Q: Which equation would you use to calculate the percent composition of an element in a compound from its chemical formula?

$$\text{Percent by mass} = \frac{\text{mass of element in 1 mole}}{\text{molar mass of compound}} \times 100$$

Question 1

A 20 g sample of sucrose contains 8.44 g of carbon. What is the percent by mass of carbon in sucrose? Use the formula $$	ext{Percent by mass} = \frac{	ext{mass of element}}{	ext{mass of compound}} 	imes 100.$$

Accepted Answer

42.2%

Question 2

Which of the following best describes the percent composition of a compound?

Accepted Answer

The percent by mass of each element in a compound.

Question 3

Sodium hydrogen carbonate ($$	ext{NaHCO}_3) is $$used in antacids. What is the molar mass of $$	ext{NaHCO}_3$$? (Na: 23.0 g/mol, H: 1.0 g/mol, C: 12.0 g/mol, O: 16.0 g/mol)

Accepted Answer

$$84.0\ 	ext{g/mol}$$

Question 4

If a compound has a molar mass of $$100\ 	ext{g/mol}$$ and contains $$40\ 	ext{g} of $$oxygen per mole, what is the percent composition of oxygen?

Accepted Answer

40%

Question 5

A chemist analyzes a sample of magnesium oxide ($$	ext{MgO}) $$and finds it contains $$24.3\ 	ext{g} of $$magnesium and $$16.0\ 	ext{g} of $$oxygen. What is the percent by mass of magnesium in the sample?

Accepted Answer

60.3%

Question 6

Which equation would you use to calculate the percent composition of an element in a compound from its chemical formula?

Accepted Answer

$$	ext{Percent by mass} = \frac{	ext{mass of element in 1 mole}}{	ext{molar mass of compound}} 	imes 100$$

Question 7

A student wants to maximize the yield of carbon dioxide in a reaction using sodium hydrogen carbonate ($$	ext{NaHCO}_3). If $$she uses $$84\ 	ext{g} of$$ $$	ext{NaHCO}_3$$, how many grams of carbon are present in her sample? (Na: 23.0 g/mol, H: 1.0 g/mol, C: 12.0 g/mol, O: 16.0 g/mol)

Accepted Answer

12.0 g

Question 8

Which of the following statements is TRUE regarding percent composition?

Accepted Answer

Percent composition can be determined from the chemical formula and molar masses.

Question 9

A $$50\ 	ext{g}$$ sample of a compound contains $$10\ 	ext{g} of $$hydrogen. What is the percent by mass of hydrogen?

Accepted Answer

20%

Question 10

Which real-life scenario best illustrates the importance of percent composition calculations?

Accepted Answer

Determining the amount of active ingredient in a medication.

Question 11

A sample of calcium chloride ($$	ext{CaCl}_2) $$weighs $$110\ 	ext{g}. If $$the molar mass of $$	ext{CaCl}_2 is$$ $$110\ 	ext{g/mol}$$, what is the percent by mass of calcium in the compound? (Ca: 40.1 g/mol, Cl: 35.5 g/mol)

Accepted Answer

36.5%

Question 12

Which of the following is NOT a step in calculating percent composition from a chemical formula?

Accepted Answer

Divide the number of atoms of each element by the total number of atoms.

Question 13

A $$10\ 	ext{g}$$ sample of a compound contains $$2\ 	ext{g} of $$nitrogen. What is the percent by mass of nitrogen?

Accepted Answer

20%

Question 14

If a compound contains $$12\ 	ext{g} of $$carbon and $$32\ 	ext{g} of $$oxygen, what is the percent by mass of carbon in the compound?

Accepted Answer

27.3%

Question 15

A $$100\ 	ext{g}$$ sample of a compound contains $$60\ 	ext{g} of $$sodium. What is the percent by mass of sodium?

Accepted Answer

60%

Question 16

Which of the following would increase the percent by mass of an element in a compound?

Accepted Answer

Increasing the mass of the element while keeping the total mass constant.

Question 17

A $$25\ 	ext{g}$$ sample of a compound contains $$5\ 	ext{g} of $$potassium. What is the percent by mass of potassium?

Accepted Answer

20%

Question 18

Which of the following is a correct application of the percent composition formula in a medical context?

Accepted Answer

Calculating the percent by mass of iron in a blood supplement to ensure proper dosage.

Question 19

A $$60\ 	ext{g}$$ sample of a compound contains $$15\ 	ext{g} of $$sulfur. What is the percent by mass of sulfur?

Accepted Answer

25%

Question 20

A $$100\ 	ext{g}$$ sample of a compound contains $$30\ 	ext{g} of $$chlorine. What is the percent by mass of chlorine?

Accepted Answer

30%

Percent Composition in Compounds: Calculations and Applications

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