Chemistry

Week 19

Check homework: molar mass conversions answer key

Quiz- ask me questions if you don’t understand!

Section 3, QUESTION 29

29. Zinc chloride (ZnCl₂) is used in soldering flux, an alloy used to join two metals together. Determine the moles of Cl^- ions in 2.50 mol ZnCl₂.

Answer:

2.50 mol ZnCl₂ x 2 mol Cl^- = 5.00 mol Cl^-

1 mol ZnCl₂

Section 3, question 34

34. Determine the molar mass of each ionic compound.

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1. NaOH
2. CaCl₂
3. KC₂H₃O₂

Answer:

1. 1 mol Na x its molar mass

1 mol O x its molar mass

1 mol H x its molar mass

Then add!

40.00 g/mol

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• 110.98 g/mol
• 98.14 g/mol

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Section 3, question 37

5. The United States chemical industry produces more sulfuric acid (H₂SO₄), in terms of mass, than any other chemical. What is the mass of 3.25 mol of H₂SO₄?

Answer: 319 g

Use molar mass of H₂SO₄ (98.079 g/mol)

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Section 3, question 40

40. Determine the number of moles present in each compound.

1. 22.6 g AgNO₃
2. 6.50 g ZnSO₄
3. 35.0 g HCl

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Answer:

Use the molar masses of each compound- grams on bottom.

1. 0.133 mol
2. 0.0403 mol
3. 0.960 mol

Section 3, question 42

42. Ethanol (C₂H₅OH), a domestically produced field source, is often blended with gasoline. A sample of ethanol has a mass of 45.6 g.

1. How many carbon atoms does the sample contain?
2. How many hydrogen atoms are present?
3. How many oxygen atoms are present?

Answer: Step 1: find molar mass of compound

Step 2: Use molar mass to find moles of compound.

Step 3: Use Avogadro’s number to find number of formula units of compound.

Step 4: Use ratios from formula to find number of atoms of each element

1. 1.19 x 10²⁴ C atoms
2. 3.58 x 10²⁴ H atoms
3. 5.96 x 10²³ O atoms

Percent by mass using experimental data

Percent by mass = mass of element x 100

Mass of compound

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Percent by mass from the chemical formula

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

molar mass of compound

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Section 4, question 54

54. What is the percent composition of phosphoric acid (H₃PO₄)?

Answer: 3.08% H, 31.61% P, 65.31% O

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Step 1: Find the molar mass of H₃PO₄.

Step 2: Mass of element in 1 mol compound/molar mass of compound x 100 for each element.

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Section 4, question 58

58. The circle graph at the right gives the percent composition for a blue solid. What is the empirical formula for this solid? (63.16% O, 36.84% N)

Answer:

Step 1: Convert mass to moles using molar mass, g on bottom. 3.947 mol O, 2.63 mol N

Step 2: Divide each by the smallest amount of moles present.

1.50 mol, 1 mol

Step 3: Multiply each number by the smallest number that gives a ratio of all whole numbers. X 2 for each means 3.0, and 2.0

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N₂O₃

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Section 4, question 62

62. A compound was found to contain 49.98 g of carbon and 10.47 g of hydrogen. The molar mass of the compound is 58.12 g/mol. Determine the molecular formula.

Answer:

Step 1: Convert grams to moles for each element.

Step 2: Divide moles by smallest value present.

Step 3: Multiply by smallest number possible to get whole numbers and establish empirical formula.

Step 4: Use the molar mass of each element to find the molar mass of the compound.

Step 5: Divide the experimentally found molar mass (58.12) by the mass of the empirical formula.

Step 6: Multiply the subscripts by the answer to 5. Answer: C₄H₁₀

Section 4, question 68

68. Analysis of a compound composed of iron and oxygen yields 174.86 g of Fe and 75.14 g of O. What is the empirical formula for this compound?

Answer: Step 1: Convert grams to moles for each element.

Step 2: Divide moles by smallest value present.

Step 3: Multiply by smallest number possible to get whole numbers and establish empirical formula.

Fe₂O₃

Section 4, question 69

69. An oxide of aluminum contains 0.545 g of Al and 0.485 g of O. Find the empirical formula for the oxide.

Answer: Step 1: Convert grams to moles for each element.

Step 2: Divide moles by smallest value present.

Step 3: Multiply by smallest number possible to get whole numbers and establish empirical formula.

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Al₂O₃

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Analyze chewing gum (p. 342)

Are sweetening and flavoring added as a coating or mixed throughout chewing gum?

1. Unwrap 2 pieces of chewing gum. Place each piece on a weighing paper. Measure and record each mass using a balance.
2. Add 150 mL of cold tap water to a 250 mL beaker. PLace one piece of chewing gum in the water, and stir with a stirring rod for 2 minutes.
3. Pat the gum dry using paper towels. Measure and record the mass of the dried gum.
4. Use scissors to cut the second piece of gum into small pieces. Repeat step 3 using fresh water. Keep the pieces from clumping together.
5. Use a piece of window screen to strain the water from the gum. Pat the gum dry using paper towels. Measure and record the mass of the dried gum.

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Analyze chewing gum (342)

1. For the uncut piece of gum, calculate the mass of sweeteners and flavorings that dissolved in the water. The mass of the sweeteners and flavorings is the difference between the original mass of the gum and the mass of the dried gum.
2. For gum cut into small pieces, calculate the mass of dissolved sweeteners and flavorings.
3. For each piece of gum, determine the percent of the original mass from the soluble sweeteners and flavorings.
4. What can you infer from the two percentages? Is the gum sugar-coated or are the sweeteners and flavorings mixed throughout?

Moon Rock Data

More information on lunar oxides

https://www.youtube.com/watch?v=VbK8GR7vWcI

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Stoichiometry mole ratios

https://www.youtube.com/watch?v=S6UQX7ZdkTg

Stoichiometry mass to mass

https://www.youtube.com/watch?v=3-rk3axcoYw

homework

Tuesday: Percent Composition and Empirical Formula Sheet 1 & 2

Wednesday: Percent Composition and Empirical Formula Sheet 3 & 4

Open Note Quiz only on Percent Composition

Thursday: Chapter 11, Section 1, pages 368-372, answer questions 1a, 2a, 5

Friday: Chapter 11, Section 2, pages 373-378, answer questions 11, 13, 15, 17, 18

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