☁️☾✧AS Level Chemistry: Paper 3 (Practical Exam) // 9701/31/O/N/20 // Question 1 - Titration♡☁️

Summary

In this educational video, the host guides viewers through a chemistry experiment aimed at determining the value of 'x' in the formula of hydrated sodium thiosulfate. The process involves preparing a salt solution, performing a titration with iodine, and conducting several calculations, including determining the number of moles of iodine, sulfate ions, and hydrated sodium thiosulfate. The video explains important concepts like concentration, volume, and significant figures, providing step-by-step instructions to solve the related chemistry questions. It also highlights key tips for handling uncertainties and errors in the experiment.

Takeaways

• 😀 The experiment involves determining the value of x in the formula of hydrated sodium thiosulfate by performing a titration with iodine solution.
• 😀 The ionic equation for the reaction between sodium thiosulfate and iodine is provided, which is crucial for the calculations.
• 😀 The method begins with weighing the sample of sodium thiosulfate, followed by dissolving it in distilled water and transferring it into a volumetric flask to prepare the solution.
• 😀 After preparing the solution, the titration process involves adding iodine (F3) into the conical flask containing sodium thiosulfate solution (F2) until a color change is observed.
• 😀 A starch indicator is used to detect the endpoint of the titration, where the solution changes from yellow to blue-black and then back to colorless when titration is complete.
• 😀 Multiple titrations are performed to ensure accuracy, with readings recorded to the nearest 0.05 cm³ to minimize error.
• 😀 The average titration volume is used in the calculations to find the number of moles of iodine and subsequently the number of moles of thiosulfate ions.
• 😀 The number of moles of sodium thiosulfate in the original solution is calculated by using the ratio of iodine to thiosulfate ions.
• 😀 The final calculation determines the value of x (the number of water molecules in the hydrated salt) by finding the molar mass of the hydrated sodium thiosulfate and subtracting the mass of the water.
• 😀 For accuracy, the uncertainty in the mass measurements is considered, and the percentage error is calculated to determine the minimum possible relative formula mass of sodium thiosulfate.
• 😀 In the final part, a comparison is made between titrations using hydrated and anhydrous sodium thiosulfate, explaining that a higher concentration of thiosulfate leads to a smaller titration value.

Q & A

What is the main goal of the experiment described in the video?

The main goal is to determine the value of x in the formula of hydrated sodium thiosulfate by performing a titration with iodine and using stoichiometric calculations.

What is the role of iodine in the titration process?

Iodine is used as the titrant in the experiment, where it reacts with thiosulfate ions to determine the amount of thiosulfate present in the solution.

How is the salt solution prepared before the titration?

The salt is first weighed and dissolved in distilled water, then transferred to a volumetric flask, and the solution is made up to 250 cm³ to create a standard solution (fa2).

Why is a starch indicator used in the titration?

The starch indicator is used to visually signal the end point of the titration. Once iodine is consumed, the solution turns blue-black, which fades when the reaction is complete.

How should the titration be repeated to ensure accuracy?

The titration should be repeated at least three times, ensuring that the burette readings are accurate and within 0.1 cm³ of each other to calculate an average volume.

What is the formula used to calculate the number of moles of iodine in the titration?

The formula is: number of moles = concentration × volume. Volume must be converted from cm³ to dm³ for consistency in units.

What is the stoichiometric relationship between iodine and thiosulfate ions in this experiment?

For every mole of iodine, two moles of thiosulfate ions react. This is crucial for calculating the moles of thiosulfate based on iodine usage.

How do you calculate the value of x in the hydrated sodium thiosulfate formula?

The value of x is determined by calculating the moles of hydrated sodium thiosulfate in the solution and relating it to the molar mass of sodium thiosulfate, considering the water of crystallization.

What is the significance of calculating the maximum percentage error in the mass measurement?

Calculating the maximum percentage error helps to estimate the uncertainty in the results, ensuring that the measurements are accurate and reliable in determining the formula mass.

How would the titration results differ if anhydrous sodium thiosulfate was used instead of the hydrated form?

Using anhydrous sodium thiosulfate would result in a smaller titration volume, as there would be no water molecules in the sample to account for, affecting the concentration of the thiosulfate solution.