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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)

    Figure A shows a carbon atom that forms two, separate double bonds with two oxygen atoms.

    (b)

    Figure B shows a hydrogen atom which forms a single bond with a carbon atom. The carbon atom forms a triple bond with another carbon atom. The second carbon atom forms a single bond with a hydrogen atom.

    (c)

    Figure C shows a carbon atom forming a double bond with another carbon atom. Each carbon atom forms a single bond with two hydrogen atoms.

    (d)

    Figure D shows a sulfur atom forming single bonds with four oxygen atoms. Two of the oxygen atoms form a single bond with a hydrogen atom.

    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)

    Figure A shows a structural diagram of four carbon atoms bonded together into a chain. The two carbon atoms on the left form a double bond with each other. All of the remaining carbon atoms form single bonds with each other. The leftmost carbon also forms single bonds with two hydrogen. The second carbon in the chain forms a single bond with a hydrogen atom. The third carbon in the chain forms a single bond with two hydrogen atoms each. The rightmost carbon forms a single bond with three hydrogen atoms each.

    (b)

    Figure B shows a structural diagram of a molecule that has a chain of four carbon atoms. The leftmost carbon atom forms a single bond with three hydrogen atoms each and single bond with the second carbon atom. The second carbon atom forms a triple bond with the third carbon atom. The third carbon atom forms a single bond to the fourth carbon atom. The fourth carbon atom forms a single bond to three hydrogen atoms each.

    (c)

    Figure C shows a structural diagram of two silicon atoms are bonded together with a single bond. Each of the silicon atoms form single bonds to two chlorine atoms each and one hydrogen atom.

    (d)

    Figure D shows a structural diagram of a phosphorus atom that forms a single bond to four oxygen atoms each. Three of the oxygen atoms each have a single bond to a hydrogen atom.

    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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