3.3.1 Finding Empirical and Molecular Formulas

Question 1

Finding Empirical and Molecular Formulas

Answer 1

• Analysis by mass is used to determine the ratio of elements in a sample.
• The empirical formula is the chemical formula of a compound written with the simplest ratio of elements.
• The molecular formula is the exact atomic formula of a compound.

Question 2

percent composition

Answer 2

• Chemists are often called upon to identify unknown substances. Analysis by mass is used to determine the ratio of elements in a sample.
• Analysis by mass gives the percent compositionof each element in a compound.
• In this example, the compound was found to contain 39.9% carbon, 6.70% hydrogen, and 53.4% oxygen.

Question 3

molar mass

Answer 3

• The percent composition can be used to determine the empirical formula of a compound.
• An empirical formula is a chemical formula that indicates only the relative numbers of atomsof each type in a molecule. Subscripts in empirical formulas are always the smallest whole numbers with no common factor grater than 1. Empirical formulas show the relative molar ratio of each element in a compound.
• The empirical formula can be conveniently determined from the percent composition by assuming a 100 gram sample, and converting the percent composition to masses of each element. These masses can be converted to molesusing the molar massof each element. Finally, the moles are converted to the simplest molar ratio by dividing by the smallest number of moles and then multiplying by a small whole number to clean and fractions.
• In this example, the empirical formula is C1H2O1 or just CH2O. However, this is the empirical formula of many different compounds, such as formaldehyde (CH2O), acetic acid (C2H4O2), and glycerine (C3H6O3).

Question 4

mass spectrometry

Answer 4

• The molecular formula is the exact atomic formula of a compound. The subscripts in the molecular formula of a compound are integer multiples of the subscripts in the empirical formula of that compound.
• Mass spectrometry is one way to determine the molar mass (the molecular formula weight) of the compound. The ratio of the molecular formula weight to the empirical formula weight is an integer (n). This integer is the number that the subscripts of the empirical formula must be multiplied by to yield the molecular formula.
• In this example, the empirical formula is CH2O. The molecular formula is a multiple of that formula, CnH2nOn. The integer n is found to be 2 by dividing the molecular formula weight by the empirical formula weight. Therefore, the molecular formula is C2H4O2.
• Even the molecular formula does not uniquely identify the compound. C2H4O2 could be vinegar, but it could also be other compounds. Further tests would have to be performed to uniquely identify the compound.

Question 5

The mineral zircon contains the following elements in the mass proportions indicated:

49. 8% Zr
15. 3% Si
34. 9% O

What is the empirical formula of zircon?

Answer 5

ZrSiO4

Question 6

What is the minimum information necessary in order to determine the molecular formula of an unknown compound?

Answer 6

the empirical formula plus the molecular weight of the compound

Question 7

A 5.00 gram sample of a compound consisting of only C, H and O, contains 2.00 grams carbon and 0.335 grams hydrogen.
What is the empirical formula for the substance?

Answer 7

CH2O

Question 8

What is molecular formula weight?

Answer 8

the molar mass of a molecular formula

Question 9

When the mass percent of each element making up an unknown compound is given, what other information is needed to determine the empirical formula?

Answer 9

the molar mass of each element

Question 10

Which statement accurately describes the empirical formula for formaldehyde, CH2O?

Answer 10

The empirical formula for formaldehyde is also the molecular formula for the compound.

Question 11

Which statement accurately describes the relationship between the molecular formula weight and empirical formula weight of penicillin? (C16H18N204S)

Answer 11

The molecular formula weight of penicillin equals the empirical formula weight.

Question 12

Which of the following statements about the empirical formula and the molecular formula of a compound is not true?

Answer 12

A compound’s empirical formula cannot be the same as its molecular formula

Question 13

What is the relationship between acetic acid (C2H4O2 ) and glyceraldehyde (C3H6O3 )?

Answer 13

Both molecules have the same empirical formula.

Question 14

Naphthalene, the chemical compound in mothballs, consists of 93.7% C atoms and 6.30% H atoms (by mass). What is the empirical formula for naphthalene?

Answer 14

C5H4