You need 20\ mL of a 0.25\ mol dm^-3 solution from a 1.0\ mol dm^-3 stock. Find the volume of stock needed. [2 marks]

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How do you measure out a few millionths of a litre accurately, and make up a solution to the concentration you need?

Describe how to use a micropipette accurately and how to prepare a solution of a required concentration

A focused answer to the O-Level outcome on micropipetting and solutions. Using a micropipette correctly, avoiding errors, and calculating how to make a solution of a required concentration.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

This outcome asks you to use a micropipette accurately and to prepare a solution of a required concentration. These are bread-and-butter laboratory skills in biotechnology: almost every practical depends on transferring tiny, precise volumes and on making up solutions correctly, so examiners test both the technique and the calculation.

The answer

Using a micropipette

A micropipette measures and transfers very small volumes, typically in microlitres. To use one accurately:

Set the volume on the dial, staying within the pipette's range.
Fit a clean tip.
Press the plunger to the first stop and hold it.
Place the tip just below the liquid surface and release the plunger slowly to draw the liquid up.
Move to the destination and press to the first then the second stop to expel all the liquid.
Use a fresh tip when changing liquids to avoid contamination.

Sources of inaccuracy

Common errors that spoil the volume include:

Drawing the liquid up too fast or releasing the plunger too quickly, causing bubbles or the wrong volume.
Setting the wrong volume, or going outside the pipette's range.
Not changing the tip, or using a leaking or wrong-sized tip.

Concentration

The concentration of a solution is the amount of solute dissolved in a given volume of solution. A more concentrated solution has more solute per unit volume.

The dilution relationship

To make a less concentrated solution from a more concentrated stock, use:

C_1 V_1 = C_2 V_2

where $C_1$ and $V_1$ are the concentration and volume of the stock, and $C_2$ and $V_2$ are the concentration and volume of the diluted solution. Rearrange to find whichever quantity you need, then top up to the final volume with water.

Preparing a diluted solution

You need $50\ \text{mL}$ of a $0.1\ \text{mol dm}^{-3}$ solution from a $0.5\ \text{mol dm}^{-3}$ stock. Find the volume of stock and the volume of water.

Step 1: Write down the known values

$C_1 = 0.5$ , $C_2 = 0.1$ , and $V_2 = 50\ \text{mL}$ . We want $V_1$ , the volume of stock.

Step 2: Rearrange the dilution equation

V_1 = \frac{C_2 V_2}{C_1}

Step 3: Substitute and calculate

V_1 = \frac{0.1 \times 50}{0.5} = \frac{5}{0.5} = 10\ \text{mL}

Step 4: Find the water and check

The water needed is the final volume minus the stock: $50 - 10 = 40\ \text{mL}$ . So mix $10\ \text{mL}$ of stock with $40\ \text{mL}$ of water. As a check, the stock is five times stronger and is diluted fivefold ( $10\ \text{mL}$ into $50\ \text{mL}$ ), which matches. Using $C_1V_1 = C_2V_2$ and finding the water by subtraction is what the question rewards.

Examples in context

Example 1. Setting up a PCR. A PCR is assembled by micropipetting a few microlitres each of template, primers, nucleotides and polymerase into a tiny tube. Accurate micropipetting and a fresh tip for each reagent are what make the reaction work and prevent contamination.

Example 2. Making a growth medium. To prepare a culture medium at the right strength, a concentrated stock is diluted using the dilution equation, then topped up with sterile water. Getting the concentration right ensures the microorganisms grow as expected.

Try this

Q1. State two steps that help a micropipette give an accurate volume. [2 marks]

Cue. Set the correct volume within range and draw the liquid up slowly (using a clean, correctly fitted tip and expelling to the second stop are also acceptable).

Q2. You need $20\ \text{mL}$ of a $0.25\ \text{mol dm}^{-3}$ solution from a $1.0\ \text{mol dm}^{-3}$ stock. Find the volume of stock needed. [2 marks]

Cue. $V_1 = \dfrac{C_2 V_2}{C_1} = \dfrac{0.25 \times 20}{1.0} = 5\ \text{mL}$ .

Q3. For the solution in Q2, state the volume of water to add. [1 mark]

Cue. Final volume minus stock: $20 - 5 = 15\ \text{mL}$ .

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksDescribe how to use a micropipette to accurately transfer

50\ \mu\text{L}

of a liquid, and state two things that would make the measurement inaccurate.

Show worked answer →

Examiners want the correct technique and two genuine sources of error.

Set the micropipette to the required volume, $50\ \mu\text{L}$ , and fit a clean tip. Press the plunger down to the first stop and hold it. Place the tip just below the surface of the liquid, then release the plunger slowly to draw the liquid up. Move the tip to the destination and press the plunger down to the first and then the second stop to expel all the liquid. Use a fresh tip when changing liquids.

Two things that cause inaccuracy: drawing the liquid up too fast, or releasing the plunger too quickly, which can cause bubbles or an incorrect volume; and setting the volume wrongly or going past the dial range. Not changing the tip, or a leaking or wrong-sized tip, would also cause errors.

What markers reward: setting the volume, using a clean tip, pressing to the first stop before drawing up slowly, expelling to the second stop, changing tips between liquids, and two valid error sources such as drawing up too fast (bubbles) or setting the wrong volume.

Original5 marksYou need to make

100\ \text{mL}

of a

0.2\ \text{mol dm}^{-3}

salt solution from a

1.0\ \text{mol dm}^{-3}

stock. Calculate the volume of stock and the volume of water needed, showing your working.

Show worked answer →

The answer should use the dilution relationship and find the water by subtraction.

Use $C_1 V_1 = C_2 V_2$ , where $C_1 = 1.0$ , $C_2 = 0.2$ , and $V_2 = 100\ \text{mL}$ .

Rearranging for the volume of stock: $V_1 = \dfrac{C_2 V_2}{C_1} = \dfrac{0.2 \times 100}{1.0} = 20\ \text{mL}$ .

The volume of water needed is the final volume minus the stock: $100 - 20 = 80\ \text{mL}$ .

So mix $20\ \text{mL}$ of stock with $80\ \text{mL}$ of water to make $100\ \text{mL}$ of $0.2\ \text{mol dm}^{-3}$ solution.

What markers reward: using $C_1 V_1 = C_2 V_2$ , calculating $20\ \text{mL}$ of stock, and finding $80\ \text{mL}$ of water by subtracting from the final volume.