Study Guide for Chapter 9



Review Problems Ch. 10.4 (Empirical & Molecular formulas) & 10.5 Hydrates

Ch 10.4 Percent Composition and Empirical & Molecular Formulas

1. Percentage Composition: percentage by mass of each element in a compound

Can be found from experimental data or from the chemical formula:

a) From experimental data:

% element in cmpd = Mass of element in sample of compound x 100

Mass of sample of compound

b) From the chemical formula:

% by mass = mass of element in 1 mol of compound x 100

molar mass of compound

2. Empirical Formula: Shows smallest whole number ratio of atoms of the elements in a compound

• May/may not be same as molecular formula – if not, mol. formula is a multiple of Emp F.

How to Find Empirical Formula:

1. Find mass (or %) of each element (assume 100.0 gram sample of compound if %)

2. Find moles of each element (divide mass by molar mass to get moles)

3. Divide moles by the smallest # of moles in step #2 to find whole number subscripts

4. If step 3 not whole #, multiply subscripts from step 3 by smallest factor to convert all to whole #

3. Molecular Formula:

• “True Formula” - the actual number of atoms in a compound

• a simple whole number multiple of empirical formula, if not same as empirical formula

• Can be found from empirical formula if molecular mass of molecular formula is given

• Mass of molecular formula = n(mass of empirical formula) & solve for whole # factor n

Chapter 10.4 & 10.5 Test Review Problems: (show all work)

1. What is the percent composition of 80.0 grams of a compound that is found experimentally to contain 23.5 grams of calcium and the remainder oxygen?

2. What is the percent composition of each element of phosphoric acid (H3PO4)?

3. Which has the larger percent by mass of sulfur: H2SO3 or H2S2O8?

4. Determine the empirical formula of a compound that contains 36.84% N and 63.16% O.

5. Cinnamon contains cinnamaldehyde. A molecule of cinnamaldehyde contains 81.79% C, 6.10 % H, and 12.11 % O. Determine its empirical formula.

6. Nicotine has a molecular mass of 162.26 g/mol and an empirical formula of C5H7N. What is its molecular formula?

7. What is the empirical formula of ascorbic acid C6H8O6?

8. What is the empirical formula and name of a hydrate that is 75.5% by mass CaCl2 and 24.5% by mass H2O?

Chapter 10.1 - 10.3 Test Review Practice

9. How many molecules in 0.254 mol of diarsenic trioxide

10. Convert 1.56 x 1023 formula units of sodium hydroxide to moles

11. How many moles of oxygen atoms in 2.50 mol of KMnO4

12. Determine the molar mass of cobalt II chloride

13. Acetaminophen, a common aspirin substitute has the formula C8H9NO2. Determine the number of molecules of acetaminophen in a 500 mg tablet.

14. Air contains several gases. When resting every breath you take contains approximately 0.600 g of air. If argon makes up 0.934% of the air, calculate the number of argon atoms inhaled with each breath.

15. Analysis of skunk spray yields a molecule with 44.77%C, 7.46%H and 47.76%S. what is the chemical formula for this molecule found in the spray from skunks that is partly responsible for the strong odor?

Answer Key:

1. 29.4% Ca, 70.6% O

2. 3.08% H, 31.61% P, 65.31% O

3. H2SO3

4. N2O3

5. C9H8O

6. C10H14N2

7. C3H4O3

8. CaCl2( 2 H2O, calcium chloride dihydrate

9. CuSO4( 5H2O, copper(II) sulfate pentahydrate

10. 1.53X1023 molecules

11. 2.59x10-1mol

12. 10.0 mol

13. 129.83 g/mol

14. 2x1021 molecules

15. 8.44x1019 atoms Ar per breath

16. C5H10S2

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