Stoichiometry Involves using balanced chemical equations and mole calculations to make quantitative predictions for reactions.

example 1 18 g of nitrogen gas reacts completely with hydrogen. a) What mass of ammonia is produced? b) What mass of hydrogen is required to completely use up the nitrogen?

Step 1 Write a balanced chemical equation for the reaction. N 2 + 3H 2 → 2NH 3

Step 2 Write given and required information under the equation N 2 + 3H 2 → 2NH 3 m = 18 g m = ?

Step 3 Convert given information into moles. N 2 + 3H 2 → 2NH 3 m = 18 g m = ?

Step 4 Use the molar ratio from the balanced equation to go sideways. N 2 + 3H 2 → 2NH 3 m = 18 g m = ? Write the ratio using the number of moles over the balancing number.

Step 5 Convert moles into the required information. N 2 + 3H 2 → 2NH 3 m = 18 g m = ? Round off to the correct number of Sig Digs This is the answer for part a).

Part b) Mass of H 2 required. For part b) we need to repeat steps N 2 + 3H 2 → 2NH 3 m = 18 g m = ? Write the ratio. Now solve for mass of H 2

Most Stoich question involve 3 math steps (after writing the equation and listing given data). Step 1 - Turn given info into moles. Step 2 - Use the mole ratio to go sideways. Step 3 - Turn moles into the required info. It′s as easy as that!

Note: When calculating molar masses for compounds with a balancing number, for example the NH 3 below, N 2 + 3H 2 → 2NH 3 do not multiply by the balancing number. Remember molar mass means the mass of 1 mole.  molar mass of NH 3 = g/mol Balancing numbers are only used when you go sideways following the mole ratio.

example 2 - This time try to do the steps yourself before they are revealed g of aluminum metal reacts with excess copper (II) chloride solution. a) What mass of copper metal is produced? b) What mass of the other product would be made?

First write a balanced chemical equation and list the given and required data. 2Al + 3CuCl 2 → 2AlCl 3 + 3Cu m = 16.8 g m = ?

Step 1 - Turn given information into moles. 2Al + 3CuCl 2 → 2AlCl 3 + 3Cu m = 16.8 g m = ?

Step 2 - Go sideways. (Find moles of Cu) 2Al + 3CuCl 2 → 2AlCl 3 + 3Cu m = 16.8 g m = ? Write the mole ratio.

Step 3 - Turn moles into required info. (Find mass of Cu) 2Al + 3CuCl 2 → 2AlCl 3 + 3Cu m = 16.8 g m = ?

Part b) Step 2 - Find moles of AlCl 3. (Step 1 is already done) 2Al + 3CuCl 2 → 2AlCl 3 + 3Cu m = 16.8 g m = ? This is a 1 to 1 ratio. You can do it in your head, or write the mole ratio.

Part b) Step 3 - Find mass of AlCl 3. 2Al + 3CuCl 2 → 2AlCl 3 + 3Cu m = 16.8 g m = ?

Stoichiometry is Fun! Here′s your homework. Do the practice problems on pages 183, 184 and 186 #8