Unit 5: The Mole and Stoichiometry

What Can A Coefficient Equal?• A coefficient represents the number of elements

or molecules in a chemical equation. It is different from a subscript which measures the number of atoms or polyatomic ions within a molecule.

• 2 CO + O 2 2 CO 2

C oefficients S ubscripts

• Come in front of an element or molecule

• Big numbers • Means that every element or molecule in a compound get multiplied by it.

• Come after an element or polyatomic ion

• Small numbers• Only tell how many of an element there are within a compound

A Coefficient can represent two different things:

1. Number of molecules of a reactant or product in an equation.

2. Number of moles of a reactant or product in an equation.

• In either case, the coefficients give the ratio between the numbers in a chemical reaction (this is why balancing is important)!

• 2CO(g) + 1O2(g) 2CO2

• The reactant ratio is 2:1

Using the Mole

• Coefficients in balanced chemical equations represent MOLES or Molecules

• Zn + 2 HCl ZnCl2 + H2

1 mole (or atom) of Zn reacts with 2 moles (or molecules) of hydrochloric acid to produce 1 mole(or molecule) of zinc chloride and 1 mole(or molecule) of hydrogen gas.

Back to the reaction• Zn + 2 HCl ZnCl2 + H2

• To carry out this reaction, how much zinc and HCl will be needed to make the reaction go to completion?

• 1 mole of Zn and 2 moles of HCl

• How many moles of HCl will be needed if I have 2 moles of zinc? 4 moles of HCl

• How many moles of Copper (I) Sulfide could be produced from 6 moles of Copper (I) Chloride reacting with an excess of H2S gas?

1. Write a balanced equation. Determine what you start with and what you want to know.

H2S + CuCl Cu2S + HCl2 2

2. Covert moles of what you start with to moles of what you want to know using a mole ratio.

• *A mole ratio compares two products or reactants using the coefficients of the balanced equation

• (in this case 2 moles CuCl:1 mole Cu2S).

6 moles CuCl

H2S + 2CuCl Cu2S + 2HCl

1 mole Cu2S2 moles CuCl

= 3 moles Cu2S

H2S + 2CuCl Cu2S + 2HCl

• How many moles of HCl would be produced starting with the same amount?

6 moles CuCl 2 moles HCl

2 moles CuCl

= 6 moles HCl

Al + H2CO3 Al2(CO3) 3 + H2

• Suppose Carlos gets 10 moles of Aluminum for his birthday. How much of each product can he make?

2 3 3

10 moles Al 1 mole Al2(CO3)3

2 moles Al= 5 moles Al2(CO3)3

10 moles Al 3 moles H2

2 moles Al= 15 moles H2