Lesson Worksheet: Percentage Composition Chemistry • 10th Grade

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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. [C=12g/mol, O=16g/mol, Ca=40g/mol]

  • 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. [H=1g/mol, O=16g/mol]

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?

  • APercentagemassofX = massofXmassofcompoundY××100%
  • BPercentagemassofX = massofcompoundYmassofX×100%
  • CPercentagemassofX = massofXmassofcompoundY
  • DPercentagemassofX = massofXmassofcompoundY×100%
  • EPercentagemassofX = massofcompoundYmassofX

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?

  • APercentagemassofX = relativeatomicmassofXrelativeformulamassofcompoundY××100%
  • BPercentagemassofX = relativeatomicmassofXrelativeformulamassofcompoundY×100%
  • CPercentagemassofX = relativeatomicmassofXrelativeformulamassofcompoundY
  • DPercentagemassofX = relativeformulamassofcompoundYrelativeatomicmassofX×100%
  • EPercentagemassofX = relativeformulamassofcompoundYrelativeatomicmassofX

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?

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