Dilution calculations

Answer three multiple-choice questions about dilution math: prepare a dilution, determine whether withdrawal changes concentration, and calculate a serial dilution factor.

Question 1 of 3

You need 1,000 µL of a 1:5 dilution, defined as one specimen part in five total parts. Which volumes prepare it?

200 µL specimen + 800 µL diluent

Correct. The specimen volume is 1,000 ÷ 5 = 200 µL. The remaining 800 µL is diluent.

250 µL specimen + 750 µL diluent

Incorrect. These volumes total 1,000 µL, but 1,000 ÷ 250 gives factor 4.

200 µL specimen + 1,000 µL diluent

Incorrect. These volumes total 1,200 µL. The factor is 1,200 ÷ 200 = 6.

Read the lesson: Parts and dilution factor (opens in a new tab)

Question 2 of 3

A well-mixed tube contains 1,000 µL of solution at 50 mg/L. After 500 µL is withdrawn, what is the concentration of the remaining solution? Assume no evaporation or reaction.

25 mg/L

Incorrect. Withdrawal halves the dissolved amount and the volume together. Their ratio stays 50 mg/L.

50 mg/L

Correct. A representative aliquot removes solute and solvent in the same proportion. The remaining concentration is unchanged.

100 mg/L

Incorrect. The solute leaves with the withdrawn liquid, so it does not all remain in the smaller volume.

Read the lesson: Withdrawal and mixing (opens in a new tab)

Question 3 of 3

A specimen is diluted 1:5, and that diluted sample is diluted 1:5 again. Which dilution factor converts a result measured on the second tube into the original specimen concentration?

Incorrect. Factor 5 reverses only the second dilution, so it gives the concentration in the first tube. Multiply by another factor of 5 to recover the original concentration.

Incorrect. Adding the factors gives 10, but the second dilution acts on a mixture that is already one fifth of the original concentration. One fifth of one fifth is one twenty-fifth, so the factors multiply: 5 × 5 = 25.

Correct. Each 1:5 step leaves one fifth of the previous concentration, and one fifth of one fifth is one twenty-fifth. The result measured on the second tube multiplied by 25 gives the original specimen concentration.

Read the lesson: Serial factors multiply (opens in a new tab)