Terry W. answered • 03/01/18
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The first thing to do is to balance the chemical equation.
You start with the elements that appears only once on both sides of the equation which is carbon and hydrogen.
There are 4 carbon atoms in each molecule of butane, so for every molecule of butane burned you should get 4 molecules of CO2. There are 10 hydrogen atoms in a butane molecule and 2 in a water molecule, so for every butane molecule burned, you should get 10/2 = 5 molecules of water.
C4H10 + O2 -> 4CO2 + 5H2O
Then we add enough oxygen atoms to the left side to balance the equation. 4 CO2 molecules have 8 oxygen atoms and 5 H2O molecules have 5 oxygen atoms for a total of 13. That means we need 6.5 O2 molecules on the left.
C4H10 + 6.5 O2 -> 4CO2 + 5H2O
This equation is balanced and is actually more useful but doesn't look very pretty with that non-integer stoichiometry. For the sake of reality, we can't have partial molecules so we multiply both sides by 2 to get whole numbers:
2C4H10 + 13O2 -> 8CO2 + 10H2O
This is the final balanced and properly written chemical equation and we are ready to go on.
Since the stoichiometry of the equation above is in molar quantities we need the mass of reactants in moles not grams. We first look up the molecular weight of butane which is 58.12 g/mol. Then we convert 11.6g of butane to moles:
11.6/58.12 = 0.20 mol
Going back up to our more useful but not so nice looking first chemical equation, we see that 1 mole of butane when burned should generate 4 moles of CO2 and 5 moles of H2O.
Thus, 0.20 moles of butane should generate (0.20)(4) = 0.8 mole CO2 and (0.20)(5) = 1 mole of H2O
From there we use the definition of a mole and Avogadro constant (6.022 x 1023) to find how many molecule of each is produced:
CO2: (0.8)(6.022 x 1023) = 4.818 x 1023 molecules CO2
H2O: (1)(6.022 x 1023) = 6.022 x 1023 molecules H2O
