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CHEMISTRY, M2 EQ-Bank 10 MC

When measuring very low concentrations of pollutants in water, which unit is most suitable?

  1. Molarity (mol L\(^{-1}\))
  2. Parts per million (ppm)
  3. Percentage by mass
  4. Volume percent (% v/v)

 

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\(B\)

Show Worked Solution

→ Parts per million (ppm) is a unit used to express very low concentrations. One ppm means that there is 1 part of solute per 1,000,000 parts of solution. This unit is ideal for detecting small amounts of pollutants in large volumes of water, where concentrations are generally too low to be expressed effectively in molarity or percentage terms.

→ Molarity is a common unit for concentration but is better suited for more moderate to high concentrations in laboratory and industrial contexts, rather than for trace pollutants in water.

→ Percentage by mass and volume percent (% v/v) are typically used for relatively larger concentrations and are not as effective for expressing concentrations as low as parts per million.

\(\Rightarrow B\)

CHEMISTRY, M2 EQ-Bank 11

A student is investigating the concentration of copper ions in a water sample collected from a local river. They use an instrument to determine that the sample contains copper ions at a concentration level of 1.75 ppm.

  1. Calculate the mass of copper ions in a 2 L sample of water.   (2 marks)
  1. Explain why parts per million is a suitable unit for measuring low concentrations of ions in environmental samples like river water.   (2 marks)
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a.    \(3.5\ \text{mg}\)

 

b.    Benefits of using ppm as a concentration measurement.

→ Parts per million (ppm) is suitable for measuring low concentrations of ions because it represents the mass of solute per million parts of solution, making it ideal for detecting trace levels of substances.

→ This unit allows for easy comparison of small concentrations, which is especially useful in environmental studies where contaminants are present in very low amounts.

Show Worked Solution

a.    \(1\ \text{ppm} = 1\ \text{mg/L}\)

\(\therefore \ce{[Cu^{2+}]} = 1.75\ \text{mg/L}\)

\(m\ce{(Cu^{2+})} = 1.75 \times 2 = 3.5\ \text{mg}\)

 

b.    Benefits of using ppm as a concentration measurement.

→ Parts per million (ppm) is suitable for measuring low concentrations of ions because it represents the mass of solute per million parts of solution, making it ideal for detecting trace levels of substances.

→ This unit allows for easy comparison of small concentrations, which is especially useful in environmental studies where contaminants are present in very low amounts.