PERCENT COMPOSITION

Curiosity,

Jeff Jordan, 2003

PERCENT COMPOSITION

EXAMPLE 1

A 45.70 g sample of an unknown substance is decomposed and found to contain 39.94 g of nitrogen and 5.76 g of hydrogen. What is the percent composition of the unknown compound?

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EXAMPLE 1

A 45.70 g sample of an unknown substance is decomposed and found to contain 39.94 g of nitrogen and 5.76 g of hydrogen. What is the percent composition of the unknown compound?

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Answer: 87.40% N and 12.60% H

EXAMPLE 2

A biologist discovers a plant that contains a substance that is a pain reliever. She collects a 45.3 g sample of the substance and has a chemist analyze it. It contains 29.2 g of carbon, 13.9 g of oxygen, and the rest is hydrogen. What is the percent composition of the unknown substance?

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EXAMPLE 2

A biologist discovers a plant that contains a substance that is a pain reliever. She collects a 45.3 g sample of the substance and has a chemist analyze it. It contains 29.2 g of carbon, 13.9 g of oxygen, and the rest is hydrogen. What is the percent composition of the unknown substance?

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EXAMPLE 2

A biologist discovers a plant that contains a substance that is a pain reliever. She collects a 45.3 g sample of the substance and has a chemist analyze it. It contains 29.2 g of carbon, 13.9 g of oxygen, and the rest is hydrogen. What is the percent composition of the unknown substance?

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EXAMPLE 2

A biologist discovers a plant that contains a substance that is a pain reliever. She collects a 45.3 g sample of the substance and has a chemist analyze it. It contains 29.2 g of carbon, 13.9 g of oxygen, and the rest is hydrogen. What is the percent composition of the unknown substance?

• How could you determine the mass of the hydrogen?
• Subtract the mass of C and O from the total mass!
• mass_H = mass_total – mass_C – mass_O

= 45.3 g – 29.2 g – 13.9 g = 2.2 g

EXAMPLE 2

A biologist discovers a plant that contains a substance that is a pain reliever. She collects a 45.3 g sample of the substance and has a chemist analyze it. It contains 29.2 g of carbon, 13.9 g of oxygen, and the rest is hydrogen. What is the percent composition of the unknown substance?

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EXAMPLE 2

A biologist discovers a plant that contains a substance that is a pain reliever. She collects a 45.3 g sample of the substance and has a chemist analyze it. It contains 29.2 g of carbon, 13.9 g of oxygen, and the rest is hydrogen. What is the percent composition of the unknown substance?

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Answer: 64.5% C, 30.7% O, and 4.9% H

USING FORMULAS TO DETERMINE PERCENT COMPOSITION

If you do not have the masses of the elements in a compound but do have the formula of the compound, you can use molar masses of each element and the total molar mass of the compound to determine the percent composition.

EXAMPLE 3

What is the percent composition of lithium sulfide?

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EXAMPLE 3

What is the percent composition of lithium sulfide?

But first, what is the formula for lithium sulfide?

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EXAMPLE 3

What is the percent composition of lithium sulfide?

But first, what is the formula for lithium sulfide?

• Li₂S

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EXAMPLE 3

What is the percent composition of lithium sulfide?

But first, what is the formula for lithium sulfide?

• Li₂S

Next, what is the molar mass of the Li₂S?

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EXAMPLE 3

What is the percent composition of lithium sulfide?

But first, what is the formula for lithium sulfide?

• Li₂S

Next, what is the molar mass of the Li₂S?

• 45.95 g/mol

EXAMPLE 3

What is the percent composition of lithium sulfide?

But first, what is the formula for lithium sulfide?

• Li₂S

Next, what is the molar mass of the Li₂S?

• 45.95 g/mol

Now we can calculate the percent composition of the compound!

EXAMPLE 3

What is the percent composition of lithium sulfide, Li₂S?

Answer: 30.2% Li and 69.77% O

YOUR TURN!

The assignment is posted on my website on today’s agenda. Do it on your own paper, and it is due tomorrow.