I'd like to start a thread will little hints and tricks that people have found useful whilst doing chem.

I'll start it off.

- When converting things use dimensional analysis (ie see them as fractions and things cancelling). like 100ml of 8.9 g/ml then if you multiply them together the mL will cancel with the denominator because its on the numerator and the denominator (ie: you don't have to think).

- A change in pH is a dilution by a factor of 10 each time. Say you started from pH 1 and a 50mL solution.

You wanna get to pH 2.

Your solution has to be made up to 50*10 so you need to add 450mL.

- If you have H ion concentration in the form of

then according to the *double decker bus* principle the pH = x.

- Multiplicity applies only when there are two different environments splitting an environment. Otherwise... n+1 rules the roost. If you have a case in which you have n hydrogens in a neighbouring environment and m hydrogens in another then:

(n+1)(m+1) peaks on High resolution NMR.

- Think of things in proportion to one another. Good example is:

Which of the following gases occupy biggest volume at STP?

A 10g of CO2

B 10 g of NO2

C 10g of SO2

D) 10g of O2

How i think is that n = v/Vm

so we are interested in V = n*Vm

Vm is a constant here. so really all we are interested in is how V varies with mol and we can clearly see that it is directly proportional, ie: a bigger volume will be the one with the biggest mol.

Now how do we maximise mol? well n = m/M, but m is constant here.

So we have to minimise M (as if you have a small denominator you get a big answer).

Now directly you just go for D as all the others have (O2 + another element) so the lowest molar mass and thus highest mol and thus greatest volume.

This is just an example to explain what i mean by thinking proportional.

- When doing gas equations get a feel for them.

ie: A fixed mass of gas has a volume of 900mL under certain conditions. The pressure and tempreature are both doubled. What is the volume of gas after these changes?

Visually if you have a fixed amount of pure gas, you have a fixed amount of mol. This means that you have a fixed volume (thereabouts). So I see a mess of particles inside a canister (representing volume). If you increase the temp by a factor of two, so will the volume by 2. So the particles are moving around more craazy. But if you increase pressure by 2 then the volume must deacrease by 2 because it needs to hit the walls in a certain ratio.

So its still. 900mL.

Let's see what the rest of the community has in their toolbox! (i'll add more when i can think of any)

edit: thanks to chansthename for correcting my fail 900 = 800