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Empirical and Molecular Formulas: Understanding the Relationship between Them, Study Guides, Projects, Research of Chemistry

Oregon Health & Science University (OHSU)Chemistry

The concepts of empirical and molecular formulas, their relationship, and how to find the empirical formula given the molecular weight or the percentage composition of a compound. Several examples are provided to illustrate the concepts.

Typology: Study Guides, Projects, Research

2021/2022

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U6LM2B-WS- Empirical and Molecular Formulas Name: ______KEY_______
1. The empirical formula of a compound is also called the simplest formula. It
represents the relative or smallest whole-number ratio of atoms in a cmpd.
2. The molecular formula represents the actual number of atoms of each element
in a molecule of the compound.
3. The empirical formula and the molecular formula are mathematically related as
follows:
Molecular formula = n x empirical formula.
4. Can the molecular formula be the same as the empirical formula? Explain.
Yes. The molecular and empirical formula may be the same when the molecular formula does not have subscripts
that can be reduced (ex: formaldeyhyde,CH2O). They are different in any case where the molecular formula may be
reduced to a smaller whole-number ratio of elements (ex: acetic acid, C2H4O2 has the same empirical formula as
formaldehyde, CH2O).
5. The molecular formula for glucose is C6H12O6.
a. What is its empirical formula? CH2O
b. The molecular formula of glucose, C6H12O6 = 6 x CH2O
c. The molecular weight of glucose is 180 g/mol. It is equal to 6 x 30 g/mol.
6. The molecular formula of benzene is C6H6.
a. What is its empirical formula? CH
b. The molecular formula of benzene, C6H6 = 6 x CH
7. The empirical formula of a compound is NH, and its molecular weight is 30.0 amu.
What is its molecular formula? N2H2
NH =15 g/mol.
n = 30.0 g/mol  15.0 g/mol = 2 2 x NH = N2H2
8. The empirical formula of a compound is CH3. Its molecular weight is 30.0 amu.
What is its molecular formula? C2H6
CH3 =15 g/mol.
n = 30.0 g/mol  15.0 g/mol = 2 2 x CH3 = C2H6
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U6LM2B-WS- Empirical and Molecular Formulas Name: ______KEY_______

1. The empirical formula of a compound is also called the simplest formula. It

represents the relative or smallest whole-number ratio of atoms in a cmpd.

2. The molecular formula represents the actual number of atoms of each element

in a molecule of the compound.

3. The empirical formula and the molecular formula are mathematically related as

follows:

Molecular formula = n x empirical formula.

4. Can the molecular formula be the same as the empirical formula? Explain.

Yes. The molecular and empirical formula may be the same when the molecular formula does not have subscripts

that can be reduced (ex: formaldeyhyde,CH 2 O). They are different in any case where the molecular formula may be reduced to a smaller whole-number ratio of elements (ex: acetic acid, C 2 H 4 O 2 has the same empirical formula as formaldehyde, CH 2 O).

5. The molecular formula for glucose is C 6 H 12 O 6.

a. What is its empirical formula? CH 2 O

b. The molecular formula of glucose, C 6 H 12 O 6 = 6 x CH 2 O

c. The molecular weight of glucose is 180 g/ mol. It is equal to 6 x 30 g/ mol.

6. The molecular formula of benzene is C 6 H 6.

a. What is its empirical formula? CH

b. The molecular formula of benzene, C 6 H 6 = 6 x CH

7. The empirical formula of a compound is NH, and its molecular weight is 30.0 amu.

What is its molecular formula? N 2 H 2

NH =15 g/ (^) mol. n = 30.0 g/ (^) mol  15.0 g/ (^) mol = 2  2 x NH = N 2 H 2

8. The empirical formula of a compound is CH 3. Its molecular weight is 30.0 amu.

What is its molecular formula? C 2 H 6

CH 3 =15 g/ (^) mol. n = 30.0 g/ (^) mol  15.0 g/ (^) mol = 2  2 x CH 3 = C 2 H 6

9. A compound is 81.7% carbon and 18.3% hydrogen.

a. What is its empirical formula? C 3 H 8

Assume 100g of the compound to simplify the problem. 81.7g C x (1 mol C / 12.01g C) = 6.803 mol C … divide both by 6.803 to reduce the ratio = 1 mol C 18.3g H x (1mol H / 1.01g H) = 18.12 mol H … to 2.663 mol H Multiplying by 3 will give the smallest whole number ratio: 3 (C 1 H2.663) = C 3 H 8

b. The formula weight of this compound is 44.0 amu. Is the molecular formula

different than the empirical formula? No. The mass of the empirical formula above is 44.

g/mol, so the empirical and molecular formulas are the same.

10. Butyric acid is 54.5% carbon, 9.09% hydrogen and 36.4% oxygen.

a. What is its empirical formula? C 2 H 4 O

54.5g C x (1 mol C / 12.01g C) = 4.538 mol C … divide all by 2.275 to reduce the ratio = 1.995 mol C 9.09g H x (1mol H / 1.01g H) = 9.000 mol H … to 3.956 mol H 36.4g O x (1mol H / 16.00g H) = 2.275 mol O … to 1 mol O This gives the smallest whole number ratio: C 2 H 4 O

b. Its molar mass is 88.0 g/mol. What is the molecular formula of butyric

acid? C 4 H 8 O 2

Empirical molar mass = 44. g/mol n = 88.0 g/ (^) mol  44.0 g/ (^) mol = 2  2 x C 2 H 4 O = C 4 H 8 O (^2)

11. Isopropyl alcohol contains C, H, and O. When we burn 11.63 g of this compound,

the products are 25.5 g CO 2 and 14.0 g H 2 O.

CxHyOz + O 2  CO 2 + H 2 O 11.63g 25.5g 14.0g Convert to moles:??? 0.5795mol 0.7778mol

a. What is the empirical formula? C 3 H 8 O

You can assume all the moles of C and all the moles of H came from the isopropanol. 0.5795 mol C x (12.01g C / 1 mol C) = 6.96g carbon 0.7778 mol H 2 O x (2 mol H / 1 mol H 2 O) = 1.5556mol H …x (1.01 g H / 1 mol H) = 1.57 g hydrogen 11.63g total – (6.96g C + 1.57g H) = 3.10g oxygen … (1 mol O / 16.00g O) = 0.1937 mol O Dividing all moles by 0.1937 to reduce the ratio gives: C 3 H 8 O

b. The molar mass of the alcohol is 60.0 g/mol. What is its molecular

formula? C 3 H 8 O

The molar mass of the empirical formula C 3 H 8 O is 60.0 g/mol, so the molecular formula is the same.