Worksheet: Percentage Composition

In this worksheet, we will practice calculating the percentage composition of a substance 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: 52.0%, Y : 48.0%
  • BX: 55.5%, Y: 44.5%
  • CX: 48.0%, Y: 52.0%
  • DX: 44.5%, Y: 55.5%
  • EX: 50.0%, Y: 50.0%

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?

  • AC∶70%, Si∶30%
  • BC∶75%, Si∶25%
  • CC∶30%, Si∶70%
  • DC∶25%, Si∶75%
  • EC∶50%, Si∶50%

Q5:

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

  • ACa: 58.9%, C: 17.6%, O: 23.5%
  • BCa: 20.0%, C: 60.0%, O: 20.0%
  • CCa: 20.0%, C: 20.0%, O: 60.0%
  • DCa: 40.0%, C: 12.0%, O: 48.0%
  • ECa: 33.3%, C: 33.3%, O: 33.3%

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?

  • APercentagemassofXmassofXmassofcompoundY=××100%
  • BPercentagemassofXmassofcompoundYmassofX=×100%
  • CPercentagemassofXmassofXmassofcompoundY=
  • DPercentagemassofXmassofXmassofcompoundY=×100%
  • EPercentagemassofXmassofcompoundYmassofX=

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?

  • APercentagemassofXrelativeatomicmassofXrelativeformulamassofcompoundY=××100%
  • BPercentagemassofXrelativeatomicmassofXrelativeformulamassofcompoundY=×100%
  • CPercentagemassofXrelativeatomicmassofXrelativeformulamassofcompoundY=
  • DPercentagemassofXrelativeformulamassofcompoundYrelativeatomicmassofX=×100%
  • EPercentagemassofXrelativeformulamassofcompoundYrelativeatomicmassofX=

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?

  • AKCl
  • BCsCl
  • CRbCl
  • DLiCl
  • ENaCl

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?

  • AH
  • BC
  • CO

Q15:

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

NameFormulaDensity (g/cm3)
NiacinCHNO6521.473
Pantothenic acidCHNO91751.266
Pyridoxal phosphateCHNOP81061.638
BiotinCHNOS1016231.267
4-Aminobenzoic acidCHNO7721.374

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.

NameFormulaDensity (g/cm3)
IbuprofenCHO131821.03
AcetaminophenCHNO8921.26
AspirinCHO9841.40
NaproxenCHO141431.20
CaffeineCHNO810421.23

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

Q17:

A vanadium compound has the molecular formula VOX3 where 𝑋 is a whole number. If the percentage by mass of V in VOX3 is 68%, what is the value of 𝑋?

Q18:

A compound with a relative formula mass of 84.2 is composed only of hydrogen and carbon. The compound is found to contain 85.6% carbon and 14.4% hydrogen by mass. What is the formula of this compound?

  • ACH612
  • BCH126
  • CCH714
  • DCH524
  • ECH816

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