Upon completion of this lesson, you should be able to:

  • define the mole and explain why the mole is a useful quantity in chemistry
  • define molar mass
  • use a data table to determine the molar mass of an element
  • calculate molar mass given chemical names or chemical formulas
  • define STP and the molar volume of a gas at STP


These items are intended to get you to think about ideas related to those in the lesson. Take a few minutes to respond to them. Use a word processor or an html editor to record your responses and submit them to your teacher.

  • What has greater mass, a dozen oxygen atoms or a dozen grapefruit? Explain.
  • Why are eggs sold by the dozen instead of by the kilogram?
  • Can atoms be counted?
  • What of properties of gases distinguish them from solids and liquids?

Throughout this section and indeed this unit, you will rely on your formula writing skills. These skills will be especially useful in the section on solution stoichiometry. You may want to book mark them if you anticipate using them frequently.

Ionic Compounds: Formula Writing

Binary Ionic Compounds

Writing chemical formulas for binary ionic compounds relies heavily on your ability to predict ion charges using the periodic table. For example, given the name sodium chloride, you would need to identify the ions in the compound and their individual charges.

Sodium is a Group 1 element, so its ions have a 1+ charge and chlorine is a Group 17 element, so its ions have a 1- charge. The chemical formula for sodium chloride is NaCl because the 1+ charge of a sodium ion is balanced completely by the 1- charge of a chloride ion and vice versa.

Now let's consider a crystal that consists of Group 2 and Group 17 ions like magnesium chloride:

Magnesium ions have a 2+ charge and chloride ions have a 1- charge. The lowest whole number ratio that produces a neutral or balanced formula is 1 Mg2+ : 2 Cl-; therefore, the chemical formula for the compound is MgCl2.

Ionic Compounds that Contain a Multivalent Species

Your Nelson Chemistry Periodic Table provides ion charges on the left side of each cell for the transition elements. What do you notice about ion charges for elements such as titanium, chromium, iron, cobalt, and copper?

Ions of a certain elements can have more than one possible charge. Such elements are called multivalent species. For example, copper is multivalent - its ions can have either a 1+ or a 2+ ion charge (Cu+ or Cu2+). The more common ion charge is list first (on top). In the case of copper it is 2+.

The names of multivalent species contain a Roman numeral to indicate the charge. Copper(II) means a copper ion with a 2+ charge while copper(I) means a copper ion with a 1+ charge.

Writing a chemical formula for a compound that includes a multivalent species requires the same approach used for other binary ionic compounds. Consider the example of iron(II) chloride:

The iron(II) ion is represented by the symbol Fe2+. The Roman numeral II indicates the charge on the iron ion. The chloride ion is Cl-. Two chloride ions are needed to balance the 2+ charge on each iron ion, so the chemical formula is FeCl2.

Polyatomic Ions

Some ionic compounds contain polyatomic ions. A polyatomic ion consists of several atoms which together have gained or lost electrons to become stable. For example, the ammonium ion is a group of bonded atoms (one nitrogen and four hydrogen atoms) that have lost one electron. A nitrate ion is one nitrogen atom and three oxygen atoms that together have gained an electron.

The data sheet on the reverse side of your periodic table (or the back inside cover of your Nelson Chemistry text) lists most of the polyatomic ions which you will encounter in this course. You do not have to memorize the names, formulas, and charges of polyatomic ions, but through sheer use you will become familiar with many of them. Refer to these tables consistently and avoid guessing the information associated with these ions.

The formula writing procedure involves writing ion symbols and balancing their charges using numerical subscripts as needed.

The main difference in writing formulas for compounds that contain polyatomic ions is that you may need to place parentheses around a polyatomic ion symbol if its numerical subscript is greater than one. Some examples are:

sodium chlorite

  • Sodium is in Group 1 and forms 1+ ions.
    • sodium is Na+
  • The -ite suffix in chlorite suggests that it may be a polyatomic ion. Find the name chlorite in the data table and list its formula and charge.
    • chlorite is ClO2-
  • Since the 1+ charge on the sodium ion is balanced by the 1- charge on chlorite, one of each ion is needed in the formula.
    • The chemical formula is NaClO2.

