CHEM101 Study Guide

Unit 4: Chemical Formulas and Equations

 

4a. Determine the formula of an ionic compound when given the name, and name the ionic compound when given a formula

  • Do you know the names and charges of the most common ions listed in this course?
  • Can you use the names and charges of the ions to determine the formula of an ionic compound when given the name?

Before you review ionic compound formulas, review Chemical Nomenclature. It is essential to know the names and charges of the ions listed here to be able to name ionic compounds.

To determine the formula of an ionic compound from the name, you must have a strong command of the names and charges of single atom ions and polyatomic ions. It may be helpful to create flashcards of the ion symbols, with their charge and names, to learn them.

When given a chemical name for an ionic compound, the first name is the cation, or positive ion and the second name is the anion, or negative ion.

First, write the formula of the cation, including charge, and then write the formula of the anion, including charge. For cations that can have different charges, the charge will be written as a Roman numeral in parentheses. Then, balance the charges. In other words, make sure the positive charge equals the negative charge in the compound. To do this, you may need to alter the number of each type of ion.

For example, let's look at the chemical copper (II) chloride. The cation is copper, and we are told it has a +2 charge by the (II). Therefore, the cation is: Cu2+. The anion is chloride, which is Cl-. Now, balance the charges. There is a +2 charge from the cation and a -1 charge from the anion. Therefore, we need two chloride ions to get a -2 charge. The formula for copper (II) chloride is CuCl2.

Review this material in Chemical Nomenclature.

 

4b. Use mass percent data and formula mass to determine empirical and molecular formulas

  • Can you use a chemical formula to determine mole ratios of atoms in a compound?
  • Can you use a chemical formula to determine the percent composition of a compound?
  • Can you calculate the percent composition of carbon in C10H16?
  • Can you determine the molecular formula of glycoaldehyde given the following information?
    • Its molar mass is 60 g/mol
    • Its percent composition is 40% C, 6.71% H, and 53.28% O

A list of the percent by mass of each element in a compound is known as the percent composition. As an example, calculate the mass percent of carbon in CH4. To perform this calculation you need the number of carbons present (1C) times the mass of carbon (12 amu) and then you divide the number by the molar mass of the compound (CH4 = 16 amu). Since it is a percentage, you will then multiply by 100%. Here is an example:

[(1C) × (12 amu)] / (16 amu) × 100% = 75% C in CH4

We can also use the chemical formula to determine the mass fraction or percent composition of the elements in a given compound.

After knowing the percent composition of a molecule, we can use it to calculate the empirical formula and molecule formula.

For more examples of percent composition, review Calculating Mass Percent, Chemical Formulas and their Arithmetic, and Molecular and Empirical Formulas from Percent Composition.

 

4c. Name binary molecular compounds using prefixes.

  • What are the prefixes for binary molecular compounds?
  • Given a formula, can you write the correct binary molecular compound name?
  • Given a name, can you write the correct binary molecular compound formula?

Many molecular compounds are binary, which means they consist of two types of atoms.

See the chart of numerical prefixes in the section Naming the Binary Molecules, in Chemical Nomenclature. Consider making flashcards to help you memorize the numerical prefixes and their numerical values.

To write the name of a binary molecular compound from the formula, you need to know the names of the atoms involved. Write the names of the elements in the order they appear in the formula. The second element should end in -ide rather than the element name. Sometimes these are polyatomic ions rather than elements. Then, add the numerical prefixes from the chart to the names of the elements.

For example, we can name P4S3. The first element is phosphorus and the second element is sulfur. We change the second element to sulfide. Now, we add the numerical prefixes. There are four phosphorus atoms so it is tetraphosphorus. There are three sulfur atoms so it is trisulfide. The name of the molecule is tetraphosphorus trisulfide.

Given a name of a binary compound we can determine the formula. Consider dinitrogen tetroxide. Here, we use the prefixes and element names to determine the chemical formula. The first element is nitrogen, and from the di prefix we know there are two nitrogens. The second element is oxygen, and from the tetra prefix we know there are four oxygens. Therefore, the formula is N2O4.

Review this material in Chemical Nomenclature.

 

4d. Balance a chemical equation

  • Can you identify reactants and products in a chemical reaction?
  • Why must chemical reactions be balanced?
  • Can you balance a chemical equation?

Chemical equations express the net change of composition that occurs during a chemical change. Understanding how chemists write chemical reactions is an important part of the language of chemistry.

In a chemical reaction, reactants are transformed into products. It is convention in chemistry that we write the reactants on the left side of the equation and the products on the right.

We write an arrow going from reactants to products to signify the chemical change:

Reactants → Products

We need to balance a chemical equation to comply with the Law of Conservation of Mass, which states that matter (mass) in a chemical reaction must be conserved. This means you cannot make or lose mass during a chemical reaction. Balancing a chemical equation ensures the amount of reactants equals the amount of products. You need to ensure an equal number of each type of atom appears on both sides of the equation (reactant and product).

Let's examine the combustion of propane, C3H8. The unbalanced chemical equation for this reaction is:

C3H8 + O2 → H2O + CO2

To begin, you should tally up the number of each type of atom on each side of the equation.

  • Reactant side: three carbons (C), eight hydrogens (H), two oxygens (O);

  • Product side: one carbon (C), two hydrogens (H), and three oxygens (O).

