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# 2.2.5: Writing Chemical Formulas (Problems)

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PROBLEM $$\PageIndex{1}$$

Explain why the symbol for an atom of the element oxygen and the formula for a molecule of oxygen differ.

Answer

The symbol for the element oxygen, O, represents both the element and one atom of oxygen. A molecule of oxygen, O2, contains two oxygen atoms; the subscript 2 in the formula must be used to distinguish the diatomic molecule from two single oxygen atoms.

PROBLEM $$\PageIndex{2}$$

Write the molecular and empirical formulas of the following compounds:

(a)

(b)

(c)

(d)

Answer a

molecular: CO2

empirical: CO2

Answer b

molecular: C2H2

empirical: CH

Answer c

molecular: C2H4

empirical: CH2

Answer d

molecular: H2SO4

empirical: H2SO4

PROBLEM $$\PageIndex{3}$$

Write the molecular and empirical formulas of the following compounds:

(a)

(b)

(c)

(d)

Answer a

molecular: C4H8

empirical: CH2

Answer b

molecular: C4H6

empirical: C2H3

Answer c

molecular: H2Si2Cl4

empirical: HSiCl2

Answer d

molecular: H3PO4

empirical: H3PO4

PROBLEM $$\PageIndex{4}$$

Determine the empirical formulas for the following compounds:

(a) caffeine, C8H10N4O2
(b) fructose, C12H22O11
(c) hydrogen peroxide, H2O2
(d) glucose, C6H12O6
(e) ascorbic acid (vitamin C), C6H8O6

Answer a

C4H5N2O

Answer b

C12H22O11

Answer c

HO

Answer d

CH2O

Answer e

C3H4O3

PROBLEM $$\PageIndex{5}$$

Determine the empirical formulas for the following compounds:

(a) acetic acid, C2H4O2
(b) citric acid, C6H8O7
(c) hydrazine, N2H4
(d) nicotine, C10H14N2
(e) butane, C4H10

Answer a

CH2O

Answer b

C6H8O7

Answer c

NH2

Answer d

C5H7N

Answer e

C2H5

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