Hi, I'm doing some practice for analytical chemistry and I was having difficulty with these problems:

http://img838.imageshack.us/img838/1487/practice.jpg

If you could give me some advice on how to approach these problems I would appreciate it, thanks.

Q 1- HOCl (aq)  ------>   H3O+ (aq) +  OCl- (aq)    Ka =  3.5  × 10-8
HOCl is a weak acid
hence you can calculate the concentration of H3O + by using Equilibrium constant expression
You need to make ICE table to calculate the concentration of H3O +

In Q 4
Calculate the number of moles of Hg 2+ used up in titration
no of moles = molarity X volume in L
now No of moles of thiourea = 4 X number of moles of Hg 2+ used up in titration
convert these moles in mass in gm and then take out the %
= mass of thiourea X 100 % /mass of organic sample

In q 6
we are using back titration
first calculate how many moles of Fe2+ are used to remove extra dichromate ions
no of mole = M X V in L
from the balanced equation no of moles of dichromate = 1/6 X moles of Fe 2+

subtract these moles of dichromate from total moles of dichromate added
= 0.025 X 0.002107 -1/6 X moles of Fe 2+
now no of moles of fe 2+ in 1oo mL of water = 6 X moles of dichromate
now convert this in ppm
means in 10^6 mL of water