Thursday, March 10, 2011

Molarity Stoichiometry and Gas Stoichiometry

Molarity Stoichiometry

*Remember the Molarity Formula

Molarity = Moles/ Litres

There are three steps to complete when given a molarity calculation in stoichometry.
Step 1: Write a balanced equation
Step 2: Map the solution (find the route to get to the answer)
Step 3: Calculation

Example 1:

Given this chemical reaction, FeCl3 + Mg(SCN)2 à Fe(SCN)3 + MgCl2, how many moles of MgCl2 would be formed if 50.0ml of 0.200 M FeCl3 is reacted with sufficient Mg(SCN)2?

Step 1: Write a balanced equation

2FeCl3 + 3Mg(SCN)2 à 2Fe(SCN)3 + 3MgCl2

Step 3: Calculations

0.0500 L FeCl3 X 0.200 mole  = 0.0100 moles    ß Find the amount of moles in FeCl3
                               1L FeCl3

0.0100 moles FeCl3 X 3 mole MgCl2  = 0.0150 moles  ß convert moles of FeCl3 to
                                        2 mole FeCl3                                 moles of MgCl2

Answer: 0.0150 moles of MgCl2

Check out this youtube video for more problems on molarity and stoichiometry!

Gas Stoichiometry

*Remember: One mole of gas = 22.4L at STP

Example 1:

Given the formula Zn + HCl à ZnCl2 + H2, at STP what volume of H2 gas would be produced from 21.2g of HCl?

Step 1: Balance the equation
Zn + 2HCl à ZnCl2 + H2

Step 2: Change grams of HCl to moles of HCl
               2.12g HCl X 1 mole = 0.0581 mol HCl
                                  36.5g HCl

            Convert moles of HCl to moles of H2
            0.0581 moles HCl X 1 H2  = 0.2904 mol H2
                                              2 HCl

            Convert the moles of H2 to the volume of H2
            0.02904 mol H2 X 22.4L  = 0.651L
                                          1 mole

Answer: 0.651 L

If you are still confused check out this Youtube Video!!

But when in doubt follow your mole map!

Remember that you can convert between compounds or elements using the mole ratio!!!!


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