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2.4 Chemical formulas  (Page 4/10)

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Figure A shows a structural diagram of acetic acid, C subscript 2 H subscript 4 O subscript 2. Acetic acid contains two carbon atoms connected by a single bond. The left carbon atom forms single bonds with three hydrogen atoms. The carbon on the right forms a double bond with an oxygen atom. The right carbon atom also forms a single bond to an oxygen atom which forms a single bond with a hydrogen atom. Figure B shows a structural diagram of methyl formate, C subscript 2 H subscript 4 O subscript 2. This molecule contains a carbon atom which forms single bonds with three hydrogen atoms, and a single bond with an oxygen atom. The oxygen atom forms a single bond with another carbon atom which forms a double bond with another oxygen atom and a single bond with a hydrogen atom.
Molecules of (a) acetic acid and methyl formate (b) are structural isomers; they have the same formula (C 2 H 4 O 2 ) but different structures (and therefore different chemical properties).

Many types of isomers exist ( [link] ). Acetic acid and methyl formate are structural isomers , compounds in which the molecules differ in how the atoms are connected to each other. There are also various types of spatial isomers    , in which the relative orientations of the atoms in space can be different. For example, the compound carvone (found in caraway seeds, spearmint, and mandarin orange peels) consists of two isomers that are mirror images of each other. S -(+)-carvone smells like caraway, and R -(−)-carvone smells like spearmint.

The top left portion of this 2 row, 4 column figure shows a structural diagram of positive carvone, C subscript 10 H subscript 14 O. This molecule has a carbon atom which forms a double bond with a C H subscript 2 group and a C H subscript 3 group. The carbon atom also forms a single bond with another carbon atom which is part of a ring. This carbon atom, being part of the ring, forms single bonds with a hydrogen atom, a C H subscript 2 group, and a C H subscript 2 group. The first C H subscript two group forms a single bond with C H which forms a double bond with a carbon atom. This carbon atom forms a single bond with a C H subscript 3 group. The carbon atom forming part of the ring forms a single bond with a carbon atom which forms a double bond with an oxygen atom and a single bond with a C H subscript 2 group to complete the ring. Below the structural diagram of carvone is a photo of caraway seeds. Column 2 contains identical ball and stick representations of the structural diagram in the top left position. The top right portions of these images each contains the letter “S” and there is an arrow pointing downward from the top image to the bottom image. Columns 3 and 4 are representations of negative carvone. The top row in column three depicts a mirrored image of the ball and stick structure to its left, reflected across the y axis. There is a downward pointing arrow to the image below, which is the same structure rotated counter clockwise 180 degrees. Both images in column 3 have an “R” in the top right corner. The image in the first row of column 4 is the same as the lewis structure in the first row of column 1, reflected across the y axis. Below this negative carvone structural diagram is a photo of spearmint leaves.
Molecules of carvone are spatial isomers; they only differ in the relative orientations of the atoms in space. (credit bottom left: modification of work by “Miansari66”/Wikimedia Commons; credit bottom right: modification of work by Forest&Kim Starr)

Select this link to view an explanation of isomers, spatial isomers, and why they have different smells (select the video titled “Mirror Molecule: Carvone”).

Key concepts and summary

A molecular formula uses chemical symbols and subscripts to indicate the exact numbers of different atoms in a molecule or compound. An empirical formula gives the simplest, whole-number ratio of atoms in a compound. A structural formula indicates the bonding arrangement of the atoms in the molecule. Ball-and-stick and space-filling models show the geometric arrangement of atoms in a molecule. Isomers are compounds with the same molecular formula but different arrangements of atoms.

Chemistry end of chapter exercises

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

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

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Explain why the symbol for the element sulfur and the formula for a molecule of sulfur differ.

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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.

(a) molecular CO 2 , empirical CO 2 ; (b) molecular C 2 H 2 , empirical CH; (c) molecular C 2 H 4 , empirical CH 2 ; (d) molecular H 2 SO 4 , empirical H 2 SO 4

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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.

