Chemical Formula questions

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

Solution

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.

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

Solution

Sulfur is a polyatomic element: in its natural form it exists as a molecule of 8 atoms, which would be written as $S_8$. The symbol for an element always represents just one atom, for sulfur that is $S$.

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 two carbon atoms that are each triple bonded to each other as well as singly bonded to one hydrogen atom each

(c) Figure C shows two carbon atoms that are double bonded to each other as well as singly bonded to two hydrogen atoms each

(d) Figure D shows a molecule with a central sulfur atom. This sulfur atom forms four single bonds to four individual oxygen atoms. Two of these oxygen atoms also have a single bond out to one hydrogen atom each.

Solution

(a) molecular CO2, empirical CO2
(b) molecular C2H2, empirical CH
(c) molecular C2H4, empirical CH2
(d) molecular H2SO4, empirical H2SO4

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.

Solution

  Molecular Formula Empirical Formula
(a) $ C_4 H_8$ $ CH_2$
(b) $ C_4 H_6$ $ C_2 H_3$
(c) $ H_2 Si_2 Cl_4 $ $ H SiCl_2$
(d) $ H_3 PO_4$ $ H_3 PO_4$

Determine the empirical formulas for the following compounds:

(a) caffeine, C8H10N4O2

(b) sucrose, C12H22O11

(c) hydrogen peroxide, H2O2

(d) glucose, C6H12O6

(e) ascorbic acid (vitamin C), C6H8O6

Solution

(a) C4H5N2O
(b) C12H22O11
(c) HO
(d) CH2O
(e) C3H4O3

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

Solution

(a) $$ CH_2 O$$

(b) $$C_6 H_8 O_7 $$

(c) $$ NH_2$$

(d) $$C_5 H_7 N$$

(e) $$C_2 H_5 $$

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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.

Solution

(a) $CH_2 O$ (b) $C_2H_4 O$

Open the Build a Molecule simulation and select the “Larger Molecules” tab. Select an appropriate atom’s “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?

Solution

The compound $C_2 H_6$ can be arranged into only one actual molecule that follows all chemical rules: butane.

Use the Build a Molecule simulation to repeat the question above, 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).?

Solution

(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.