A water sample was diluted 10-fold before analysis by colorimetry. The diluted sample had an absorbance of 0.40, which a calibration curve showed is a concentration of 2.0 mg/L. What was the concentration of the original sample?

Answer and explanation

Correct answer: D. 20 mg/L

  1. Step 1: The calibration curve gives the concentration of the solution that was measured: 2.0 mg/L.
  2. Step 2: The sample was diluted 10-fold, so multiply by the dilution factor: 2.0 x 10 = 20 mg/L.
Answer
20 mg/L.
Why not the others: 0.20 mg/L divides instead of multiplies; 2.0 mg/L ignores the dilution; 200 mg/L multiplies by 100.

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