Chapter 6 Stoichiometry

Stoichiometry... I know what you're thinking. Sounds like some more hard chemistry right? Well lucky for you, you're wrong :) Once you get the hang of it you'll be flying through your stoichiometry questions. Anyways, stoichiometry is calculations involving reactions. (I know.. surprising right? =P) "Stoichio" is a greek word meaning element and "metry" means measurement. It is the study of the relationship between the amount of reactants used and the amount of products produced in the reaction.

An example would be..
1Zn + 2HCl -> 1ZnCl2 + 1H2
1:2:1:1 = the mole ratio

So...
1 molecule of Zn reacts with 2 molecules of HCl to produce 1 molecule of ZnCl2 and 1 molecule of H2.
Or...
1 mole of Zn reacts with 2 moles of HCl to produce 1 mole of ZnCl2 and 1 mole of H2.

*Always remember to balance the equation before you do anything else!!!
- This tells us the ratio of molecules or moles of the substances in a chemical reaction.
- The coefficient in a balanced equation is the number of moles reacted or produced.
- They can also be used as conversion factors.
*What you need over what you have or where you're going over where you start.

Here's an example question.
In photosynthesis plants convert carbon dioxide and water into glucose (C6H12O6) according to the following equation:

6 CO₂ (g) + 6H₂O (l) →6O₂ (g) + C₆H₁₂O₆ (aq)
*Notice that it's already balanced!!


Suppose you determine that a particular plant consumes 37.8 g CO2 in one week. Assuming that there is more than enough water present to react with all the CO2, what mass of glucose (in grams) can the plant synthesize from the CO2?

Given: mass of CO2 = 37.8 g
Unknown: mass of C6H12O6 (glucose)

It’s usually always a good idea to immediately convert all your quantities into moles right away:
37.8 g CO2 = 0.86 moles CO2
44.0 g/mol CO2

Look at the balanced chemical equation and use the mole ratio between CO2 and C6H12O6 just like the cake example. The mol ratio on the left is taken from the chemical equation

6 moles CO2 = 0.860 moles CO2
1 mole C6H12O6 x moles C6H12O6

X = 0.14 moles of glucose (C6H12O6)

Since we were asked to find the mass of glucose, multiply the moles of glucose produced by its molecular weight:
(Moles of glucose)(molecular weight of glucose) = mass of glucose
(0.143 moles)(180.2 g/mol) = 25.8 grams of glucose

Here's a video if you need someone to explain it further:

http://www.youtube.com/watch?v=Ouvzp_TMl-4

Thanks for reading!!