Empirical Formula

The empirical formula represents the simplest formula for a compound. 

Examining Empirical Formulas

Concept: Empirical Formula vs. Molecular Formula

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Video Transcript

We're going to say that the empirical formula is also known as the simplest formula. We're going to say it represents the simplest whole number ratio of moles of each element in a compound. The empirical formula can sometimes be different than the molecular formula. The molecular formula is also known as the actual formula. We're going to say it represents the actual or true whole number ratio of each element in the compound. Let's just see what this means.
Beside this, we have a box of C6H12O6. This is the formula for glucose. A simple monosaccharide sugar that our body uses for energy. We're going to say that's the actual formula of glucose. That represents the molecular formula of glucose.
Now, though, we might be asked what is the empirical formula for glucose. Just think of this in the same way you would with algebra. If your professor says, I give you two-fourths and they want the actual answer, the actual answer would not be two-fourths. We can reduce this even further. We can reduce it to one-half. The two-fourths would be, in a sense, the molecular formula. The one-half would be the most reduced form of our answer. That would represent our empirical formula.
I tell you the molecular formula for glucose is C6H12O6. We're going to say that all those numbers are divisible by six so that gives us a new formula, CH2O. That CH2O would represent the empirical formula of glucose. It's the lowest, most reduced form of each of those elements, still keeping them in whole number form.

Example: What is the empirical formula of dimethylhydrazine, C2HN2, a colorless liquid used as a rocket fuel? 

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The empirical formula represents the lowest ratio of moles of each element within the compound. 

Example: Elemental analysis of a sample of an ionic compound showed 2.82 g of Na, 4.35 g of Cl, and 7.83 g of O. What is the empirical formula and name of the compound?

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Whenever you are given the mass or percentages of elements you can assume it is out of 100 grams. 

Example: After a workout session, lactic acid (M = 90.08 g/mol) forms in muscle tissue and is responsible for muscle soreness. Elemental analysis shows that this compound contains 40% C, 6.7% H and 53.3% O. Determine the molecular formula. 

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Empirical Formula Additional Practice Problems

What is the empirical formula of molecule A below?

a. C4H10O4

b. C2H5O2

c. C4H10O2

d. C2H4O

e. C2H5O

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A compound is composed of 63.81 % C, 6.42 % H, and 29.77 % N by mass. You may round element molar mass to 2 decimal places.

a) The Empirical formula of this compound is ___.

 

 

 

b) If the molar mass of the compound in a) is 188.2 g/mol, the molecular formula of this compound is ___.

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A strip of pure copper metal weighing 3.178 g is heated in the presence of oxygen gas. In the process, all of the copper reacts with oxygen to form 3.978 g of a black copper oxide. What is the empirical formula for this copper oxide?

Given that the molar mass of the compound is about 80 g, name this compound.

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Determine the empirical formula for a compound that contains C, H, and O. It contains 52.14% C and 34.73% O by mass.

 

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Glucuronolactone is found in many plant gums and is added to some energy drinks.  A sample of this compound is analyzed and it is found to consist of the following mass percentages:  40.91 percent carbon, 4.58 percent hydrogen, and 54.51 percent oxygen.    

What is the empirical formula of this compound?

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If the molar mass of hydrazine is 32 g/mol and its empirical formula is NH2, the molecular formula is

A. NH3

B. N2H4

C. NH2

D. N4H8

E. N2H2

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Dichlorobenzene contains 49.02 mass % C, 2.74 mass % H, and the remainder is Cl. Determine its empirical formula.

A. C3H3Cl2

B. CHCl

C. C2H2Cl2

D. C3H2Cl

E. C3H3Cl2

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A molecule is found to contain 47.35% by mass C, 10.60% by mass H, and 42.05% by mass O. What is the empirical formula for this molecule?

a. C2H6O

b. C3H4O

c. C3H8O2

d. C4H6O2

e. C4H8O3

 

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The molar mass of caffeine is 194.2 g/mol. Its empirical formula is C4H5N2O.
What is its chemical formula?

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Peroxyacylnitrate (PAN) is one of the components of smog. It is a compound of C, H, N, and O. Determine the percent composition of oxygen and the empirical formula from the following percent composition by mass: 19.8 percent C, 2.50 percent H, 11.6 percent N. What is its molecular formula given that its molar mass is about 120 g?

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Determine the molecular formula of a compound that has molar mass of 183.2 g/mol and an empirical formula of C2H5O2.

a. C2H5O2

b. C6H15O6

c. C3H7O3

d. C4H10O4

e. C8H20O8

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The combustion of 4.16 grams of a compound which contains only C,H,O and F yields 7.7 g CO2 and 2.52 g H2O. Another sample of the compound with a mass of 3.63 g is found to contain 0.58 g F. What is the empirical formula of the compound? 

