F6 CHEMISTRY

1. Suppose originally there is x moles of reactant and assume the volume (V) is constant for the reaction.
Now, when the no. of moles of reactant is doubled, i.e. it becomes 2x
Original concentration = x/V = M
New concentration = 2x/V = 2(x/V) = 2M
The concentration is also doubled when no. of moles is doubled, assuming volume is constant throughout the reaction.
Hence when no. of moles increases/decreases, the concentration increases/decreases proportionally, assuming volume is constant.

2. Starch forms an water-insoluble complex with iodide ion irreversibly in the titrimetric analysis when starch is added too early. The amount of thiosulphate used for titration with iodine is reduced.

3. The colour change for end point is not sharp enough if the starch indicator is not added. Although the brown colour is observed at the beginning of the reaction, but soon it turns to pale yellow when concentration of iodine is reduced (just before the end point), and it turns to colourless at the end point. Therefore, it is difficult to observe the colour change from the pale yellow to colourless.
If you use starch indicator, the colour change is more obvious, originally the colour of solution is deep blue just before the end point and the colour turns into colourless when end point is attained.

1yes

2,starch can react with hypochlorite

3the brown colour is difficult to observe.since iodine can make starch turn in blue black,therefor we can determine iodine inside easily.