Worksheet: Percentage Composition

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In this worksheet, we will practice calculating the percentage composition of a material using chemical formulas or the masses of constituent elements.

Q1:

A 5.0 g sample of spinal fluid contains 3.75 mg (0.00375 g) of glucose. What is the percent by mass of glucose in spinal fluid?

Q2:

A bottle of a tile cleanser contains 135 g of HCl and 775 g of water. What is the percent by mass of HCl in this cleanser?

Q3:

A compound contains only the elements X and Y. A sample of the compound with a mass of 12.5 g is found to contain 6.50 g of X. What is the percentage composition of the compound?

  • AX: 44.5%, Y: 55.5%
  • BX: 50.0%, Y: 50.0%
  • CX: 55.5%, Y: 44.5%
  • DX: 48.0%, Y: 52.0%
  • EX: 52.0%, Y: 48.0%

Q4:

A 10 g sample of silicon carbide (SiC) is found to contain 3.0 g of carbon. What is the percentage composition of silicon carbide?

  • A C ∶ 7 0 % , S i ∶ 3 0 %
  • B C ∶ 7 5 % , S i ∶ 2 5 %
  • C C ∶ 3 0 % , S i ∶ 7 0 %
  • D C ∶ 2 5 % , S i ∶ 7 5 %
  • E C ∶ 5 0 % , S i ∶ 5 0 %

Q5:

What is the percentage composition of calcium carbonate (CaCO)3? State percentages to 1 decimal place.

  • A C a : 5 8 . 9 % , C : 1 7 . 6 % , O : 2 3 . 5 %
  • B C a : 2 0 . 0 % , C : 6 0 . 0 % , O : 2 0 . 0 %
  • C C a : 2 0 . 0 % , C : 2 0 . 0 % , O : 6 0 . 0 %
  • D C a : 4 0 . 0 % , C : 1 2 . 0 % , O : 4 8 . 0 %
  • E C a : 3 3 . 3 % , C : 3 3 . 3 % , O : 3 3 . 3 %

Q6:

To 1 decimal place, calculate the oxygen content of a water molecule as a percentage of the total mass.

Q7:

A compound Y contains a known mass of element X. What is the formula for expressing the mass of X in compound Y as a percentage of the total mass?

  • A P e r c e n t a g e m a s s o f X m a s s o f X m a s s o f c o m p o u n d Y = × × 1 0 0 %
  • B P e r c e n t a g e m a s s o f X m a s s o f c o m p o u n d Y m a s s o f X = × 1 0 0 %
  • C P e r c e n t a g e m a s s o f X m a s s o f X m a s s o f c o m p o u n d Y =
  • D P e r c e n t a g e m a s s o f X m a s s o f X m a s s o f c o m p o u n d Y = × 1 0 0 %
  • E P e r c e n t a g e m a s s o f X m a s s o f c o m p o u n d Y m a s s o f X =

Q8:

A compound Y contains one atom of element X per formula unit. The relative formula mass of Y is known. What is the formula for expressing the mass of X in compound Y as a percentage of the total mass?

  • A P e r c e n t a g e m a s s o f X r e l a t i v e a t o m i c m a s s o f X r e l a t i v e f o r m u l a m a s s o f c o m p o u n d Y = × × 1 0 0 %
  • B P e r c e n t a g e m a s s o f X r e l a t i v e a t o m i c m a s s o f X r e l a t i v e f o r m u l a m a s s o f c o m p o u n d Y = × 1 0 0 %
  • C P e r c e n t a g e m a s s o f X r e l a t i v e a t o m i c m a s s o f X r e l a t i v e f o r m u l a m a s s o f c o m p o u n d Y =
  • D P e r c e n t a g e m a s s o f X r e l a t i v e f o r m u l a m a s s o f c o m p o u n d Y r e l a t i v e a t o m i c m a s s o f X = × 1 0 0 %
  • E P e r c e n t a g e m a s s o f X r e l a t i v e f o r m u l a m a s s o f c o m p o u n d Y r e l a t i v e a t o m i c m a s s o f X =

Q9:

A sample of a compound composed of only elements X and Y has a mass of 50 g. After elemental analysis, it is shown that the sample contains 45 g of X. To the nearest percentage unit, what is the mass of X in the sample as a percentage of the total mass?

