chemical_formulae

Question 1

Define empirical formula.

Answer 1

The empirical formula is the chemical formula which represents the simplest whole number ratio of atoms in the compound.

Question 2

Define molecular formula.

Answer 2

The molecular formula is the chemical formula which represents the actual formula of a compound.

Question 3

The molecular formula of hydrogen peroxide is H₂O₂. What is the empirical formula?

Answer 3

HO is the empirical formula for hydrogen peroxide (H₂O₂).

Question 4

HO is the empirical formula for hydrogen peroxide. What additional information would you need to determine the molecular formula?

Answer 4

Given the empirical formula (HO), one needs the molar mass to determine the molecular formula. In this case the molar mass for hydrogen peroxide is about 34 g/mol. Given the empirical formula mass is 17 g/mol and 34/17 = 2, the molecular formula is 2 x HO = H₂O₂.

Question 5

The molecular formula of glucose is C₆H₁₂O₆. What is the empircal formula?

Answer 5

CH₂O is the empirical formula for glucose (C₆H₁₂O₆).

Question 6

CH₂O is the empirical formula for glucose. What additional information would you need to determine the molecular formula?

Answer 6

Given the empirical formula (CH₂O), one needs the molar mass to determine the molecular formula. In this case the molar mass for glucose is about 180 g/mol. Given the empirical formula mass is 30 g/mol and 180/30 = 6, the molecular formula is 6 x CH₂O = C₆H₁₂O₆.

Question 7

The molecular formula of aspirin (acetyl salicylic acid) is C₉H₈O₄. What is the empirical formula?

Answer 7

C₉H₈O₄ is already the simplest whole number ratio and therefore this is the empirical formula.

Question 8

What is the empirical formula for magnesium chloride?

Answer 8

The empirical formula for magnesium chloride is MgCl₂.

In this question we are not providing data to determine the empirical formula but rather we are asking you to predict the formula based on your knowleged of the charges of magnesium (Mg²⁺) and chloride (Cl^-). Usually we would simply ask you to provide the “formula” for magnesium chloride.

Question 9

Explain how to distinguish between an ionic and a molecular compound.

Answer 9

An ionic compound usually consist of a metal cation and a non metal anion. A molecular compound consists of nonmetal atoms. An exception to this rule of thumb is when the cation is the ammonium ion (NH₄⁺). The compound ammonium chloride (NH₄Cl) is ionic even though there is no metal cation.

Question 10

Is the compound barium sulfate BaSO₄ ionic or molecular?

Answer 10

Barium sulfate is considered to be an ionic compound because it consists of the barium cation (Ba²⁺) and the polyatomic nonmetal anion sulfate (SO₄^2-).

Question 11

Is the compound ammonium phosphate (NH₄)₃PO₄ ionic or molecular?

Answer 11

Ammonium phosphate is considered to be an ionic cmpound because it consists of the ammonium cation (NH₄⁺) and the polyatomic nonmetal anion phosphate (PO₄^3-). It is important to recognize the ammonium cation as one of the few cations which are not derived from metals.

Question 12

When is it necessary to use a roman numeral in naming a compound?

Answer 12

A roman numeral is used for an ionic compound when the metal cation can have multiple charges (Fe²⁺ and Fe³⁺ for example). The roman numberal indicates the charge.

Essentially all transition elements can have multiple charges - with the exception of Zn²⁺, Cd²⁺ and Ag⁺ (actually strictly speaking Zn and Cd are not considered to be transition elements - but this is not an important detail for this course). Other elements that can have multiple charges, which you need to know, are lead (Pb²⁺ and Pb⁴⁺) and tin (Sn²⁺ and Sn⁴⁺).

Question 13

When is it necessary to use a greek prefix in naming a compond?

Answer 13

A greek prefix is usually used in naming a binary molecular compound. This is because the nonmetals in such compounds can be combined in multiple ratios which are not predictable as with ionic compounds. The greek prefixes are required to indicate how many of each atom in a compound.

As an example the following two compounds (NO and N₂O) are called nitrogen monoxide and dinitrogen monoxide. Note that if there is only one atom of the first element, the prefix “mono” is not used. Greek prefixes are also used in the naming of some polyatomic ions. For example H₂PO₄^- is called dihydrogen phosphate.

Question 14

What is a hydrate?

Answer 14

Most ionic compounds exist naturally as hydrates. That is the ionic crystals absorb water from the atmosphere in definite molar proportions.

For example sodium thiosulfate in its natural state has 5 moles of H₂O molecules per 1 formula unit of Na₂S₂O₃. The formula is normally written as Na₂S₂O₃ • 5H₂O and is called sodium thihosulfate pentahydrate. The water molecules are loosely bound and can be temrporarily removed by heating (with a bunsen burner for example). If the compound is no longer heated and if the compound is in contact with the atmosphere, it will once again adsorb back the water in the appropriate proportions.

Question 15

Give the name for the following compound and indicate if it is ionic or molecular: Ag₂SO₃

Answer 15

Silver sulfite (ionic)

Question 16

Give the name for the following compound and indicate if it is ionic or molecular: K₃PO₄

Answer 16

Potassium phosphate (ionic)

Question 17

Give the name for the following compound and indicate if it is ionic or molecular: CoBr₂

Answer 17

Cobalt (II) bromide (ionic)

Question 18

Give the name for the following compound and indicate if it is ionic or molecular: NBr₃

Answer 18

Nitrogen tribromide (molecular)

Question 19

Give the name for the following compound and indicate if it is ionic or molecular: NaCN

Answer 19

Sodium cyanide (ionic)

Question 20

Give the name for the following compound and indicate if it is ionic or molecular: FeSO₄

Answer 20

Iron (II) sulfate

Question 21

Give the name for the following compound and indicate if it is ionic or molecular: ICl₃

Answer 21

Iodine trichloride (molecular)

Question 22

Give the name for the following compound and indicate if it is ionic or molecular: KMnO₄

Answer 22

Potassium permanganate (ionic and note that the polyatomic anion has a metal + oxygen)

Question 23

Give the name for the following compound and indicate if it is ionic or molecular: Na₂Cr₂O₇

Answer 23

Sodium dichromate (ionic and note that the polyatomic anion has a metal + oxygen)

Question 24

Give the name for the following compound and indicate if it is ionic or molecular: Hg₂S

Answer 24

Mercury (I) sulfide (ionic)

Question 25

Give the name for the following compound and indicate if it is ionic or molecular: Zn(NO₃)₂

Answer 25

Zinc nitrate (ionic)

Question 26

Give the name for the following compound and indicate if it is ionic or molecular: Ba(NO₂)₂

Answer 26

Barium nitrite (ionic)

Question 27

Give the name for the following compound and indicate if it is ionic or molecular: P₂O₅

Answer 27

Diphosphorus pentaoxide (molecular)

Question 28

Give the name for the following compound and indicate if it is ionic or molecular: MgI₂

Answer 28

Magnesium iodide (ionic)

Question 29

Give the name for the following compound and indicate if it is ionic or molecular: FeCl₃ • 6H₂0

Answer 29

Iron (III) chloride hexahydrate (ionic)

Question 30

Give the name for the following compound and indicate if it is ionic or molecular: NH₃

Answer 30

nitrogen trihydride - commonly known as ammonia (molecular)