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Chemistry AS last minute help

When an atom 'expands the octet' do you just treat the bonding and lone pairs as normal? And, when drawing shapes of molecules, sometimes the mark scheme just has solid line when I would expect them to be dotted or wedged?

What is the formula for percentage error? (Is it uncertainty / minimum reading x100)

Does anyone have any tips for,titration method questions? E.g. Errors , hazards etc

And I am mostly stuck on questions that include velocity, mass etc in a TOF mass spec and you have to calculate the distance ? Any known resources?

Sorry if these are brief , any help would be greatly appreciated. Thank you :smile:
Reply 1
Avatar for Arabius
Arabius
3
I also have an exam tomorrow, yet I know not what you mean by 'expanding the octet', last I knew, atoms become inert when they have a full electron configuration. Unless you're forcing it like electron affinity.

Tips on titration, check the ratios before you do anything. Use proportion whenever needed and know your formulas.

The percentage error varies with the apparatus you want to use. A burrette is usually uncertainty*2/reading*100
Reply 2
Avatar for Mil23333
Mil23333
9
Best to draw the bonds wedged because they won't mark you down (as long as it is correct) but it might say it on the mark scheme or it might just be straight lines, %uncertainty or %error: if you are given the uncertainty for example of a pippette might be 0.01 then you do that divided by your value so this could be 25cm^3 then multiplied by 100, if you have to work out the uncertainty yourself from a table of results it will be max value-min value divided by the number of readings, than you use that value of uncertainty to calculate the %uncertainty by dividing it by one of the readings and the multiplying by 100
Reply 3
Avatar for Mil23333
Mil23333
9
With TOF they might want you to use E= 1/2mv^2 so just rearrange it for the value you need and sub in the values you are given (not sure if that helps at all lol) I also have my exam today :smile:
Reply 4
Avatar for Mil23333
Mil23333
9
With titrations you can reduce errors in the pippette by using a more dilute titre or a more concentrated substance that u are titration for so that you will need a larger volume of titre in your pippette meaning the errors are reduced (saw this on a mark scheme)

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