g/mole
For S: 1 x g/mole = g/mole
For O: 4 x g/mole = g/mole
For H2SO4: g/mole

8. Molar mass (NH4)3PO4 Calculating molar mass
If the subscript follows a parenthesis, then multiply everything inside the parentheses by the subscript
(NH4)3PO4
For N: x = g/mole
For H: 12 x = g/mole
For P: 1 x = g/mole
For O: x = g/mole
For (NH4)3PO4: g/mole

9. Molar mass (NH4)3PO4 Calculating molar mass
If the subscript follows a parenthesis, then multiply everything inside the parentheses by the subscript
(NH4)3PO4
For N: x g/mole = g/mole
For H: 12 x g/mole = g/mole
For P: 1 x g/mole = g/mole
For O: x g/mole = g/mole

10. Molar mass (NH4)3PO4 Calculating molar mass
If the subscript follows a parenthesis, then multiply everything inside the parentheses by the subscript
(NH4)3PO4
For N: x g/mole = g/mole
For H: 12 x g/mole = g/mole
For P: 1 x g/mole = g/mole
For O: x g/mole = g/mole
For (NH4)3PO4: g/mole

11. Molar mass
What is the molar mass of aluminum carbonate, Al2(CO3)3?

12. Molar mass What is the molar mass of aluminum carbonate, Al2(CO3)3?
Asked: The molar mass
Given: The name of the formula
Relationships: The name of the formula can be used to determine the ratios of elements in the formula. Then the periodic table can be used to get the molar mass of each of the elements used.

13. Molar mass What is the molar mass of aluminum carbonate, Al2(CO3)3?
Asked: The molar mass
Given: The name of the formula
Relationships: The name of the formula can be used to determine the ratios of elements in the formula. Then the periodic table can be used to get the molar mass of each of the elements used.
Solve: Aluminum carbonate is ionic, so the formula is Al2(CO3)3.

14. NaCl Percent composition
You can identify a compound using the percent mass of each element.
% by atom number
Na: 50% Cl: 50%
NaCl

15. NaCl Percent composition
You can identify a compound using the percent mass of each element.
% by atom number
Na: 50% Cl: 50%
NaCl
% by mass
Na: 39% Cl: 61%

16. NaCl Percent composition
You can identify a compound using the percent mass of each element.
% by atom number
Na: 50% Cl: 50%
NaCl
1 mole of Cl
is heavier than
1 mole of Na
% by mass
Na: 39% Cl: 61%

17. Two types of problems Calculate % composition using a chemical formula
Find the % composition of Ca3PO4.
Calculate % composition with a measured sample
We know the % composition of a white powder. Determine if this powder is common table sugar.

18. Two types of problems Calculate % composition using a chemical formula
Find the % composition of Ca3(PO4)2.
Calculate % composition with a measured sample
We know the % composition of a white powder. Determine if this powder is common table sugar.

19. Percent composition Find the % composition of Ca3(PO4)2.
1. Calculate the molar mass
2. Calculate the percent of each element in a mole of the substance
For Ca: x = g/mole
For P: 2 x = g/mole
For O: x = g/mole
For Ca3PO4: g/mole

20. Percent composition Find the % composition of Ca3(PO4)2.
1. Calculate the molar mass
2. Calculate the percent of each element in a mole of the substance
Ca: g/mole
P: g/mole
O: g/mole
Ca3PO4:
g/mole

21. Two types of problems Calculate % composition using a chemical formula
Find the % composition of Ca3PO4.
Calculate % composition with a measured sample
We know the % composition of a white powder. Determine if this powder is common table sugar.

22. Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose

23. Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
1.05 g
0.16 g
1.29 g
2.50 g C:
H2:
O2:
Total of
unknown:

24. Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
Total of unknown: 2.50 g

25. Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
For sucrose:

26. Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
1. Add up the individual mass measurements to get the total mass
2. Calculate the percent composition of the unknown
3. Compare with the percent composition of sucrose
For sucrose:

27. Percent composition
A white powder can be broken down into 1.05 g of C, 0.16 g of H2, and 1.29 g of O2. Is this powder common table sugar, also known as sucrose (C12H22O11)?
YES!
The percent composition is very close,
so the unknown is probably sugar
% composition of sucrose:
% composition of unknown:
% C: %
% H: %
% O: %

28. Empirical formulas Na6Cl6 NaCl
The empirical formula is the one with the simplest ratio Na Na Cl Cl Cl Na
Na6Cl6
NaCl Na Cl Na Na Cl
6:6 ratio
1:1 ratio Cl Cl Na

29. Empirical formulas Calculating an empirical formula
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Assume you have a sample that is g hydrogen and 4.00 g oxygen.

