General Chemistry – Stoichiometry – Percent Composition of Compounds


In this video we’re gonna calculate percent
composition. Percent composition is the percentage of each element present in a pure substance.
Let’s take a simple problem. Calculate the percent composition by mass of glucose. C6
H12 O6. The first step is to actually calculate the molar mass of glucose. To do this we’ll
look at the carbon, hydrogen, and oxygen and look at the number of moles of each of these
elements in the compounds so C6 H12 O6. We can see that we have these quantities to get
the amounts on each one of these we’re going to need to multiply by their atomic mass in
grams per mole so 12.011 grams per mole, for oxygen 1.008 grams per mole, and for oxygen
15.999 grams per mole. Moles’ll cancel out. We’ll end up with 72.066 grams of carbon per
mole of glucose. For hydrogen 12.096 grams of hydrogen per mole of glucose and for oxygen
95.994 grams of oxygen per mole of glucose. We can add these three together and we’ll
get a molar mass of glucose of 180.156 grams per mole. So that gets the smaller mass. To
get the percent composition what we’ll have to do is look at the mass percentage of each
of these elements. For carbon the mass percent of carbon is going to be equal to the mass
of the carbon in glucose 72.066 grams of carbon per mole of glucose divided by 180.156 grams
per mole of glucose times as we want this in percentage times 100% for carbon it’ll
give us 40.001%. If we do the same calculation for hydrogen–mass percent of hydrogen is
going to be equal to the weight of the hydrogen in glucose over the molar mass equals in this
case times 100% will equal in this case 6.7142% and finally if we look at the mass percent
for oxygen we’ll have 95.994 grams of oxygen per mole of glucose over the molar mass 180.156
grams per mole of glucose times 100% and we will do the calculation and we get 53.284%.
So therefore the percentage composition for glucose is going to be 40% carbon 6.7142%
for hydrogen and 53.284% for oxygen.

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