Question 1

Which statements regarding combustion analysis to determine percent composition are not correct? I. The mass of oxygen in the sample compound can be determined from the mass of carbon dioxide that is produced. II. If a compound contains an element other than carbon and hydrogen, combustion analysis cannot be used to determine its empirical formula. III. Combustion analysis data alone is not sufficient to determine the molecular formula with certainty. IV. If some CO is produced rather than all CO₂, then the empirical formula that is calculated will be too high in carbon. A)I and II are not correct. B)I, II, and III are not correct. C)II, III, and IV are not correct. D)II and III are not correct. E)I, II, and IV are not correct.

Accepted Answer

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Question 2

Caffeine has an elemental analysis of 49.48% carbon, 5.190% hydrogen, 16.47% oxygen, and 28.85% nitrogen. It has a molar mass of 194.19 g/mol. What is the molecular formula of caffeine? A)C₄H₅N₂O B)C₈H₁₀N₄O₂ C)C₁₀H₁₄N₂O₂ D)C₆H₆NO₂ E)C₅H₇NO

Accepted Answer

1) Find the empirical formula:
- Assume 100g sample, then it contains:
49.48g C / 12.01 g/mol = 4.120 mol C
5.190g H / 1.008 g/mol = 5.150 mol H
16.47g O / 16.00 g/mol = 1.029 mol O
28.85g N / 14.01 g/mol = 2.064 mol N
- Divide by smallest value (1.029 mol):
C4.120H5.150N2.064O1.029
- Multiply by 3 to simplify the formula:
C12.36H15.45N6.192O3.087
2) Calculate the empirical formula mass:
12.36 g/mol (C) + 15.45 g/mol (H) + 6.192 g/mol (N) + 3.087 g/mol (O) = 37.08 g/mol
3) Calculate the factor to obtain the molecular formula mass:
194.19 g/mol / 37.08 g/mol = 5.238
4) Multiply the empirical formula by the factor:
C12.36 x 5.238 = 64.7428 ≈ 65
H15.45 x 5.238 = 80.9871 ≈ 81
N6.192 x 5.238 = 32.449 ~ 32
O3.087 x 5.238 = 16.159 ~ 16
- Molecular formula: C65H81N32O16
- Simplify the formula by 2:
Caffeine: C8H10N4O2

Question 3

A hydrocarbon molecule contains carbon and hydrogen atoms in equal numbers. Its molar mass is 130.18 g/mol. What is the molecular formula for the hydrocarbon? A)C₇H₇ B)C₁₂H₁₂ C)C₈H₈ D)C₁₀H₁₀ E)CH

Accepted Answer

The molar mass of the hydrocarbon is 130.18 g/mol. Since the hydrocarbon contains carbon and hydrogen atoms in equal numbers, the empirical formula is CH. The molecular formula is a multiple of the empirical formula. The molar mass of the empirical formula is 13 g/mol. Therefore, the molecular formula is C10H10.

Question 4

Phthalocyanine is a large molecule used in printing inks and dyes for clothing due to its insolubility in most solvents, its chemical stability, and its intense blue color. Elemental analysis showed that it consists of 74.69% C, 3.525% H, and 21.77% N, and has a molar mass of 514.54 g/mol. What is the molecular formula for phthalocyanine? A)C₁₆H₉N₄ B)C₃₀H₈N₁₀ C)C₃₂H₁₈N₈ D)C₃₂H₄N₉ E)C₁₆H₁₈N₄

Accepted Answer

1) Calculate the empirical formula: 
- C: 74.69 g / 12.01 g/mol ≈ 6.22 ≈ 6 
- H: 3.525 g / 1.008 g/mol ≈ 3.5 ≈ 3 
- N: 21.77 g / 14.01 g/mol ≈ 1.55 ≈ 2 
Empirical formula: C6H3N2 
2) Calculate the molecular mass: 
- Empirical formula mass: (6 x 12.01) + (3 x 1.008) + (2 x 14.01) = 226.26 g/mol 
- Actual molecular mass: 514.54 g/mol 
- Factor: 514.54 g/mol / 226.26 g/mol ≈ 2.28 ≈ 2 
3) Multiply the empirical formula by the factor: 
Empirical formula: C6H3N2 
Molecular formula: (C6H3N2) x 2 = C12H6N4 
The only choice that matches the molecular formula is C.