Q10:

A sample of a compound has a mass of 80 g and is composed entirely of elements X and Y. Elemental analysis shows that the sample contains 20 g of X. To the nearest percentage point, what is the mass of Y in the sample as a percentage of the total mass?

Q11:

A 1.497 g sample of a tin alloy is dissolved in nitric acid and solid metastannic acid (HSnO)23 is precipitated. Heating of the precipitate generates 0.4909 g of tin(IV) oxide, SnO2. Calculate, to 1 decimal place, the mass of tin in the original alloy as a percentage of the total sample mass.

Q12:

A 2.5624 g sample of a pure solid alkali metal chloride is dissolved in water and treated with excess aqueous silver nitrate. The resulting precipitate, filtered and dried, weighs 2.1813 g.

To 4 significant figures, what is the mass of the chloride ion in the alkali metal salt as a percentage of the total sample mass?

What is the molecular formula of the alkali metal salt?

  • A K C l
  • B C s C l
  • C R b C l
  • D L i C l
  • E N a C l

Q13:

Ethanol has the formula CHOH25.

As a percentage by mass, what is the oxygen content of the ethanol molecule, to 2 significant figures?

As a percentage by mass, what is the carbon content of the ethanol molecule, to 2 significant figures?

As a percentage by mass, what is the hydrogen content of the ethanol molecule, to 2 significant figures?

Q14:

Ethylene glycol has the formula HOCHCHOH22. Which element constitutes the largest percentage by mass of the ethylene glycol molecule?

  • A H
  • B C
  • C O

Q15:

The molecular formulas and densities of five B vitamins are shown in the table.

Name Formula Density (g/cm3)
Niacin C H N O 6 5 2 1.473
Pantothenic acid C H N O 9 1 7 5 1.266
Pyridoxal phosphate C H N O P 8 1 0 6 1.638
Biotin C H N O S 1 0 1 6 2 3 1.267
4-Aminobenzoic acid C H N O 7 7 2 1.374

Which of these vitamins contains the most molecules per unit volume?

  • ABiotin
  • B4-Aminobenzoic acid
  • CPyridoxal phosphate
  • DPantothenic acid
  • ENiacin

Which of these vitamins contains the fewest atoms per unit volume?

  • APyridoxal phosphate
  • BNiacin
  • C4-Aminobenzoic acid
  • DPantothenic acid
  • EBiotin

Which of these vitamins contains the most hydrogen atoms per unit volume?

  • ABiotin
  • BPantothenic acid
  • CPyridoxal phosphate
  • D4-Aminobenzoic acid
  • ENiacin

Which of these vitamins contains the most nonhydrogen atoms per unit volume?

  • ABiotin
  • BNiacin
  • CPyridoxal phosphate
  • D4-Aminobenzoic acid
  • EPantothenic acid

Q16:

The molecular formulas and densities of five compounds found in analgesic drugs are shown in the table.

Name Formula Density (g/cm3)
Ibuprofen C H O 1 3 1 8 2 1.03
Acetaminophen C H N O 8 9 2 1.26
Aspirin C H O 9 8 4 1.40
Naproxen C H O 1 4 1 4 3 1.20
Caffeine C H N O 8 1 0 4 2 1.23

Which of these compounds contains the fewest molecules per unit volume?

  • AIbuprofen
  • BAspirin
  • CAcetaminophen
  • DNaproxen
  • ECaffeine

Which of these compounds contains the most atoms per unit volume?

  • AAspirin
  • BNaproxen
  • CCaffeine
  • DIbuprofen
  • EAcetaminophen

Which of these compounds contains the fewest hydrogen atoms per unit volume?

  • AAcetaminophen
  • BAspirin
  • CIbuprofen
  • DNaproxen
  • ECaffeine

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