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Determine the empirical formulas for the following compounds:

(a) caffeine, C 8 H 10 N 4 O 2

(b) fructose, C 12 H 22 O 11

(c) hydrogen peroxide, H 2 O 2

(d) glucose, C 6 H 12 O 6

(e) ascorbic acid (vitamin C), C 6 H 8 O 6

(a) C 4 H 5 N 2 O; (b) C 12 H 22 O 11 ; (c) HO; (d) CH 2 O; (e) C 3 H 4 O 3

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Determine the empirical formulas for the following compounds:

(a) acetic acid, C 2 H 4 O 2

(b) citric acid, C 6 H 8 O 7

(c) hydrazine, N 2 H 4

(d) nicotine, C 10 H 14 N 2

(e) butane, C 4 H 10

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Write the empirical formulas for the following compounds:

(a)
Figure A shows a structural diagram of two carbon atoms that form a single bond with each other. The left carbon atom forms single bonds with hydrogen atoms each. The right carbon forms a double bond to an oxygen atom. The right carbon also forms a single bonded to another oxygen atom. This oxygen atom also forms a single bond to a hydrogen atom.
(b)
Figure B shows a structural diagram containing a leftmost carbon that forms single bonds to three hydrogen atoms each. This leftmost carbon also forms a single bond to a second carbon atom. The second carbon atom forms a double bond with an oxygen atom. The second carbon also forms a single bond to a second oxygen atom. This oxygen atom forms a single bond to a third carbon atom. This third carbon atom forms single bonds with two hydrogen atoms each as well as a single bond with another carbon atom. The rightmost carbon atom forms a single bond with three hydrogen atoms each.

(a) CH 2 O; (b) C 2 H 4 O

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Open the Build a Molecule simulation and select the “Larger Molecules” tab. Select an appropriate atoms “Kit” to build a molecule with two carbon and six hydrogen atoms. Drag atoms into the space above the “Kit” to make a molecule. A name will appear when you have made an actual molecule that exists (even if it is not the one you want). You can use the scissors tool to separate atoms if you would like to change the connections. Click on “3D” to see the molecule, and look at both the space-filling and ball-and-stick possibilities.

(a) Draw the structural formula of this molecule and state its name.

(b) Can you arrange these atoms in any way to make a different compound?

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Use the Build a Molecule simulation to repeat [link] , but build a molecule with two carbons, six hydrogens, and one oxygen.

(a) Draw the structural formula of this molecule and state its name.

(b) Can you arrange these atoms to make a different molecule? If so, draw its structural formula and state its name.

(c) How are the molecules drawn in (a) and (b) the same? How do they differ? What are they called (the type of relationship between these molecules, not their names).

(a) ethanol

A Lewis Structure is shown. An oxygen atom is bonded to a hydrogen atom and a carbon atom. The carbon atom is bonded to two hydrogen atoms and another carbon atom. That carbon atom is bonded to three more hydrogen atoms. There are a total of two carbon atoms, six hydrogen atoms, and one oxygen atoms.

(b) methoxymethane, more commonly known as dimethyl ether

A Lewis Structure is shown. An oxygen atom is bonded to two carbon atoms. Each carbon atom is bonded to three different hydrogen atoms. There are a total of two carbon atoms, six hydrogen atoms, and one oxygen atom.

(c) These molecules have the same chemical composition (types and number of atoms) but different chemical structures. They are structural isomers.

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Use the Build a Molecule simulation to repeat [link] , but build a molecule with three carbons, seven hydrogens, and one chlorine.

(a) Draw the structural formula of this molecule and state its name.

(b) Can you arrange these atoms to make a different molecule? If so, draw its structural formula and state its name.

(c) How are the molecules drawn in (a) and (b) the same? How do they differ? What are they called (the type of relationship between these molecules, not their names)?

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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