Question 5

One form of asbestos called chrysotile is considered to be a human carcinogen. Mass analysis reveals that it has the empirical formula Mg₃Si₂H₄O₉. If the molar mass is 831 g/mol, which molecular formula is correct? A)MgSiO₄(OH)₂ B)Mg₂Si₄O₈(OH)₄ C)Mg₃Si₂O₅(OH)₄ D)Mg₆Si₄O₁₀(OH)₈ E)Mg₉Si₆O₁₅(OH)₁₂

Accepted Answer

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Question 6

A reaction vessel contains equal masses of iron and oxygen. How much FeO could theoretically be produced?
A)an amount equal to the sum of the masses of iron and oxygen
B)an amount greater than the sum of the masses of iron and oxygen
C)an amount less than the sum of the masses of iron and oxygen
D)There is no way to tell.
E)It depends on the reaction conditions.

Accepted Answer

The theoretical yield of FeO would be equal to the limiting reagent, which in this case is either iron or oxygen, whichever is present in lesser amount. Therefore, only a part of either iron or oxygen would be used, and the remaining amount of the other reactant would remain unreacted. Therefore, the amount of FeO produced would be less than the sum of the masses of iron and oxygen.

Question 7

Phosphorus pentoxide is a common drying agent of organic molecules. Mass analysis reveals that it has the empirical formula P₂O₅. If the molar mass is 283.89 g/mol, which molecular formula is correct? A)P₂O₅ B)P₃O₄ C)P₄O₁₀ D)P₅O₆ E)P₆H₁₅

Accepted Answer

The empirical formula P₂O₅ has a molar mass of approximately 141.94 g/mol. The molecular formula is a multiple of the empirical formula. Since 283.89 g/mol is roughly twice 141.94 g/mol, the molecular formula is P₄O₁₀.

Question 8

Which molecules shown below have the same empirical formula? 
A)II, III, and IV are the same.
B)Only II and III are the same.
C)Only I and III are the same.
D)I, II, and III are the same.
E)I and III are the same, and II and IV are the same.

Accepted Answer

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Question 9

Chemical analysis of an organic compound found the following composition: 40.0% C, 53.5% O, and 6.7% H. If the molar mass is 180.2 g/mol, how many empirical formula units are there in the molecular formula?
A)2
B)4
C)8
D)6
E)7

Accepted Answer

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Question 10

Which molecules shown below have the same molecular formula? 
A)Only I and II are the same.
B)Only I and IV are the same.
C)I, II, and IV are the same.
D)Only II and IV are the same.
E)All are different.

Accepted Answer

The answer of Which molecules shown below have the same...

Question 11

A gas is produced when fruit ripens. That gas can be used in mixtures with nitrogen gas to ripen bananas just before they are put on the grocery shelf. This mixture is sold as banana gas. The mass percentages of carbon and hydrogen in this gas are 85.62% for C and 14.38% for H. The molar mass was found to be twice the empirical formula unit molar mass. What is the molecular formula of this gas? A)CH₂ B)C₂H₂ C)C₂H₄ D)C₂H₆ E)C₄H₄

Accepted Answer

The empirical formula based on the mass percentages is CH2. The molar mass of CH2 is 14 g/mol. Since the molecular formula's molar mass is twice the empirical formula's molar mass, it is 28 g/mol, which corresponds to C2H4.

Question 12

When are an empirical formula and a molecular formula the same?
A)always
B)when the mass of the empirical formula unit is equal to the molecular mass
C)when the mass percentages of all of the elements in the two formulas are the same
D)never
E)when the molecular mass is an integer multiple of formula unit mass

Accepted Answer

The empirical formula is the simplest whole number ratio of atoms in a compound, while the molecular formula gives the exact number of atoms of each element in a molecule. The molecular formula can be a multiple of the empirical formula, but they will be the same if the molecular formula mass is a multiple of the empirical formula unit mass. Therefore, when the mass of the empirical formula unit is equal to the molecular mass (which is an integer multiple of the empirical formula mass), the empirical formula and the molecular formula will be the same.

Question 13

Allicin, a potent antibacterial compound, is formed by the enzyme alliinase when garlic is chopped or damaged. Allicin contains carbon, hydrogen, oxygen, and sulfur atoms. In combustion analysis, a 10.00 mg sample of allicin produced 16.27 mg of CO₂, 5.55 mg of water, and 7.90 mg of SO₂. What is the empirical formula of allicin? A)C₆H₁₀S₂O B)C₃H₅SO C)C₁₂H₂₀S₄O D)C₆H₁₀SO E)The moles of oxygen in the sample cannot be determined, so the empirical formula cannot be determined.