A. C3H8O2F

B. C4H10OF

C. C4H8OF

D. C5H8O2F

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Gahnite is a naturally occurring mineral containing aluminum (Al), oxygen (O), and zinc (Zn). A sample of the pure mineral was analyzed, and the following results obtained (percentages are by mass):

% Al = 29.54 %                  % O = 34.75 %                   % Zn = 35.71 %

Based on this information find the empirical formula for gahnite.

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How many of the following formulas are empirical formulas?

Hg2(NO3)2      H2SO4             P4O10              C2­H4O          

A) 0                      

B) 1                

C) 2                

D) 3                

E) 4       

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Determine the empirical formula for a compound that is 36.86% N and 63.14% O by mass.

A) NO2

B) N2O3

C) N2O

D) NO

E) NO3

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Determine the empirical formula for a compound that is found to contain 10.15 mg P and 34.85 mg Cl.

A) P2Cl3

B) P3Cl

C) PCl3

D) PCl

E) PCl2

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2.404 g of a compound were burned in oxygen and found to produce only 3.520 g CO(g), 2.564 g SO2 (g) and 1.440 g H2O (l). Determine the empirical formula of the compound.

A) C2SH4

B) C2SH2

C) CSH

D) C4SH2

E) C2S2H4

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Combustion analysis of 2.400 g of an unknown compound containing carbon, hydrogen, and oxygen produced 4.171 g of CO2 and 2.268 g of H2O. What is the empirical formula of the compound?

A) C2H5O2

B) C2H5O

C) C2H10O3

D) C3H8O2

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Combustion analysis of an unknown compound containing only carbon and hydrogen produced 4.554 g of CO2 and 2.322 g of H2O. What is the empirical formula of the compound?

A) C4H10

B) CH2

C) C2H5

D) C5H2

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A compound responsible for the odor of garlic has a molecular weight of 146 g/mol. A 0.650 g sample of the compound contains 0.321 g of carbon, 0.044 g of hydrogen, and 0.285 g of sulfur. What is the molecular formula of the compound?

A) C6H10S2

B) C3H15S3

C) CH5S

D) C3H5S

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What is the empirical formula of a substance that contains 2.64 g of C, 0.444 g of H, and 3.52 g of O?

A) CH2O

B) C2H4O3

C) C2H4O2

D) C3H4O4

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What is the empirical formula for perfluoropropane if the compound contains 81% fluorine and 19% carbon by mass?

A) C19F81

B) C2F8

C) C3F8

D) CF3

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What is the empirical formula for ethyl fluoride if the compound contains 49.97% carbon, 10.51% hydrogen, and 39.52% fluorine by mass?

A) C4H10F4

B) C2H5F

C) C25F2

D) C4H10F2

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What is the empirical formula for C 4H10O2?

A) C2H5

B) CHO

C) CHO2

D) CH2O

E) C2H4O

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An organic compound contains C, H, and O. The percentages of H and C are 6.73% H, and 39.99% C by mass. The molar mass is 60.06 amu. What is the molecular formula of the compound?

A) CH2O

B) C4H8O4

C) C2H3O2

D) C2H4O2

E) C3H6O3

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A 4.05 g sample of a compound containing only C, H, and O was burned completely. The only combustion products were 10.942 g CO2 and 4.476 g H2O. What is the empirical formula of the compound?

A) C7H7O

B) C7H14O7

C) CH2O

D) C7H14O

E) C6H12O

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A sample contains 43.7% phosphorus and 56.3% oxygen. The molar mass is 284 g. What is the molecular formula of the compound?

A) P4O5

B) P4O10

C) P5O8

D) PO3

E) P2O5

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Combustion analysis of an unknown compound containing only carbon and hydrogen produced 0.2845 g of CO2 and 0.1451 g of H 2O. What is the empirical formula of the compound?

A) C4H5

B) C4H10

C) C5H2

D) CH2

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Which one of the following is not an empirical formula?

A) CH2O

B) CHO

C) C2H4O

D) C2H4O2

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Equilin is an estrogen that is commonly used in hormone replacement therapy. A sample of equilin is sent for elemental analysis and is determined to contain 80.546% carbon, 7.510% hydrogen, and 11.943% oxygen. If the molar mass of equilin is 268.34 g/mol, what is the molecular formula for equilin?

a. C20H20O2

b. C18H20O2

c. C9H20O

d. C27H30O3

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Which pair of the following compounds does  NOT have the same empirical formula?

A) C2H5COOCH3, CH3CHO

B) C2H2, C6H6

C) C2H4, C3H6

D) C2H4O2, C6H12O6

E) CO, CO2

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Butyric acid has an unpleasant odor and is found in milk, cheese, butter. It causes the main distinctive smell in human vomit. A sample of butyric acid is sent for elemental analysis and is determined to contain 54.52% carbon, 9.151% hydrogen and 36.32% oxygen.

