[AleggsMac]

Hi! I have no idea what information to include, so I'll try and include as much as possible...

A 0.2mmol/L solution of PNP was put into a spectrophotometer at a wavelength of 400nm, and it's absorbance was measured at 0.842. It's pH was 6.87

Then 1mL of 0.2mmol/L PNP was put into a seperate test tube, and 'a few' uL of 1mol/L NaOH and 3mL of water was added. The absorption of this was then measured at 0.751 and a pH of 7.06.

From this I have to Calcualate

1) A_diss
2) A₀
3) A_undiss
4) pK_a

Anybody know where on earth I start with this? I think Le Chatelier is involved, possibly Bronsted-Lowry, or possibly neither. I am so confused, and would be so grateful of some step-by-step help. If you need more information, I'll try to provide it.

My lecturer goes waaaaaay too fast to really get these various principles and equations properly, and any tutorial sessions I've been to have been completely unhelpful

Thanks in advance!

[Borek]

You are probably expected to assume that first solution contains almost only protonated form (note that pH is almost equal 7, as if there was no acid added).

The only way you can find the answer is assuming that neutralized form is colorless. Use this information to calculate ratio of acid and conjugate base.