Acid-Base Titrations & Standard Solutions | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive guide on acid-base titrations and standard solutions, crucial concepts in A-level chemistry. Understanding these processes is essential for experiments that determine the concentration of unknown solutions and for solving related calculations in your chemistry coursework.

Step 1: Understanding Titrations

Titrations are a form of volumetric analysis used to determine the concentration of an unknown solution by reacting it with a solution of known concentration.

• Key components:

  • Known solution (titrant): The solution with a known concentration.
  • Unknown solution (analyte): The solution you are testing.

• Purpose: To find unknowns such as concentration, molar mass, or the formula of a compound.

Step 2: The Titration Process

Follow these steps to perform a titration:

1. Measure an appropriate volume of the unknown solution using a pipette and transfer it to a conical flask.
2. Add a suitable indicator to the flask to observe the endpoint of the reaction.
3. Fill a burette with the known solution.
4. Open the burette valve and allow the known solution to flow into the flask gradually.
5. Continue adding until you reach the endpoint, where the indicator changes color.
6. Record the volume of the known solution used to reach the endpoint.
7. Perform calculations to determine the unknown concentration or other values.

Step 3: Making a Standard Solution

A standard solution has a precisely known concentration, essential for titrations.

Steps to Prepare a Standard Solution:

1. Accurately weigh the required mass of the solute.
2. Dissolve the solute in a beaker using a suitable solvent.
3. Transfer the solution to a volumetric flask, rinsing the beaker with the solvent and adding the washings to the flask.
4. Add solvent to the flask but do not fill to the graduation line initially.
5. Slowly add solvent drop by drop until the bottom of the meniscus sits exactly on the graduation line.
6. Mix the solution thoroughly by inverting the flask multiple times.

Initial Calculations for Standard Solutions

1. Determine the number of moles required for your desired concentration.
2. Calculate the molar mass of the solute.
3. Use the molar mass to find the mass of the solute needed.

Step 4: Performing Titration Calculations

Calculations are used to find unknown values based on the titration results.

Example Calculation for Unknown Concentration:

• Given: 25.0 cm³ of 0.1 mol/dm³ NaOH titrated with 22.5 cm³ of HCl.

1. Write the balanced reaction equation.
2. Calculate moles of NaOH used:
   • Moles = Volume × Concentration
3. Use the stoichiometry of the reaction to find moles of HCl.
4. Calculate the concentration of HCl:
   • Concentration = Moles / Volume of HCl used.

Example Calculation for Unknown Molar Mass:

• Given: 1.5 g of an acid dissolved to make 250 cm³ solution.

1. Write the balanced reaction equation.
2. Calculate moles of NaOH used in the titration.
3. Determine moles of the acid from the titration results.
4. Calculate molar mass of the acid:
   • Molar Mass = Mass / Moles of acid.

Conclusion

In this tutorial, we covered the concepts of titrations, the process of preparing standard solutions, and the essential calculations involved in titration analysis. Mastering these skills will enhance your understanding of volumetric analysis in chemistry. Next steps could include practicing titration experiments and solving varied titration problems to ensure proficiency.