Experimental techniques and chemical analysis - Acid-base titrations (3)
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1.
Question 3
Describe an acid-base titration to determine the concentration of a strong acid using a burette, a volumetric pipette, and a suitable indicator. Explain why it is important to perform multiple titrations to ensure accuracy.
Apparatus Required:
- Burette (50.0 cm3)
- 25.0 cm3 volumetric pipette
- Conical flask
- Distilled water
- Indicator (e.g., methyl orange, bromothymol blue)
- White tile
- Stirring rod
Procedure:
- Carefully pipette 25.0 cm3 of the strong acid solution into a clean conical flask. Rinse the pipette thoroughly with distilled water.
- Add 2-3 drops of the chosen indicator to the conical flask.
- Fill the burette with distilled water, ensuring no air bubbles are present. Record the initial burette reading.
- Slowly add the strong acid from the burette to the conical flask while swirling the flask continuously.
- Continue adding the acid dropwise until a permanent colour change occurs, indicating the endpoint of the titration.
- Record the final burette reading.
- Repeat the titration at least three times to obtain concordant results.
Importance of Multiple Titrations:
Performing multiple titrations is crucial for ensuring accuracy because it helps to reduce the impact of random errors. Each titration may have a slightly different result due to minor variations in the experimental conditions (e.g., slight variations in the amount of indicator added, slight variations in the precision of reading the burette). By taking multiple measurements and averaging them, these random errors are minimised, and a more reliable and accurate result is obtained. The more titrations performed, the more reliable the average concentration will be.
2.
An iodine solution is used to titrate potassium thiocyanate. The reaction is: KSCN(aq) + I2(aq) → KI(aq) + SCN-(aq). A student is carrying out a titration and uses the indicator ferroin. Explain why ferroin is a suitable indicator for this titration. Include a description of the colour change associated with the end-point.
Ferroin is a suitable indicator for the titration of potassium thiocyanate with iodine because it exhibits a distinct colour change over the pH range of the reaction. The reaction between iodine and thiocyanate results in the formation of a complex ion, which is a relatively weak acid. The pH of the solution changes as the reaction proceeds.
Ferroin is a diprotic indicator, meaning it has two distinct colour forms depending on the pH. It typically appears colourless in acidic solutions (pH 6.2). The reaction between iodine and thiocyanate produces HI, which is a strong acid. As the reaction proceeds, the pH of the solution decreases. The end-point is reached when the pH falls below 2.3, causing the ferroin to change from colourless to a deep blue colour. This sharp colour change signals the complete consumption of iodine by the thiocyanate.
3.
A student is carrying out a titration to determine the concentration of hydrochloric acid (HCl) using a sodium carbonate (Na2CO3) solution. They use phenolphthalein as the indicator. State why phenolphthalein is a suitable indicator for this titration. Also, describe what the colour change at the end-point indicates about the reaction.
Phenolphthalein is a suitable indicator for the titration of hydrochloric acid with sodium carbonate because it changes colour over a specific pH range that coincides with the pH change during the reaction. Phenolphthalein is colourless in acidic solutions (pH 8.3).
In the reaction between hydrochloric acid and sodium carbonate, a neutralization reaction occurs, producing water and carbon dioxide. The carbon dioxide produced reacts with the water to form carbonic acid (H2CO3), which is a weak acid. This leads to an increase in the pH of the solution. The end-point is reached when the pH rises above 8.3, causing the phenolphthalein to change from colourless to pink. This pink colour indicates that the hydrochloric acid has been completely neutralised by the sodium carbonate. The amount of pink colour is proportional to the amount of acid present.