30. Empirical formulas Calculating an empirical formula
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Assume you have a sample that is g hydrogen and 4.00 g oxygen.

31. smallest number of moles
Empirical formulas
Calculating an empirical formula
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Assume you have a sample that is g hydrogen and 4.00 g oxygen.
smallest number of moles

32. smallest number of moles
Empirical formulas
Calculating an empirical formula
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Assume you have a sample that is g hydrogen and 4.00 g oxygen.
smallest number of moles

33. smallest number of moles
Empirical formulas
Calculating an empirical formula
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Assume you have a sample that is g hydrogen and 4.00 g oxygen.
H2O
smallest number of moles

34. Empirical formulas
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula

35. Empirical formulas
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula

36. smallest number of moles
Empirical formulas
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
smallest number of moles

37. Empirical formulas Cu2O
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
1. Convert grams to moles
2. Simplify the mole ratio
3. Write the formula
Cu2O

38. Empirical formulas Cu2O Copper(I) oxide Cu2+ O2– Cu+ O2–
A jar of powder is labeled copper oxide, but you do not know whether the copper is Cu+ or Cu2+. After taking a small sample you find that it is made from 2.96 g of copper and 0.37 g of oxygen. What is the correct name and formula for the ionic compound in that jar?
Copper(I) oxide
Cu2+
O2–
Cu+
O2–
Cu2O
+4 –2
+2 –2

39. Molecular formulas
The empirical formula can be the same as the molecular formula…
Empirical
formula
Molecular formula
CH2O
CH2O
Formaldehyde

40. Molecular formulas
The empirical formula can be the same as the molecular formula…
Empirical
formula
Molecular formula
CH2O
CH2O
Formaldehyde
…but not always.
Glucose
CH2O
C6H12O6

41. Molecular formulas CH2O C6H12O6
The molecular mass will always be equal to, or a multiple of, the empirical formula mass.
Empirical
formula
Molecular formula
x 6
CH2O
C6H12O6
Glucose
x 6
30.03 g/mole
g/mole

42. Molecular formulas
Given the following empirical formula and molar mass, determine the molecular formula for CH (78.11 g/mole).

43. Molecular formulas CH C?H? ? g/mole 78.11 g/mole
Given the following empirical formula and molar mass, determine the molecular formula for CH (78.11 g/mole).
Empirical
formula
Molecular formula CH
C?H?
? g/mole
78.11 g/mole

44. Molecular formulas CH C?H? 13.018 g/mole 78.11 g/mole
Given the following empirical formula and molar mass, determine the molecular formula for CH (78.11 g/mole).
Empirical
formula
Molecular formula CH
C?H?
g/mole
78.11 g/mole
For CH: = g/mole

45. Molecular formulas CH C?H? 13.018 g/mole 78.11 g/mole
Given the following empirical formula and molar mass, determine the molecular formula for CH (78.11 g/mole).
Empirical
formula
Molecular formula CH
C?H?
x 6
g/mole
78.11 g/mole

46. Molecular formulas CH C6H6 13.018 g/mole 78.11 g/mole
Given the following empirical formula and molar mass, determine the molecular formula for CH (78.11 g/mole).
Empirical
formula
Molecular formula CH
C6H6
x 6
g/mole
78.11 g/mole
The molecular formula is C6H6

47. Two types of problems:
Calculate % composition using a chemical formula
Find the % composition of Ca3(PO4)2.
Calculate % composition with a measured sample
We know the % composition of a white powder. Determine if this powder is common table sugar.

48. The empirical formula can be the same as the molecular formula…
CH2O
CH2O
Formaldehyde
…but not always.
x 6
CH2O
C6H12O6
Glucose
x 6
30 g/mole
180 g/mole