# A level chemistry help

Q15.
The apparatus in the figure below was set up to measure the time taken for 20.0
cm3 of sodium thiosulfate solution to react with 5.0 cm3 of hydrochloric acid in a
100 cm3 conical flask at 20 °C. The timer was started when the sodium
thiosulfate solution was added to the acid in the flask. The timer was stopped
when it was no longer possible to see the cross on the paper.
AQA Chemistry A-Level - Kinetics QP PhysicsAndMathsTutor.com
Page 19 of 22
What is likely to decrease the accuracy of the experiment?
A Rinsing the flask with acid before each new experiment.
B Stirring the solution throughout each experiment.
C Using the same piece of paper for each experiment.
D Using different measuring cylinders to measure the volumes

The answer is A - can somebody explain why?
Original post by mitostudent
Q15.
The apparatus in the figure below was set up to measure the time taken for 20.0
cm3 of sodium thiosulfate solution to react with 5.0 cm3 of hydrochloric acid in a
100 cm3 conical flask at 20 °C. The timer was started when the sodium
thiosulfate solution was added to the acid in the flask. The timer was stopped
when it was no longer possible to see the cross on the paper.
AQA Chemistry A-Level - Kinetics QP PhysicsAndMathsTutor.com
Page 19 of 22
What is likely to decrease the accuracy of the experiment?
A Rinsing the flask with acid before each new experiment.
B Stirring the solution throughout each experiment.
C Using the same piece of paper for each experiment.
D Using different measuring cylinders to measure the volumes
The answer is A - can somebody explain why?

Essentially it's to ensure no other factors could affect the concentration of HCl. If you were to use more acid to rinse out the conical flask, the volume of acid used would increase. Accuracy is how close to the real value you can get.

If in doubt, first try to eliminate other answers. Using the same piece of paper doesn't change reaction conditions, as does using different measuring cylinders to measure volumes.

So you now have gotten to a 50/50 between A and B. If at worst, that's a better shot than 1/4.

Hope this helps!