Mar. 29-Apr.3

This week, I learned about stoichiometry. This is the ratio of chemical reactants and products to one and another. For example, if 2NaCl + CO2 form 2(Na2)O and 1C(Cl2), then the ratios for NaCl to CO2 are 1:1, the ratios for CO2 to (Na2)O are 1:2, etc.

Anyone can say that 1 CO2 + 2 NaCl mean that there is one mole of carbon dioxide while there are two moles of sodium chloride. This, however, can only be true if the molar mass is equivalent to the number of moles (e.g. 1 mole oxygen=16 g). Therefore, it is possible for compounds to have different molar masses, hence number of moles as well.

A good example of this would be with 140 g of CO2 from the above chemical equation. For every 140g, how many moles of CO2 are there? To calculate the molar mass per mole, you must add the atomic masses of each element to each other. Knowing that diatomic oxygen has atomic mass of 32 (16*2) and that carbon has a molar mass of 12.11, you combine 32g to 12.11g to form 44.11 g/mole. But, as stated above, for every 140g of CO2, how many moles are there? To answer this question, set up the molar mass to 140g/X moles as a proportion. The answer is 3.17 moles of CO2. The steps are below:

But, this was, of course, is what we already know. This is just review. Now, let's move on to stoichiometry. In order to represent stoichiometry, we need to know the ratios first in the chemical reaction. Otherwise, if you don't balance the equation, you won't know the ratios, making it only harder to use stoichiometry. Even though it would be possible to still calculate the molar mass of the compounds, you wouldn't be able to identify the quantity of moles.

The next step after balancing the chemical equation is to calculate the molar mass of one of the reactants and find the moles of the other reactant and the products based on the ratios. In order to do so, represent this process using the BCA chart (bottom of the page).

In the problem above, propane is combined with oxygen gas. They yield the products of carbon dioxide and water. To balance the equation, look at the number of carbon in CO2 and propane. In propane, there are three, and in CO2, there is one. In order for this ratio to change from 3:1 to 3:3, there must be 3 CO2. The next step is to look at the hydrogen in propane and water. In propane, there are eight, and in water, there are 4 diatomic hydrogens. In order to keep the same number of hydrogens represented in the formulas and knowing water has diatomic hydrogen, there must be four diatomic hydrogen in order to have the same number of atoms as H8. Since the number of propane didn't have to change, there is only one molecule of propane. With 4 H2, there are four molecules of water. With three carbons, there are three molecules of carbon dioxide.

With one propane and five diatomic oxygen forming three carbon dioxide and four water, we can find the ratios.

However, the last part of the equation to balance is O2. In order to so, count the number of oxygen in water and carbon dioxide and combine them. Since there are three diatomic oxygen in carbon dioxide and 2 O2 (4 O=2 O2), there must have been five diatomic oxygen molecules to start out with.

The last step then is to do the calculations. Since it has been defined that there are 4.17 moles of propane, the moles of the other compounds is in sync with the ratio between propane and the other compounds. For example, propane to 5 O2 is 4.17 to 20.85 (4.17 * 5).

This is my favorite part. Under the before column, write down the number of moles the reactants start out. Then, for the products, write out 0 moles because the reactants haven't chemically combined through a combustion reaction yet. During the change column, this is the change in the number of moles from before to after (e.g. -4.17 moles of propane to represent a loss of moles on the reactant side). Since the moles are combining to form different compounds, the moles of the reactants would be zero in the after column since all the elements were used up with none to spare.

Then, for the products in the change, write the same number, except positive since it gained those moles, not lost them. In the after column, write the number of moles of the products.