999! Chem. in A level is so different! -- Chemistry

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Date Posted: 18:44:13 01/18/02 Fri
Author: Sin Chun
Subject: 999! Chem. in A level is so different!

Hello Mr Yeo,
I just had my practical(titration) this thur. and the teacher wanted us to tap the pipette at least 5 times! So my qn is why? Isn't the accuracy of the pipette is to the last drop in it? And the last drop must be in the pipette?

Then for the calculation part, she said that if we write full statements then we do not need to write units! Is this allowed by Cambridge?
e.g.
no. of moles of KOH in 1 dm3 of solution= x.xxx(no need to write units???!!!)

Lastly, I encounter a question which I don't understand, even when the lecturer had try to explain(not only me, many pple don't know what she is talking 'bout), The lecturer explain without statements, so I guess it is quite hard to understand. Could you kindly explain it to me?
Qn:
10.0cm3 of a hydrocarbon CxHy were exploded with an excess of oxygen. There was a contraction of 25.0cm3. When the product was treated with a solution of sodium hydroxide, a further contraction of 40.0cm3 occurred. All measurements of gas vol. are at r.t.p.
Deduce the values of x and y in the molecular formula of the hydrocarbon.

Thank you!

Sin Chun
p.s. Wei Ling , Ameline and I misses your way of teaching!:)

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