Need Urgent Help in Chemistry (0620)-Extended

[Shamu M]

Can anyone please tell me how to calculate the time in the this question:This question is from paper 32 of May/June seesion 2011.Sorry I didn't know how to upload the table that was given,please see the question paper for the table for the 3rd question.


3 ) The equation for the reaction between sodium thiosulfate and hydrochloric acid is given
below.
Na2S2O3(aq)+2HCl(aq)→ 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

The speed of this reaction was investigated using the following experiment. A beaker
containing 50 cm3 of 0.2mol / dm3 sodium thiosulfate was placed on a black cross. 5.0cm3
of 2.0mol /dm3 hydrochloric acid was added and the clock was started.look down at cross on paper,paper solution turns from colourless to cloudy sodium thiosulfate and hydrochloric acid cross on paper view looking down
Initially the cross was clearly visible. When the solution became cloudy and the cross could
no longer be seen, the clock was stopped and the time recorded.
(a) The experiment was repeated with 25cm3
of 0.2mol/dm3
sodium thiosulfate and 25cm3
of water. Typical results for this experiment and a further two experiments are given in
the table.


(i) Explain why it is necessary to keep the total volume the same in all the
experiments.
....................................................................................................................................
....................................................................................................................................
.............................................................................................................................. [2]
(ii) Complete the table.

 

[AllisonMack]

it is necessary to keep the total volume constant,
concentration of thiosulfate is proportional to volume of thiosulfate solution added (when
total volume is same in all experiments) / concentration of acid always the same
u can also rite "for fair comparison" but tht will get u onli 1 mark
for completing the table,
it makes sense,as when u see..
conc 50 ,time taken is 48 ...when it falls half (divide by 2)as in conc is 25,time is doubles (into 2),ie96..
so if conc falls one fifth, ie 50--to--10(divides by 5),the time taken will be 48 into 5.ie 240 s

 

[Shamu M]

Thanks allison,but I'm done with the exam, fortunately I saw ur reply through my phone, Thanks once again!!!!!!!!!