iron(III) sulfate

  • The Roman numeral III indicates that the iron ion has a 3+ charge.
    • iron(III) is Fe3+
  • Sulfate has the -ate ending so it is probably a polyatomic ion. A quick look at the data table confirms this
    • sulfate is SO42-
  • Since the charges are odd and even, you can apply the LCM method to balance the charges.
    • Multiply charge values: 2 x 3 = 6
    • Divide product by charge of iron: 6 ÷ 3 = 2
    • The iron symbol gets the subscript 2: Fe2
    • Divide product by charge of sulfate: 6 ÷ 2 = 3
    • The sulfate symbol gets the subscript 3: (SO4)3. Note the use of parentheses to separate the 4 from the 3. Without them the polyatomic ion formula would show 43 oxygen atoms: SO43!
    • The chemical formula for iron(III) sulfate is Fe2(SO4)3.

ammonium permanganate

  • Ammonium is a polyatomic ion. A IUPAC convention is that cation names should end in -ium.
    • The polyatomic ions table lists ammonium as a 1+ ion: NH4+.
  • The -ate suffix in permanganate suggests that it may be a polyatomic ion. Locate the name permanganate in the data table and list its formula and charge:
    • permanganate is MnO4-.
  • Since the 1+ charge on the ammonium ion is balanced by the 1- charge on permanganate, one of each ion is needed in the formula.
    • The chemical formula is NH4MnO4.
  • Note that parentheses are not required because just one of each ion is needed to produce a neutral unit.

Ionic Hydrates

An ionic hydrate is a compound that has water as part of its crystalline structure. Bluestone contains five water molecules per copper(II) sulfate unit in the crystal. Its IUPAC name is copper(II) sulfate pentahydrate (CuSO4·5H2O). Anhydrous means without water, thus the name for the anhydrous form of this compound is simply copper(II) sulfate (CuSO4).

A hydrate like rock salt, sodium chloride monohydrate (NaCl·1H2O), can be formed when a salt water lakes dries up leaving behind solid salt. However, not all of the water evaporates. Some water molecules became part of the salt crystals giving rise to hydrated crystals.

In order to convert IUPAC names for ionic hydrates into chemical formulas, you will need to use a prefix system. A prefix changes the meaning of a term or a name. Memorize these prefixes and the numbers associated with them:

  • mono = 1
  • di = 2
  • tri = 3
  • tetra = 4
  • penta = 5
  • hexa = 6
  • hepta = 7
  • octa = 8
  • nona = 9
  • deca = 10


sodium thiosulfate pentahydrate

  • Break the name down into three parts:
    • the -ate ending in thiosulfate suggests that the anion is polyatomic: S2O32-
    • the cation - sodium - is a member of Group 1 and has a 1+ charge: Na+
    • the label pentahydrate indicates the presence of five water molecules per formula unit of sodium thiosulfate: ·5H2O.
  • Balance the ion charges:
    • two Na+ ions are needed for each S2O32- ion.
  • Put all this information together to get:
    • Na2S2O3·5 H2O

Notice that a dot separates water from the rest of of the formula.


  1. If name contains a metal ion name and nonmetal ion name, then it is a binary ionic compound. Use the periodic table to predict the ion charges. Write the balanced formula by ensuring that the total positive charge is balanced by the total negative charge. Use the LCM method if the charges do not balance easily. Be sure the formula is the lowest whole number ratio (e.g. X1Y2 not X2Y4).
  2. If the cation name is followed by a Roman numeral, then you know the cation's charge. Use the periodic table or the table of polyatomic ions to determine the charge of the anion. Write the balanced formula by ensuring that the total positive charge is balanced by the total negative charge. Use the LCM method if necessary.
  3. If the name contains one or two polyatomic ion names, look up their ion formulas and charges in the polyatomic ions table. Write the balanced formula by ensuring that the total positive charge is balanced by the total negative charge. Use the LCM method if necessary.
  4. If the name contains the term hydrate with a prefix, then write the formula name using the appropriate rules and add · X H2O to the formula where X is the number corresponding to the prefix.
Molecular Compounds: Formula Writing

Binary Molecular Compounds

A binary molecular compound consists of atoms from two different elements. Carbon dioxide is a good example. It consists of carbon atoms and oxygen atoms.