Accepted Answer

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Question 14

You are given a liquid sample that contains either methanol (CH₃OH), ethanol (CH₃CH₂OH), or a mixture of both. You use combustion analysis of a 0.336 g sample and obtain 0.462 g of CO₂. What did you learn about your sample? A)It is methanol. B)It is ethanol. C)It is more methanol than ethanol. D)It is more ethanol than methanol. E)It contains equal amounts of methanol and ethanol.

Accepted Answer

The combustion analysis shows that the sample contains carbon and hydrogen. The ratio of CO₂ to H₂O produced in the combustion is 1:1, which indicates that the sample is an alcohol. The molecular weight of methanol is 32 g/mol, and the molecular weight of ethanol is 46 g/mol. The sample weight is 0.336 g, and the weight of CO₂ produced is 0.462 g. The number of moles of CO₂ produced is 0.462 g / 44 g/mol = 0.0105 mol. The number of moles of carbon in the sample is 0.0105 mol. The number of moles of hydrogen in the sample is 2 x 0.0105 mol = 0.021 mol. The empirical formula of the sample is CH₂O. The molecular weight of CH₂O is 30 g/mol, which is close to the molecular weight of methanol (32 g/mol). Therefore, the sample is most likely methanol.

Question 15

Combustion analysis of an organic compound to determine the percentages of carbon, hydrogen, and oxygen in the compound depends on which of the following assumptions?
A)The compound burns completely to form carbon dioxide and water.
B)All the oxygen in the products comes from the added oxygen gas.
C)The mass of the carbon dioxide produced equals the mass of the carbon in the sample.
D)The moles of water produced equal the moles of hydrogen in the sample.
E)The mass of oxygen in the sample is the difference between the mass of the sample and the sum of the masses of carbon dioxide and water produced.

Accepted Answer

The combustion analysis assumes that the compound burns completely to form carbon dioxide and water, which allows for the determination of the percentages of carbon, hydrogen, and oxygen in the sample based on their masses. Assumptions B, C, D, and E are all true statements about the process of combustion analysis, but they are not fundamental assumptions that the method is based on.

Question 16

Chlorhexidine is an antibacterial drug found in mouthwash and has an elemental analysis of 52.28% carbon, 5.98% hydrogen, 14.03% chlorine, and 27.71% nitrogen. It has a molar mass of 505.45 g/mol. What is the molecular formula of chlorhexidine? A)C₁₁H₁₅ClN₅ B)C₂₂H₃₀Cl₂N₁₀ C)C₁₀H₁₅ClN₆ D)C₂₀H₃₀Cl₂N₁₂ E)C₃₀H₄₅Cl₃N₁₈

Accepted Answer

1) Calculate the empirical formula:
- Assume 100 g of chlorhexidine, which gives the following masses of each element:
  - Carbon: 52.28 g
  - Hydrogen: 5.98 g
  - Chlorine: 14.03 g
  - Nitrogen: 27.71 g
- Convert each mass to moles:
  - Carbon: 52.28 g / 12.01 g/mol = 4.35 mol
  - Hydrogen: 5.98 g / 1.01 g/mol = 5.92 mol
  - Chlorine: 14.03 g / 35.45 g/mol = 0.396 mol
  - Nitrogen: 27.71 g / 14.01 g/mol = 1.98 mol
- Divide each mole value by the smallest mole value (0.396 mol) to get the empirical formula:
  - C11H30ClN2
2) Calculate the molecular formula:
- Divide the given molar mass by the empirical formula mass to get the scaling factor:
  - Scaling factor = 505.45 g/mol / (12.01 g/mol x 11 + 1.01 g/mol x 30 + 35.45 g/mol + 14.01 g/mol x 2) = 5
- Multiply the subscripts in the empirical formula by the scaling factor to get the molecular formula:
  - C55H150Cl5N10

Question 17

Azobenzene is a highly colored organic compound. Many diazo dyes used in coloring of clothing are based on an azobenzene core. The elemental analysis of azobenzene provides 79.10% carbon, 5.53% hydrogen, and the rest is nitrogen. It has a molar mass of 182.23 g/mol. What is the molecular formula of azobenzene? A)C₆H₅N B)C₁₂H₁₀N₂ C)C₅H₅N₂ D)C₁₀H₁₀N₄ E)C₃H₂N

Accepted Answer

The percentages of carbon and hydrogen in the compound suggest the presence of 12 carbon and 12 hydrogen atoms, respectively. The molar mass of 182.23 g/mol suggests that the molecular formula contains multiple units of the empirical formula. To determine the number of nitrogen atoms, we subtract the mass contributed by carbon and hydrogen from the molar mass - this gives us the mass contributed by nitrogen to be 45.64 g/mol. Dividing this mass by the molar mass of nitrogen (14.01 g/mol) gives us approximately 3.26. Therefore, the molecular formula contains 3 units of the empirical formula, C6H5N=NC6H5, which corresponds to choice B.

