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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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