What are some useful tidbits in chemistry that you've learned?

[CrimpJiggler]

I've been learning cheistry for a very long time, and I often come across bits of knowledge that becomes permanently engrained in my mind (no revision required) due to the empowering nature of it. I'm going to keep updating this thread every time I remember another one of these things, or come across a new one. I'm hoping other people here will do the same, because I'd love to learn more of these things.

Alright

1.) If you add silver to an organic halide, it binds to the halogen atom, and turns it into an excellent leaving group. When it leaves, the silver halide precipitates out, leaving you with a solution of carbocations.

2.) Grignard reagents let you generate carbanions which you can use as nucleophiles in nucleophilic additions. I know that's basic high school knowledge, but I added it anyway due to how useful it is.They're just the two off the top of my head, I'll have way more when I remember them.

 

[Borek]

$$\frac {[HA]}{[A^-]} = 10^{pK_a-pH}$$

Works for every acid, always. In words - when you are 1 pH unit below pK_a ratio of concentrations of HA/A^- is 10, when you are 2 pH units below ratio is 100, when pH is 1 unit above pK_a ratio is 0.1 and so on. Makes wonders when you try to estimate what ions are present in the acid solution of known pH.