Most compounds are made up of different kinds

of atoms. For instance, water is made of of hydrogen atoms and oxygen atoms. One water

molecule, H2O, is made up of 3 atoms: 1 oxygen atom and 2 hydrogen atoms. But would we say

that water is ⅓ oxygen and ⅔ hydrogen? No, not if we’re talking about % composition

by mass. That’s because hydrogen and oxygen atoms weigh different amounts – they have

different gram atomic masses, as you can see from the periodic table. the Percent composition of a compound is the percent of the total mass of the compound that is due to each

component. The atoms of each element weigh different amounts, so we need to look at the

periodic table to find the percent composition by mass. We’ll calculate the mass of a mole

of the compound, as well as the mass due to each element in the compound. If you add up

the % compositions for all the elements in a compound, it should sum to 100%. Let’s do this for water. First, we find the molar mass of water from

the periodic table by summing up the gram atomic masses from each element. 2x (1.008

g/mol) (that’s the hydrogens) + 1x (15.999 g/mol) (that’s the oxygen)=18.015 g/mol.

Now what percent of the total molar mass of water is due to hydrogen? The hydrogen % composition

is the mass due to hydrogen divided by the total molar mass, times 100%. In each water

molecule, there are 2 atoms of hydrogen. So the total mass of hydrogen in a mole of water

is 2(1.008)=2.016 grams. What percent of the total molar mass of water is that? Divide

it by the molar mass of water, and multiply by 100%. 2.016/18.015 x 100%=11.19%. The rest of the mass must be due to oxygen,

right? 100% – 11.19%=88.81% of the mass of water is due to oxygen. But let’s check,

just to make sure. There is one atom of oxygen in each water molecule, so that is 1 x 15.999

=15.999 grams of oxygen in each mole of water. To find the % of water’s molar mass due

to oxygen, we’ll divide that by the total molar mass and multiply by 100%. 15.999/18.015

x 100%=88.81%, on the nose. The percentages DO add up to 100% (11.19 + 88.81=100%),

so we didn’t mess up anywhere. For a more complicated example, let’s look

at GLUCOSE. We usually think of glucose as a single thing – it’s one sugar, a monosaccharide

– and it is ONE molecule. But it’s made up of three kinds of atoms: carbon, hydrogen,

and oxygen. Its formula is C6H12O6. So one molecule of glucose has 6 atoms of carbon,

12 atoms of hydrogen, and 6 atoms of oxygen. What is the % composition by mass of glucose?

That is to say, what % of its mass is from carbon, what % of its mass is hydrogen, and

what % of its mass is oxygen? Step 1:

To find the molar mass of glucose, C6H12O6, sum up the gram atomic masses of the individual

atoms. Again, we get that from the Periodic Table. There are 6 atoms of carbon, 12 atoms

of hydrogen, and 6 atoms of oxygen, so, the molar mass of glucose equals:

(6 x 12.011 g/mol) + (12 x 1.008 g/mol) + (6 x 15.999 g/mol )=180.156 g/mol. Step 2: Find the % Mass of each element in

Glucose We will take the mass of each element in 1

mole of glucose, divide it by the molar mass of glucose, then multiply by 100%. First, % mass from carbon:

The mass of Carbon in 1 mole of glucose divided by the molar mass of glucose (the mass of

1 mole of glucose) times 100% is 6(12.011)g / 180.156g x 100%.

=72.066g/ 180.156g x 100%=0.400 x 100%

=40.0% So Glucose is 40% carbon, by mass. Next, the % mass from hydrogen

The mass of Hydrogen in 1 mole of glucose divided by the molar mass of glucose times

100% is 12(1.008) g / 180.156g x 100%

=12.096g/180.156g x 100%=0.067 x 100%

=6.7% Glucose is 6.7% hydrogen, by mass. And finally, the % mass from oxygen:

The mass of oxygen in 1 mole of glucose divided by the molar mass of glucose, times 100% is

6(15.999)g / 180.156g x 100%=95.994g /180.156g x 100%

=0.533 x 100%=53.3% Glucose is 53.3% oxygen, by mass. Let’s check to make sure these three % compositions

add to 100%: 40% + 6.7% + 53.3%=100%.

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