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• 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 isoxygen andhydrogen?

• No, not if were 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. Well 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, well 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%.

Most compounds are made up of different kinds of atoms. For instance, water is made of of

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B2 glucose molar mass molar hydrogen oxygen mole

# Chemistry: Percent Composition with 2 examples | Homework Tutor

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林宜悉 posted on 2020/03/06
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