1. Using Chemical Formulas
Chapter 7
Using Chemical Formulas

2. Formula Mass
Formula Mass—the sum of the average atomic masses of all atoms represented in the formula.
Numerically equal to molar mass (different units).
Difference is molar mass is the total mass of 1 mol of the substance (g/mol), and formula mass is the mass of only 1 unit of that formula (amu).

3. Formulas
Subscript numbers tell how many moles of each atom are in 1 mol of the compound or molecule.
Example: Al2S3 shows there are 2 mol of Aluminum and 3 mol of Sulfur in every 1 mol of Al2S3

4. Practice
Determine the number of moles of each atom present in the following formulas:
Mg(OH)2
H2SO4
Ca(NO3)2
Determine the molar mass for each compound

5. REVIEW Determine the number of moles in 2.3 g of NH3
Determine the number of grams in 5 moles of Ca(NO3)2

6. REVIEW MOLE TO MOLE RATIO
The only way to convert from one substance to another is through a mole to mole ratio!!!

7. More Review
Determine the number of moles of silver in g of Silver (II) Phosphate.
Determine the number of grams of carbon in 17.4 g of heptane.

8. BELL RINGER (5 minutes)
How many moles of phosphorus are in 16.2 grams of sodium phosphate?
How many grams of calcium are in grams of calcium nitride?

9. Review: Percent composition
mass of element in sample of compound x = % element in compound
mass of sample of compound
mass of element in 1 mole of compound x = % element in compound
molar mass of compound

10. Percent composition Examples
Determine the percent composition of KClO3
Determine the percent composition of Copper in Copper (I) Sulfide

11. Percent composition of Hydrates
Na2CO3 ● 10H2O - name?
10 water molecules are present for every 1 formula unit of Na2CO3
Find the mass percentage of water in sodium carbonate decahydrate:

12. White boarding Practice
Calculate the percent composition of each element in aluminum sulfate
Calculate the percentage of water in barium hydroxide heptahydrate

13. Percent composition Bellringer
1. Find the percent composition of each element in silver (I) nitrate.
2. Find the mass percentage of water in copper (II) sulfate pentahydrate.
3. Find the percent of iron in iron (III) chloride dehydrate.

14. Determining Chemical Formulas
Empirical formula—the formula showing the smallest whole-number mole ratio of different atoms in a compound.
Molecular formula—represents the actual number of atoms present in a molecule.

15. What’s the difference? Empirical Formula Molecular formula
Name of Molecule
Boiling Point (oC)
CH2O
Formaldehyde
-21
C2H4O2
Acetic Acid
117
C3H6O3
Glyceraldehyde
140

16. Calculation of Empirical Formulas
From percent composition
3 steps—Example: Compound containing 78.1% B and 21.9% H.
Convert all percentages to masses out of 100 g.
Convert masses to moles using molar mass of element.
Divide each number of moles by smallest mole value to get ratios for empirical formula.
If not whole numbers after dividing, multiply all values by whole numbers until all are made whole numbers.

17. Practice
Quantitative analysis shows a compound with 32.38% Na, 22.65% S, and 44.99% O. Find the empirical formula of the compound.
Analysis of a g sample of a compound containing only P and O indicates a P content of g. What is the empirical formula of the compound?

18. Empirical Formula of Hydrates
A gram sample of a certain hydrate of magnesium sulfate, MgSO4 • xH2O, is heated until all the water is driven off. The resulting anhydrous compound weighs grams. What is the formula of the hydrate?

19. Calculation of Molecular Formulas
Empirical formula = smallest ratio of whole numbers, therefore the molecular formula must be thus:
x(empirical formula) = molecular formula
𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒇𝒐𝒓𝒎𝒖𝒍𝒂 𝒎𝒂𝒔𝒔 𝒆𝒎𝒑𝒊𝒓𝒊𝒄𝒂𝒍 𝒇𝒐𝒓𝒎𝒖𝒍𝒂 𝒎𝒂𝒔𝒔 =𝒙
x = the number by which to multiply ratios in empirical formula to get molecular formula

20. Practice
Determine the molecular formula of a compound with an empirical formula of CH and a molecular mass of amu.
A sample with a formula mass of amu consists of 0.44 g of H and 6.92 g O. Find the molecular formula.