Then, add whole number coefficients to the molecules to ensure the number of each type of atom on each side of the equation is equal. Note that you can only alter the number of molecules – you cannot change the formulas of the molecules by changing the number of individual atoms in the molecule.

Let's start by balancing carbon. To balance carbon, put a coefficient of three in front of CO2 in the products. Then re-tally the atom count.

C3H8 + O2 → H2O + 3CO2

Reactants: 3C, 8H, 2O; Products, 3C, 2H, 7O

Note that since CO2 has two oxygen atoms, the three CO2 molecules have six oxygen atoms. There is also an oxygen in the water in the products.

Next, repeat the process of balancing a different atom and calculating a new atom tally. Continue until the number of each type of atom on the reactant side is the same as the product side.

Secondly, let's balance hydrogen. Since eight hydrogen atoms are on the reactant side and two are on the product side, you should put a coefficient of four in front of the water to make eight hydrogens on the product side.

C3H8 + O2 → 4H2O + 3CO2

Reactants: 3C, 8H, 2O; Product: 3C, 8H, 10O

Finally, let's balance oxygen. Since two oxygen atoms are on the reactant side and 10 are on the product side, you should put a coefficient of five in front of the oxygen on the reactant side to balance.

C3H8 + 5O2 → 4H2O + 3CO2 

Reactants: 3C, 8H, 10O; Products: 3C, 8H, 10O

The equation is balanced.

Review balancing a chemical equation in Combustion of Propane and Combustion of Ethane. For more review, see Balancing Chemical Reactions.

 

4e. Use molar mass or formula mass in conversions between grams and moles

  • Given a chemical formula, can you determine the number of each type of atom in the compound?
  • Can you use the chemical formula to determine molecular mass?
  • Can you convert the molecular mass of a compound to moles?

The chemical formula specifies the types of atoms in a chemical compound and the number of each type of atom in the compound. The chemical formula defines the compound.

When we know the chemical formula for a compound, we can determine its molecular mass, and molar mass.

Molecular mass, or molecular weight, is the mass of the compound in atomic mass units (amu). This is also called formula mass or formula weight. We can use the chemical formula to determine molecular mass by adding up the atomic masses of all atoms in the compound.

The chemical formula for a compound also allows us to calculate the mole ratios of elements for the compound. The atomic ratios in a formula are also the mole ratios of the atoms in the formula.

For example, in methane, CH4, there are four hydrogen atoms for every carbon atom. There are also four moles of hydrogen for every one mole of carbon.

For example, let's look at ethanol, C2H6O, which is the type of alcohol found in alcoholic beverages. The formula tells us ethanol has two carbons, six hydrogens, and one oxygen. To identify the molecular mass, we multiple the number of each element by the mass on the periodic table.

2 mol C = 2 × 12 g/mol = 24g of C

6 mol H = 6 × 1 g/mol = 6g of H

1 mol O = 1 × 16 g/mol = 16g of O

Molar mass of C2H6O is equal to 24 + 6 + 16 = 46 g/mol.

Now that we know the molar mass of a molecule, we can use it to convert grams of a molecule to moles of a molecule. Here is an example of determining the number of moles of ethanol in 10g of ethanol.

To solve this problem, start with the 10g of ethanol and divide it by the molar mass of ethanol (46 g/mol):

10g ethanol × 1 mol ethanol/46 g ethanol = 0.22 mol of ethanol.

Can you determine the number of moles in 16g of ethanol?

To review, see Formula Weight and Molar Mass, Mole Ratio Calculation, Mole Fraction, and Mole Percent. For more information, review Calculating Molar Mass and Number of Moles and Chemical Formulas and their Arithmetic.

 

4f. Perform stoichiometry calculations, including stoichiometry calculations when there is a limiting reactant

  • Can you convert nomenclature into a chemical reaction?
  • Can you balance a chemical reaction?
  • Can you identify the limiting reactant in a balanced chemical reaction?
  • Can you perform stoichiometry using a limiting reactant?

When we write a chemical equation, we are showing how elements or compounds react to form new compounds. We must make sure the number of each element on the reactant side equals the number of each element on the product side. This is called balancing a chemical equation, and it ensures that the Law of Conservation of Mass is not violated.

When chemical reactions occur, the reactants react in whole-number mole ratios with each other. Products are formed in whole-number mole ratios. We read the equation for the hydrolysis of water (2H2O → 2H2 + O2) as "two moles of water react to yield two moles of hydrogen and two moles of oxygen".

The coefficients before each formula are called stoichiometric coefficients. These coefficients are the mole ratios in the balanced equation. When we have a balanced equation, we can use the mole ratios of a balanced chemical equation to determine the amount of product that can be produced. These are called stoichiometry calculations.

Review this material in Stoichiometry and Stoichiometry Example Problems. For additional information on limiting reactants, review Calculating Amounts of Reactants and Products.

 

Unit 4 Vocabulary

  • Balancing a chemical equation
  • Binary molecular compound
  • Chemical equation
  • Chemical formula
  • Formula mass/formula weight
  • Law of Conservation of Mass
  • Molar mass
  • Molecular mass/molecular weight
  • Polyatomic ion
  • Product
  • Reactant