What is the empirical formula for butyric acid?

a. C5H9O3

b. C4H9O2

c C9H10O

d. C2H4O

Considering your response to the previous question, and knowing the molar mass of butyric acid is 88.11 g/mol, what is the molecular formula for butyric acid?

a. C10H18O6

b. C4H8O2

c. C3H4O3

d. C5H12O

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What is the empirical formula for the substance with the following analysis?

Na          34.9%

B            16.4%

O            48.6%

A. Na3BO3

B. NaBO2

C. Na4BO4

D. Na3B2O2

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Give the molecular formula of the compound pictured below.

A. CH

B. C2H2

C. C6H6

D. C2H3

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Which of the following statements is  false concerning the formula of a compound?

a) The molecular formula and empirical formula can be identical.

b) The molecular formula is the true ratio of atoms in a compound.

c) The number of atoms in a molecular formula is always greater than the number of atoms in an empirical formula.

d) The empirical formula is the simplest whole numbered ratio of atoms in a compound.

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Part A:

A chemist attempted to synthesize a particular compound that showed promise as a new antibiotic. If she succeeded in making and purifying the compound, then it contains only carbon, hydrogen, bromine, and oxygen. She sent a sample off for analysis, and days later the following results arrived:

carbon, 34.35%; hydrogen, 1.725%; bromine, 45.65%; oxygen, 18.27%

 

Determine the empirical formula.

 

 

 

 

 

 

 

Part B:

Because the scientist knows the compound she is trying to make, she can calculate the molar mass, assuming she synthesized the right compound. The expected molar mass is 699.9 g/mol. 

What is the molecular formula of the compound?

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An unknown compound was isolated, purified, and sent off for elemental analysis. It was suspected that the compound was a hydrocarbon of some type, so a carbon-hydrogen analysis was requested, and the following results were returned: %C, 59.94; %H, 12.08. A reasonable empirical formula was obtained by assuming that the remaining mass was due entirely to nitrogen, but the compound was never resubmitted for a nitrogen analysis.

 

a) Determine the empirical formula for the compound assuming that it consists of carbon, hydrogen, and nitrogen. Be sure to show your work, and write the empirical formula in the box provided.

 

 

 

The empirical formula is

b) It was later determined that the other element in the compound was silicon, not nitrogen. You could repeat the above analysis based on carbon, hydrogen, and silicon, but a quick glance at the periodic table should allow you to immediately write the empirical formula based on the work you did in Part A.

1. What is the empirical formula for the compound assuming that it consists of carbon, hydrogen, and silicon?

2. Explain how you can determine the empirical formula based on the work you did in Part A without repeating the full calculation.

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A hydrocarbon  with general formula CxHy was burned completely in air, yielding 0.18 g of water and 0.44 g of carbon dioxide. Which formula could give such data?

A. C2H2

B. C2H4

C. C2H6

D. C6H6

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A 4.08 g sample of a compound of nitrogen and oxygen contains 3.02 g of oxygen. What is the empirical formula?

A. NO

B. NO2

C. N2O

D. N2O5

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Thiophene contains only the elements carbon, hydrogen and sulfur. A sample weighing 7.96 g was burned in oxygen to give 16.65 g CO2 and 3.40 g of H2O. What is the empirical formula of thiophene?

a) C2H2S

b) C4H4S

c) C2H4S3

d) C2H4S

e) C4H4S3

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Determine the empirical formula of natural rubber which is 88.1% C and 11.9% H.

a) C2H3

b) C3H7

c) C2H5

d) C3H5

e) C5H8

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What is the empirical formula of diphosphorus tetrafluoride?

1. PF3            2. PF2            3. P2F2            4. P2F4

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What is the empirical formula for this structure?

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Para-cresol is used as a disinfectant. A 0.3654 g sample of this compound that contains only C, H, and O, gave 1.0420 g CO2 and 0.2437 g H2O in a combustion analysis. Its molecular mass is 108.1g/mol. What is its molecular formula?

 

a. C7H8O

b. C6H7O

c. C7H7O2

d. C6H7O2

e. C7H8O2

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Methylparaben is used in food and drugs and especially in beauty products although some studies have shown that it can disrupt endocrine production. A sample of methylparaben was decomposed to its elements – carbon, hydrogen and oxygen. The percentage of hydrogen was measured to be 5.26% and that a carbon was found to be 63.2%. An error was made in attempting to determine the percentage that was oxygen so this value was not reported. What is the empirical formula of methyparaben?

 

a. C8H8O3

b. C6H6O

c. C2H6O

d. CH2O

e. CHO

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