Stoichiometry calculations along with Molarity and STP!!

Remember molarity from chapter 4? Well, we're going to apply that to stoichiometry now!

Review: Molarity = mole

                              Litres

So, you can change it to Litres = mole       OR           mole = Molarity x Litres

                                                     Molarity

Lets try an example.

Ex.1 Consider the following reaction: Zn + 2HCl --> ZnCl2 + H2

How many moles of hydrogen gas are needed to completely react with 15 g of zinc?

Step 1:  The reaction is given to you and is already balanced.

Step 2: Your mole map route is going to be from grams of zinc--> moles of zinc --> moles of H2

Step 3: 15g of zinc  x  1 mol zinc  x  1 mol H2  = 0.23 mol of H2 (don't forget sigfigs)

                                     65.4g zinc       1 mole zinc 

Ex. 2 Using the same reaction from above,

What is the molarity of HCl if 50 L of HCl is reacted to form 6.0 moles of H2

Step 1: The reaction is balanced.

Step 2:  Map route will be from moles of H2 --> moles of HCl. Then take the molarity of HCl.

Step 3:  6.0 moles H2  x  2 moles HCl  = 12 moles HCl

                                           1 mole H2

Now that you have moles of HCl, you can find out the molarity by putting moles over litres.

Molarity = 12 moles HCl

                    50 L HCl

              = 0.24 M HCl

Now, the last thing is to calculate stoichiometry along with STP.

Ex. 3 With the following reaction: N2 + 3H2 --> 2NH3

Calculate the number of grams of nitrogen gas required to make 1.44L of ammonia at STP.

1.44 L ammonia  x  1 mole ammonia  x  1 mole N2  x  14.0 g N2  =  0.450 g of N2

                                         22.4 L              2 moles NH3   1 mole N2

Here is a video that may help you!