Skip to content
SearchProgress
Display

Display

Theme
Density
Text size

Sign in

Add your earlier progress to your account?

Study progress is waiting to be saved

Aliquots, withdrawal and mixing

12 min

  • Calculate the amount of solute in an aliquot
  • Predict the concentration after withdrawing a well-mixed aliquot
  • Mix a dilution before transferring the next aliquot

Read the full reference

Try first

Try first

A solution holds 40 mg/L of a substance. How much of the substance is in a 500 µL aliquot of it?

The next section explains it.

The next section explains it.

Right. The next section explains why.

The next section explains it.

Get the idea

Amount from concentration

A concentration is an amount per volume, so an amount is a concentration multiplied by a volume:

amount = concentration × volume

The volume has to be in the unit the concentration is written per. A concentration in mg/L needs the volume in liters: 2,000 µL is 0.002 L, and at 150 mg/L it holds 150 × 0.002 = 0.3 mg.1 Multiplying 150 by 2,000 without converting gives 300,000, a million times too large, and nothing in the arithmetic warns you.

Aliquots and withdrawal

A solution is uniform throughout, so every part of it has the same composition as the whole.1 Three things follow:

  • An aliquot, a measured portion taken from a well-mixed tube, has the same concentration as the tube. Its smaller volume holds proportionally less of the substance.
  • Adding diluent lowers a concentration because it adds volume without adding substance.1
  • Withdrawal takes volume and substance out together. Taking 300 µL out of 1,200 µL removes a quarter of the volume and a quarter of the substance, so the 900 µL left behind is exactly as concentrated as before. Withdrawing liquid for the next tube does not dilute the tube, so its step factor stays the same.

Keep two volumes apart in your records:

VolumeWhat it sets
Prepared, when the tube was madeThe tube's factor
Remaining, after withdrawalsHow much is left to use

Mix before you draw

All of this holds only once the tube is mixed. Right after a transfer, the added solution and the diluent have not yet blended, and liquid at the pipette tip can be stronger or weaker than the tube as a whole.1 An aliquot drawn then carries an unknown concentration into the next tube, and every factor calculated after it describes a series that was never made.

So each step runs in the same order:

  1. Mix the source.
  2. Transfer.
  3. Mix the new tube completely.
  4. Only then draw from it.
References
  1. Flowers P, Theopold K, Langley R, Robinson WR. Molarity. In: Chemistry 2e. OpenStax; 2019. Accessed September 23, 2026. https://openstax.org/books/chemistry-2e/pages/3-3-molarity

Watch one

A well-mixed tube holds 2,000 µL of a solution at 60 mg/L. You withdraw 800 µL for the next step. What is the concentration of the 1,200 µL left in the tube?

  1. Convert: 2,000 µL = 0.002 L, 800 µL = 0.0008 L and 1,200 µL = 0.0012 L.

    The concentration is written per liter, so the volumes must be in liters too.

  2. Amount in the tube: 60 mg/L × 0.002 L = 0.12 mg, which is 120 µg.

    Knowing the amount in the whole tube shows what the withdrawal takes away.

  3. Amount withdrawn: 60 mg/L × 0.0008 L = 0.048 mg, which is 48 µg.

    A portion of a mixed solution carries its share of the substance.

  4. What stays is 120 − 48 = 72 µg in 0.0012 L, and 0.072 mg ÷ 0.0012 L = 60 mg/L.

    Amount and volume both fell, so the concentration is what remains of each.

The remaining 1,200 µL is still 60 mg/L. The withdrawal lowered the amount of substance and left the concentration unchanged.

Your turn

Problem 1 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.

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

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

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

Hint
  1. Ask what leaves the tube along with the 500 µL of liquid.
  2. The withdrawn liquid carries half the substance, and half the liquid stays behind.
  3. Divide what remains of the substance by what remains of the volume.

Review Withdrawal and mixing

Problem 2 of 3

A mixed solution holds 250 mg/L. How many micrograms of the substance are in a 400 µL aliquot?

Hint
  1. Write the aliquot's volume in liters, the unit the concentration is per.
  2. The aliquot is 0.0004 L. Multiply it by the concentration to get milligrams.
  3. One milligram is 1,000 µg.
Show the answer

100 µg

The aliquot holds 250 mg/L × 0.0004 L = 0.1 mg, which is 100 µg. Multiplying 250 by 400 without converting the microliters gives 100,000, which is 1,000 times the correct 100 µg.

Review Concentration before a serial dilution

Problem 3 of 3

Put one step of a serial dilution in order, from the tube you already have to the aliquot for the tube after next.

  1. Mix the source tube completely.
  2. Draw the aliquot for the following tube from the new tube.
  3. Mix the new tube until the solution is uniform.
  4. Transfer the set volume from the source tube into the next tube, which holds its diluent.
Show the answer

Mix the source tube completely., Transfer the set volume from the source tube into the next tube, which holds its diluent., Mix the new tube until the solution is uniform., Draw the aliquot for the following tube from the new tube.

Each draw comes from a tube that has just been mixed: the source before the transfer, and the new tube before anything is taken from it. An aliquot from an unmixed tube carries an unknown concentration into every tube after it.

Review Withdrawal and mixing

Use it

Try this at the Dilution stationPendingOriginal · mg/L100Source concentrationThree twofold stepsDilute specimen

Keep

Sources checked