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Lesson Worksheet: Determining Purity Chemistry

In this worksheet, we will practice determining the purity of a substance based on its proportion by mass and physical properties.


What is the equation that gives percentage purity in terms of the mass of the pure chemical and the mass of the sample?

  • APercentagepurity = massofsamplemassofpurechemicalmassofsample×100%.
  • BPercentagepurity = massofsamplemassofpurechemical×100%.
  • CPercentagepurity = massofpurechemicalmassofsample×100%.
  • DPercentagepurity = massofsamplemassofpurechemical.
  • EPercentagepurity = massofpurechemicalmassofsample.


A mixture of nitrogen and oxygen gas has a mass of 1.20 kg. After 250 g of oxygen in the mixture is reacted with a metal, pure nitrogen is left behind. To 2 significant figures, what is the percentage purity of nitrogen in the original mixture?


An impure sample of aluminum bromide contains 4%‎ contaminants by weight. What is the percentage purity of the sample?


An impure sample of magnesium chloride has a mass of 50 g. After perfect purification, 45 g of magnesium chloride is recovered. What is the percentage purity of the original sample?


Which of the following statements best describes what an impurity is?

  • AAn unwanted substance mixed within a desired substance
  • BA wanted substance mixed within a desired substance
  • CA substance finely dispersed in another substance
  • DA substance dissolved in another substance
  • EA substance that can be separated from another substance


Which of the following is an effect that impurities do not typically have on the boiling or melting point of a substance?

  • AMaking the melting point occur over a larger temperature range
  • BDeviating from the reference value for the melting or boiling point
  • CMaking the boiling point occur sharply at a definite temperature
  • DIncreasing the boiling point
  • EDecreasing the melting point


Which of the following substances can be considered chemically pure?

  • ABrass
  • BSeawater
  • CAir
  • DApple juice
  • EA bar of 24-karat gold


A student obtains a sample of water and wants to test its purity by determining its melting and boiling points.

What results should the student expect to find if the sample is pure?

  • AThe sample boils at exactly 100C and freezes at exactly 0C.
  • BThe sample boils between 99C and 101C and freezes between 1C and 1C.
  • CThe sample boils and freezes over a large temperature range.
  • DThe sample boils over a large temperature range but freezes at exactly 0C.

How can the student obtain pure water if the sample is found to contain soluble impurities?

  • AFiltration
  • BTitration
  • CDistillation
  • DCrystallization


The image below shows a labeled bottle of orange juice.

Why might the company claim the orange juice is 100% pure?

  • AIt has a pH of 7, making it neutral.
  • BIt contains no added or artificial products.
  • CIt is made up of any one type of compound.
  • DThe solution is completely orange in color.
  • EThe oranges were organically grown.

Why might a chemist say the orange juice is not pure?

  • AIt only contains an artificial substance.
  • BThe particles are free to move around each other.
  • CIt does not contain just one substance.
  • DIt is slightly acidic.
  • EThe solution is not colorless.


5 g of salt is mixed with 20 g of powdered sugar. Which of the following statements best describes this mixture?

  • ASugar is considered an impurity, and its percentage is 80%.
  • BEither sugar or salt can be an impurity interchangeably.
  • CSalt is considered an impurity, and its percentage is 25%.
  • DSugar is considered an impurity, and its percentage is 75%.
  • ESalt is considered an impurity, and its percentage is 20%.

This lesson includes 17 additional questions and 4 additional question variations for subscribers.

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