[scrffy]

I have a solution (say 200L). I need to add H2O2 (@30%) to give a final H2O2 concentration of 2%w/v.
2% of 200 is 4. But as the H2O2 is 30% so I add 13.33L (4/30*100). However the final volume is then 213.33L, which means the concentration is really 1.87% ((13.33/213.33*100)*30/100).
Is there a calculation that determines th final volume required and therefore the actual weight of solution to add.

[Borek]

Tricky, but doable with a use of density tables. You will probably need a set of equations describing total mass and masses of solutions used. Difference between w/w and w/v for 2% solution is negligible.

Or use the mixing calculator built into CASC.