Grade 10 · Science · Chemistry

10th grade Percent composition of a compound worksheet

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Percent composition of a compound

Grade 10 Science Worksheet - Challenge Me

NameDate

Circle the best answer, or write your answer on the line.

A lab analysis of 10 g of table sugar finds 4.21 g of carbon. What percent carbon is that, to 0.1%, Esme?
A lab analysis of 9 g of water finds 1.01 g of hydrogen. What percent hydrogen is that, to 0.1%, Jun?
A lab analysis of 12 g of table salt finds 4.72 g of sodium. What percent sodium is that, to 0.1%, Kira?
4.
Bo says water is 66.7% hydrogen because 2 of its 3 atoms are hydrogen. Which reply fits, Luz?
- Agree, for small samples of water
- Agree: percent composition counts atoms, not their masses
- Disagree: by mass, hydrogen is just 11.2%
5.
Zee says the element percents in a compound add to 100%. Which reply fits, Noah?
- Disagree: they add to the molar mass, 180.18
- Disagree: each element is 100% of itself, so they add to 300%
- Agree: glucose's 40.0% + 6.7% + 53.3% = 100%
6.
Glucose has 6 C atoms at 12.01 g/mol in 180.18 g/mol. What percent of its mass is carbon, Pia?
- 40.0%
- 25.0%
- 6.7%
7.
Glucose has 12 H atoms at 1.01 g/mol in 180.18 g/mol. What percent of its mass is hydrogen, Ren?
- 0.6%
- 50.0%
- 6.7%
8.
Two labs test pure water from two cities. What percent hydrogen should both find, Zee?
- Different values, since each city's water is different
- 66.7%, since 2 of 3 atoms are H
- 11.2%, since pure water is always H₂O
9.
A sample sold as pure salt tests at 45.0% sodium. What does that suggest, Ari?
- It is pure salt, since salt's percent can vary
- It is not pure salt, which is 39.3% sodium
- It is pure salt from a larger sample
10.
Which percent bar fits carbon dioxide (CO₂), Bo?
- C 27.3%, O 72.7%
- C 50.0%, O 50.0%
- C 33.3%, O 66.7%

10th grade Percent composition of a compound worksheet – printable — Print: 1 2 3 4 5 6

About this worksheet

Find the percent composition of a compound. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

I can find the percent by mass of an element: its grams in 1 mol ÷ molar mass × 100 (water is 11.2% H, 88.8% O).
I can find the grams of an element in a sample from its percent, and a percent from lab masses.
I can explain that percent is by mass, not by atom count, and stays the same for any amount of a pure compound.

Standard: none (TX)

Words to know

percent composition: The share of a compound's mass that each element makes up

percent by mass: Grams of an element in each 100 g of the compound

percent bar: A bar split into each element's share of the mass

pure compound: One substance with a fixed formula, like H₂O

Common mistakes

Your child used one atom's mass. Multiply by the subscript: water has two H atoms, so 2 × 1.01 = 2.02 g.

Your child divided the wrong way. Percent = element mass ÷ whole mass × 100, so it is always under 100%.

Your child used the other element's share. Read which element the question asks about.

Questions parents ask

What is the most common mistake with percent composition of a compound?

Your child used one atom's mass. Multiply by the subscript: water has two H atoms, so 2 × 1.01 = 2.02 g.

Are these 10th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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