Question 18

The empirical formula for buckminsterfullerene is C₁, and its molar mass is 720.6 g/mol. What is its molecular formula? A)C₆₀ B)C₇₂ C)C₁₂ D)C₁ E)C₇₂₀

Accepted Answer

The empirical formula C₁ means the simplest ratio of carbon atoms. Given the molar mass of 720.6 g/mol, dividing by the atomic mass of carbon (12 g/mol) gives 60, indicating the molecular formula is C₆₀.

Question 19

Which of the following molecules have identical molecular and empirical formulas? 
A)I only
B)II only
C)III only
D)II, III, and IV only
E)All have identical formulas.

Accepted Answer

The answer of Which of the following molecules have identical...

Question 20

Nicotine is an alkaloid and a component of cigarettes that contribute to the addictive properties of tobacco. Mass analysis reveals that it has the empirical formula C₅H₇N. If the molar mass is 162.23 g/mol, which molecular formula is correct? A)C₅H₇N B)C₁₀H₁₄N₂ C)C₇H₉N₃ D)C₁₅H₂₁N₃ E)C₂₀H₂₈N₄

Accepted Answer

The empirical formula mass of C₅H₇N is approximately 81.12 g/mol. Dividing the molar mass (162.23 g/mol) by the empirical formula mass gives approximately 2, indicating the molecular formula is twice the empirical formula: C₁₀H₁₄N₂.

Quiz 3: Stoichiometry: Mass, Formulas, and Reactions

Caffeine has an elemental analysis of 49.48% carbon, 5.190% hydrogen, 16.47% oxygen, and 28.85% nitrogen. It has a molar mass of 194.19 g/mol. What is the molecular formula of caffeine?

A hydrocarbon molecule contains carbon and hydrogen atoms in equal numbers. Its molar mass is 130.18 g/mol. What is the molecular formula for the hydrocarbon?

One form of asbestos called chrysotile is considered to be a human carcinogen. Mass analysis reveals that it has the empirical formula Mg₃Si₂H₄O₉. If the molar mass is 831 g/mol, which molecular formula is correct?

A reaction vessel contains equal masses of iron and oxygen. How much FeO could theoretically be produced?

Phosphorus pentoxide is a common drying agent of organic molecules. Mass analysis reveals that it has the empirical formula P₂O₅. If the molar mass is 283.89 g/mol, which molecular formula is correct?

Which molecules shown below have the same empirical formula?

Chemical analysis of an organic compound found the following composition: 40.0% C, 53.5% O, and 6.7% H. If the molar mass is 180.2 g/mol, how many empirical formula units are there in the molecular formula?

Which molecules shown below have the same molecular formula?

When are an empirical formula and a molecular formula the same?

You are given a liquid sample that contains either methanol (CH₃OH) , ethanol (CH₃CH₂OH) , or a mixture of both. You use combustion analysis of a 0.336 g sample and obtain 0.462 g of CO₂. What did you learn about your sample?

Combustion analysis of an organic compound to determine the percentages of carbon, hydrogen, and oxygen in the compound depends on which of the following assumptions?

Chlorhexidine is an antibacterial drug found in mouthwash and has an elemental analysis of 52.28% carbon, 5.98% hydrogen, 14.03% chlorine, and 27.71% nitrogen. It has a molar mass of 505.45 g/mol. What is the molecular formula of chlorhexidine?

The empirical formula for buckminsterfullerene is C₁, and its molar mass is 720.6 g/mol. What is its molecular formula?

Which of the following molecules have identical molecular and empirical formulas?

Nicotine is an alkaloid and a component of cigarettes that contribute to the addictive properties of tobacco. Mass analysis reveals that it has the empirical formula C₅H₇N. If the molar mass is 162.23 g/mol, which molecular formula is correct?