[smilie_hugs]

Hi there, I need immediate help here because my chemistry practical examinations [18/10/04]  is next Monday, so hope that you guys/gals can help to answer my question. I'm doing the GCE 'O' levels science (chemistry) practical examination.

Question 1:
  When they ask you to test for 2 different gases [eg, oxygen and carbon dioxide] in a experiment, will there be a situation where both gases are absent? If not, does it mean that the result will always be one positive and one negative?


Question 2:
  When we test for hydrogen gas with a lighted splint, and the wooden splint extinguished without a 'pop' sound, hygrogen is not present right? Is the extinguishing of the flame could be due to other gases like carbon dioxide? So, if that is the case, can I write the observation as ' No reaction' on my answer sheet?

Question 3:
  Okay, this titration question is a bit stupid, however, it always borther me. Do you place the high-tension clip on the tip of the burette, near the top of the rubber tubing? Do anyone have any tips on how to stop the burette from leaking? I hate it when it leaks.

Question 4:
  This is also a titration question: I'm not quite sure how to read a burette, so I have found a burette picture, and I see that the liquid in the burette is 23.8 cm³. Can anyone confrim this answer with me?


- Thanks in advance -

[ssssss]

Not much idea about the practical Examination.But whenever the case of Buerrete you mentioned in the figure,you have to take the bottom point[It has something to do with the liquid meniscus,no need to go in detail].So you are right.

Secondly about Buerrete.I had Broken many during the Experiments,so dont have Idea how to use them.

[smilie_hugs]

Okay... I was going through my past titration papers. Normally, after each titration, they would ask us to do the mole calulation, but I always don't know how. I copied one of the question here, can anyone solve it?

Question:

Aim: To find out the concentration of a sample of dulite acid.


Procedure
Pipette 25.0cm3 of solution X2 into the conical flask
Titrate X2 with X1 using Methyl orange as indicator.

Materials
X1 = aolution of sulphuric acid [concentration: unknown]
X2 = aqueous sodium hydroxide [concentration: 4.0g/dm3]


Results  
Rough--? Final reading cm3 = 11.5
              Initial reading cm3 = 0.0
              Volume of X1 used cm 3 = 11.5

Titration  
Number 1 -->  Final reading cm3 = 10.0
                     Initial reading cm3 = 0.0
                      Volume of X1 used cm 3 = 10.0

Titration  
Number 2 -->  Final reading cm3 = 15.5
                     Initial reading cm3 = 5.5
                      Volume of X1 used cm 3 = 10.0

Titration number 1 and 2 are the best results


Summary
25.0 cm3 of X2 requires an average volume of 10.0 cm3 of X1 for a complete neutralisation.


Caluations:
[a] Construct a balanced equation for the reaction.
From the results obtained, calculate the concentration in mol/dm3, of Sodium hydroxide in X2
[c] Find the number of sodium hydroxide that was pipetted into the conical flask.
[d] Calcaulate the number of moles of sulphuric acid the react with 25.0 cm3 of sodium hydroxide
[e] Find the concentraion of the sulphuric acid in mol/dm3

-Thanks in advance-

[ssssss]

Sorry we are not here to solve your questions?
If you get problem in any specific step then tell us.

Which step in this problem you are unable to understand?

[movies]

Whenever you read any measurement off of glassware, then you need to estimate one further decimal place than the glassware is marked.  So in the case of your picture, I would estimate the volume to be 23.85 mL.

[smilie_hugs]

Okay, I have constructed a chemical equation and I think should be correct. The equation is 1 mole of NaoH + 2 mole of H2SO4 --> 1 mole of Na2So4 + 2 moles of H2O.

The for part , I don't know how to convert g/dm3 into mol/dm3.

[ssssss]

Okay, I have constructed a chemical equation and I think should be correct. The equation is 1 mole of NaoH + 2 mole of H2SO4 --> 1 mole of Na2So4 + 2 moles of H2O.

The for part , I don't know how to convert g/dm3 into mol/dm3.

Wrong the correct Equation is:

2NaOH + H2SO4=>Na2SO4 + 2H20

[ssssss]

Okay, I have constructed a chemical equation and I think should be correct. The equation is 1 mole of NaoH + 2 mole of H2SO4 --> 1 mole of Na2So4 + 2 moles of H2O.

The for part , I don't know how to convert g/dm3 into mol/dm3.

And for part b Divide the g/dm3 By the Molecular Mass

[smilie_hugs]

So is the answer suppose to be 2? Because RMM of NaOH is 20. Then 4 divided by 20 will be 0.2 mol/dm3.


However, according to the answer given is 0.0025dm3.

[smilie_hugs]

Thanks for all your help. Today, I sat for the paper. It was a QA question. They give us an unknown sample P and ask us to test the gas given out, but never state what gas. Throughout every small test, they never mention to test for any gas at all! So, my question is how to test? Do you test for all the 6 gases? But if we do so, we would not have enough sample to use!   To make things worst, they didn't give us a space to write our conclusion for the gas given out by sample P only, all the conclusion boxes are for other substances that are supposed to mixed with smaple P. So, how do we write down? We can't possibly test for every gas for each small test as we simply do not have enough substance to mix with smaple P and then do the test! So, what excatly do they want us to do?