Back Titrations | A level Chemistry

Summary

This chemistry video tutorial delves into back titrations, explaining their purpose, experimental procedure, and calculation steps. It illustrates how to determine the concentration of a solution by reacting it with a known concentration solution, then titrating the excess with another known solution. The video uses examples to explain calculating reactant amounts, percentage purity, and handling complex scenarios with multiple steps and dilutions, providing a comprehensive guide for students.

Takeaways

• 🔍 Back titrations are used to determine the concentration of a solution when direct measurement is not possible.
• 🧪 The process involves adding one solution from a buret to another in a conical flask, ensuring precise volume control.
• 📐 The concentration of the unknown solution can be calculated using the formula: moles = concentration × volume.
• 🔄 Back titrations work by first reacting the unknown with an excess of a known reactant, then titrating the leftover reactant.
• 🌟 An example given is reacting a base with excess acid and then titrating the leftover acid with a known base to find the original base concentration.
• 📈 Back titration can also determine the percentage purity of a substance by comparing the actual amount of substance to the expected amount.
• 💡 The script provides a step-by-step guide on how to perform and calculate back titrations, including dealing with excess reactants and dilutions.
• ⚖️ A specific example is given using calcium carbonate and hydrochloric acid, showing how to calculate purity based on reaction ratios.
• 📉 The video script also covers more complex scenarios involving multiple steps and dilutions, guiding through the calculation process.
• 📝 Advice is given on how to approach exam-style questions on back titrations, emphasizing the importance of diagrams and step-by-step calculations.

Q & A

What is the purpose of back titrations?

Back titrations are used to determine information about a reactant or chemical that cannot be measured directly, such as the number of moles or purity of a substance.

How is a back titration performed in a basic sense?

In a back titration, a substance is reacted with an excess of a reagent, and then the leftover reagent is titrated to determine how much was used in the reaction, allowing the number of moles of the initial substance to be calculated.

Why do we add an excess of acid in back titrations involving bases?

We add an excess of acid to ensure that all of the base reacts, allowing us to later titrate the leftover acid to determine how much of the acid was used.

How do you calculate the moles of a base from back titration results?

After titrating the leftover acid with a base, the moles of leftover acid can be determined. By subtracting the leftover acid from the initial amount, the moles of acid that reacted with the base can be found, which corresponds to the moles of base used, assuming a 1:1 reaction ratio.

How is percentage purity calculated using a back titration?

Percentage purity is calculated by dividing the pure moles or mass of the substance by the impure moles or mass, then multiplying by 100 to convert it to a percentage.

In the example of calcium carbonate and hydrochloric acid, why is the reaction ratio 1:2?

Calcium carbonate reacts with hydrochloric acid in a 1:2 ratio because calcium carbonate has one carbonate ion that requires two HCl molecules to form calcium chloride (CaCl2), carbon dioxide (CO2), and water (H2O).

What is the role of sodium hydroxide in the titration process?

Sodium hydroxide is used to titrate the leftover hydrochloric acid after the reaction with calcium carbonate, allowing the amount of acid that did not react to be measured.

Why might the ratio of reactants not always be 1:1 in back titrations?

In some reactions, the substances involved do not react in a 1:1 ratio, as seen with calcium carbonate and hydrochloric acid, which react in a 1:2 ratio. The reaction stoichiometry depends on the chemical properties of the reactants.

What happens if the leftover reagent in a back titration is diluted?

If the leftover reagent is diluted, only a portion of the diluted sample is used in the titration. The moles of the reagent in the full sample can be calculated by multiplying the titrated moles by the dilution factor.

What is a key tip for solving back titration problems in exams?

A helpful tip is to draw diagrams of the reaction and titration steps. This can make it easier to visualize the process and track the different stages of the calculations, especially when working with complex multi-step titrations.