Required practical 1: Make up a volumetric solution and carry out a simple acid-base titration

Introduction to Required Practical 1

Welcome to your first required practical! In this chapter, we are going to master two essential laboratory skills: making a volumetric solution (a solution with an accurately known concentration) and using it in an acid-base titration. These techniques are the "bread and butter" of analytical chemistry. Whether you are checking the purity of a medicine or testing the acidity of a river, these are the steps you would follow.

Don't worry if the math or the precision seems a bit daunting at first. We will break it down into simple, manageable steps that will help you gain full marks in your practical endorsement and your written exams.

Part 1: Making up a Volumetric Solution

Before we can find out the concentration of an unknown substance, we need a "standard" to compare it against. A standard solution (or volumetric solution) is one where we know the exact concentration.

1. Calculating the Mass Required

First, you need to work out how much solid (solute) you need. We use the formula from section 3.1.2: moles = concentration \(\times\) volume.

Example: To make \(250\text{ cm}^3\) of a \(0.100\text{ mol dm}^{-3}\) solution of sodium hydrogensulfate (\(\text{NaHSO}_4\)):

1. Convert volume to \(\text{dm}^3\): \(250 / 1000 = 0.250\text{ dm}^3\)
2. Calculate moles: \(0.100\text{ mol dm}^{-3} \times 0.250\text{ dm}^3 = 0.025\text{ moles}\)
3. Calculate mass: \(moles \times M_r = 0.025 \times 120.1 = 3.0025\text{ g}\)

2. The "Weighing by Difference" Method

To be as accurate as possible, AQA examiners want you to use this method:

1. Weigh a clean, dry weighing boat on a digital balance set to zero (or "tare" it).
2. Add your calculated mass of solid.
3. Record the precise mass of the boat + solid.
4. Tip the solid into a clean beaker.
5. Re-weigh the empty weighing boat. Any solid stuck to the boat is now accounted for by subtracting the second mass from the first. This is much more accurate than just assuming everything fell out!

3. Dissolving and Transferring

1. Add a small volume of distilled water (e.g., \(100\text{ cm}^3\)) to the beaker containing the solid.
2. Stir with a glass rod until all the solid has dissolved.
3. Using a funnel, pour the solution into a volumetric flask.
4. Rinse the beaker, the glass rod, and the funnel with distilled water, adding all the "washings" into the flask. This ensures every single molecule of your solute ends up in the flask.

4. Filling to the Graduation Mark

1. Add distilled water until the level is just below the horizontal line (the graduation mark) on the neck of the flask.
2. Use a dropping pipette for the last few drops. View the mark at eye level.
3. The bottom of the meniscus (the curve of the liquid) must sit exactly on the line.
4. Put the stopper on and invert the flask several times to ensure the concentration is uniform throughout.

Quick Review: Accuracy comes from washings, using a volumetric flask (not a beaker!), and weighing by difference.

Part 2: Carrying Out the Titration

Now that you have your standard solution, you can use it to find the concentration of another solution (the "unknown").

1. Preparing the Apparatus

This is where many students lose marks! You must rinse your equipment correctly:

1. Burette: Rinse with distilled water, then rinse with the chemical you are going to fill it with. This prevents the chemical from being diluted by leftover water.
2. Pipette: Rinse with distilled water, then rinse with the solution you are measuring out.
3. Conical Flask: Rinse only with distilled water. It doesn't matter if there is extra water in the flask, as it won't change the number of moles of the chemical you pipette into it.

2. The Titration Procedure

1. Use a pipette filler to measure exactly \(25.0\text{ cm}^3\) of your unknown solution into a conical flask.
2. Add a few drops of a suitable indicator (see below).
3. Fill the burette with your standard solution. Ensure the space below the tap is full and there are no air bubbles.
4. Rough Titration: Add the solution from the burette quickly while swirling until the indicator changes colour. This gives you an approximate "end point."
5. Accurate Titrations: Repeat the process, but as you approach the rough end point, add the solution dropwise. Swirl constantly!
6. Stop the moment the colour changes permanently. Record the final volume.

3. Concordancy: The Goal

You must repeat the titration until you have at least two concordant titres. These are volumes that are within \(0.10\text{ cm}^3\) of each other. Once you have concordant results, calculate their mean (ignore the rough titration and any non-concordant results).

Did you know? Using a white tile under your conical flask makes it much easier to see the exact moment the indicator changes colour!

Part 3: Indicators and Safety

Choosing the right indicator is vital for a successful titration. You will learn more about the theory in section 3.1.12 (Acids and Bases), but for now, remember the common ones:

Methyl Orange:
- Use for: Strong acid vs. Weak base titrations.
- Colour: Red in acid; Yellow in alkali.
- End point: Orange.

Phenolphthalein:
- Use for: Weak acid vs. Strong base titrations.
- Colour: Colourless in acid; Pink in alkali.
- End point: Pale pink (just as the colour persists).

Safety (AT k)

Many acids and alkalis used in these practicals are corrosive or irritants. Always:
- Wear safety goggles at all times.
- Fill the burette below eye level to avoid splashes to the face.
- Clean up any spills immediately with plenty of water.

Part 4: Calculating Your Results

Once you have your mean titre, you can calculate the unknown concentration using these steps:

1. Write the balanced equation for the reaction.
2. Calculate the moles of the "known" substance: \(n = c \times V\) (remember to convert \(V\) to \(\text{dm}^3\)).
3. Use the molar ratio from the equation to find the moles of the "unknown."
4. Calculate the concentration of the unknown: \(c = n / V\).

Key Takeaway: Titrations are all about precision. From the way you read the burette (bottom of the meniscus) to how you rinse your glassware, every drop counts toward an accurate final result!

Common Mistake to Avoid: Don't forget to remove the funnel from the top of the burette after filling it. Drops of liquid could fall from the funnel during the titration, making your volume reading inaccurate.