Friday, March 4, 2011

Stoichiometry (Pronounced: stoi'kē-ŏm'ĭ-trē)

The main idea of stoichiometry is to measure the amounts of elements and compounds that are involved in a reaction. (Ex/ the relationship between the amounts of reactants and products in a reaction)

Mole Ratio is something that will be used in pretty much ever stoichiometry related question that you will need to solve this year. It can be found by looking at the coefficients of a balanced equation.

4NH3 + 5O2 -> 6H2O + 4NO

Mole ration of this equation:
4 : 5 : 6 : 4

If a question asks you to use the information provided in the equation to find the grams of NO produced if 3.0 moles of O2 were combined with excess 4NH3...

3.0 mol O2          4 mol NO       30.0 g NO
                       X -------------- X -------------- = 72 g NO should be produced
                            5 mol O2       1 mol NO

**The bolded part of the work shown above is where mole ratios come in.**

  • We start with 3.0 mol O2 because that's only solid piece of information we are given. 
  • We use mole ratio to determine the amount of O2 and NO needed for a reaction to occur successfully. We do this by looking at the balanced equation. We see that the coefficient of O2 is 5 and the coefficient of NO is 4. That is the ratio of O2 : NO. 
  • Add that to your work as the next step. Doing this has allowed us to figure out the moles for NO required for 3.0 moles of O2 to react successfully in this particular situation. We now need to convert moles of NO into grams, because that what the question asked us for. 
  • Using the periodic table, we see that one mole of NO is 30.0g. Add this to the work, and calculate. 
  • We have determined the grams of NO that should be produced if 3.0 moles of O2 were to be reacted. 
If you still don't understand how to do stoichiometry, have a look at these images below which outline the key points.

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