Chemistry 9701 Paper:5

[XPFMember]

Assalamoalaikum!!

can anyone plz help me in nov. 02 paper 5 Question:2

I dont understand the reason for heating and then re heating :%) :unknown:

 

[destined007]

Reheating is done to ensure that you get a constant mass. You need to make sure that all the water of crystallization has been evaporated. If there is some water of crystallization left behind, you will get a higher mass leading to incorrect value of x.

 

[XPFMember]

if it's alright for u, cud u plz give the answer m supposed to write for this one....i m kinda confused with this question...actually just started p5 and i'm finding it hard to go on..any tips? :unknown:

 

[destined007]

Outline, by a series of numbered steps, the apparatus to be used and the experimental work
you would carry out, including the measurements to be taken, to find the mass of water
remaining in the crystals.

Here you are suppose to write about apparatus used and experiment to be carried out. The detail should be in points. Its less than 5 marks so be brief.
Start your answer like this:
1) weigh an empty evaporating dish. Let the mass be M1.
2) add small amount of sodium carbonate into the dish and reweigh. Let the mass be M2
3) heat the the evaporating dish multiple times until the constant mass is observed. let the constant mass be M3

Draw the table of results you would prepare to record the results of your experiment.

In table you must have columns for:
1) mass of empty evaporating dish/ g (M1)
2) mass of evaporating dish and sample/ g (M2)
3) constant mass/ g (M3)
4) mass of anhydrous sodium carbonate/ g (M3-M1)
5) mass of water evaporated/ g (M3-M2)


Show how you would use the results of your experiment to find the percentage of water that
has been lost by the crystals on standing in air.

First calculate the number of moles of water from the masses obtained.
Moles of water: mass of water evaporated/Mr of water let this be x
moles of anhydrous sodium carbonate: mass of sodium carbonate/Mr of sodium carbonate let this be y

this means y moles of sodium carbonate gives x moles of water
1 mole of sodium carbonate will give x/y moles of water

therefore,
% mass of water lost on standing will be: (10 - x/y)/10 x 100

 

[XPFMember]

thnx a ton!!! 8)

by the way now after seeing ur answer i think i can go and write down thee answer but i'm finding it difficult to cope on with this paper 5

any tips? i cant go on asking for help in each question :unknown:

 

[destined007]

You are welcome.
See questions are usually similar. In order to get a firm hold on these quesions, a lot of practice is required. Once you start doing past papers you will notice that the questions are related and you need to write the same thing.
Still, if there's some thing unclear, post on the forum.