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A student attempted to determine the concentration of a hydrochloric acid solution - HSC - SSCE Chemistry - Question 28 - 2013 - Paper 1
Question 28
A student attempted to determine the concentration of a hydrochloric acid solution. The following steps were performed. Step 1: A conical flask was rinsed with wate... show full transcript
Worked Solution & Example Answer:A student attempted to determine the concentration of a hydrochloric acid solution - HSC - SSCE Chemistry - Question 28 - 2013 - Paper 1
Step 1
Identify the mistake the student made in step 4 and propose a change that would improve the validity of the result.
Answer
The mistake made in step 4 was that the student blew through the pipette to transfer the sodium carbonate solution. This could introduce air bubbles and alter the volume transferred, affecting the accuracy of the titration. To improve the validity of the result, the student should have touched the end of the pipette to the surface of the conical flask to allow the solution to flow by gravity, ensuring that all of the solution is transferred without altering its volume.
Step 2
Explain the effect of the mistakes made in steps 2 and 3 on the calculation of the concentration of the hydrochloric acid solution.
Answer
In step 2, rinsing the pipette with water could decrease the number of moles of sodium carbonate (Na2CO3) it contains, as water dilutes the solution left in the pipette. This would lead to a smaller volume of sodium carbonate being used in the titration, which would then result in a lower calculated concentration of hydrochloric acid (HCl) due to fewer moles of Na2CO3 reacting.
In step 3, not properly filling the pipette entirely with the sodium carbonate solution would further reduce the amount of Na2CO3 used in the reaction. This compounding mistake would lead to a miscalculation of the HCl concentration, as the expected number of moles reacting would be less than the actual number, causing the final calculated concentration of HCl to be higher than it should be.