To write a chemical formula for a binary molecular compound:

  1. Write the symbol for each element in the compound.
  2. Use the prefix to determine the number of atoms of each element in the formula and write the appropriate number as a subscript to the right of the element's symbol.
  3. If an element name lacks a prefix, assume that there is just one atom of that element. It is not necessary to write the numerical subscript 1 - it is implied.

Example: diboron hexahydride.

Diboron means that the molecule contains two boron atoms (B2) and hexahydride means the molecule contains six hydrogen atoms (H6) . Bringing the two symbols together in the order they appear in the name gives: B2H6.

Trivial Names

Before the establishment and widespread use of IUPAC rules, a compound could have been known by several different names. In many cases, the names revealed nothing about the composition of the compound. As you can imagine, this had the potential to create confusion and miscommunication among chemists.

Today, most compounds are known by their IUPAC names. Nonetheless, you will still see chemicals like NH3, H2O2, and O3 labelled with their trivial names ammonia, peroxide, and ozone instead of nitrogen trihydride, dihydrogen dioxide, and trioxygen. The same thing happens in real life - few people know the real names of recording artists like Shaggy (Orville Richard Burrell) and Sting (Gordon Sumner).

Since some compounds are also better known by their trivial names, you will need to do some memorizing. Here are the ones you should memorize:

  • ammonia - NH3
  • hydrogen peroxide - H2O2
  • hydrogen sulfide - H2S
  • ozone - O3
  • water - H2O

Organic Compounds

Certain compounds (mainly those that contain carbon) are named using different sets of IUPAC rules. You will learn about these in the organic chemistry unit. For now it is helpful to memorize these names and formulas:

  • methane - CH4
  • propane - C3H8
  • methanol - CH3OH
  • ethanol - C2H5OH
  • glucose - C6H12O6
  • sucrose - C12H22O11

Hydrogen Compounds

Due to the special nature of hydrogen atoms, the IUPAC rules are not strictly followed for certain classes of hydrogen compounds. Group 16 and Group 17 hydrogen compounds tend to be named as if they are ionic compounds. For example, H2S is hydrogen sulfide and HCl is hydrogen chloride.

Chemical Formulas for Acids

If the word acid appears in the name of a compound, you should automatically think about applying a special set of formula writing rules. For example, given the name hydroiodic acid, you should immediately realize the need to apply a different set of rules.

Rules for Writing Chemical Formulas for Acids

In order to convert an acid name to a chemical formula:

  • convert the acid name to an ionic name
  • identify the cation and the anion from the ionic name and write their symbols with their charges
  • determine the number of hydrogen ions required to cancel the negative charge on the anion.
  • write the formula for the acid using the appropriate numerical subscript for hydrogen
  • add the aq state of matter subscript to the end of the formula


  • binary acids
  • ic acids
  • ous acids
  • special cases

Example 1: hydroiodic acid.

  • convert the acid name to the ionic name: hydrogen iodide
  • write the symbols for the two ions: H+ and I-
  • find the number of hydrogen ions needed: one H+ ion cancels the charge on one I- ion
  • write the balanced formula: HI
  • add the aq subscript: HI (aq)

Example 2: boric acid.

  • convert the acid name to the ionic name: hydrogen borate
  • write the symbols for the two ions: H+ and BO33-
  • find the number of hydrogen ions needed: three H+ ions cancel the charge on one BO33- ion
  • write the balanced formula: H3BO3
  • add the aq subscript: H3BO3 (aq)

Example 3: nitrous acid

  • convert the acid name to the ionic name: hydrogen nitrite
  • write the symbols for the two ions: H+ and NO2-
  • find the number of hydrogen ions needed: one H+ cancels the charge on one NO2- ion
  • write the balanced formula: HNO2
  • add the aq subscript: HNO2 (aq)

Special Cases

Some acid names deviate from the rules because of historical usage. The four cases you should memorize are:

  • H2SO4(aq) - sulfuric acid
  • H2SO3(aq) - sulfurous acid
  • H2S(aq) - hydrosulfuric acid
  • H3PO4(aq) - phosphoric acid

The syllables in italics are added because of the way these names have been pronounced in the past.