[MitchTwitchita]

O.k. I mixed 30 mL of 1.0 M hydrogen peroxide (H2O2) with 120 mL distilled water.  I then took 20 mL of this solution and mixed it with 40 mL buffered KI, 10 mL sodium thiosulfate and another 10mL of distilled water.  So, now there's 80 mL of this new solution.  How would I calculate the moles/Liter of H2O2?

M1V1 = M2V2
(1.0 mol/L)(0.03 L) = M2(0.150 L)
M2 = 0.20 mol/L

Then, M1V1 = M2V2
(0.20 mol/L)(0.020 L) = M2(0.080 L)
=0.05 mol/L?

Hmm, there's got to be a better way.

[Borek]

Result is OK - and while there are some tricks that can be used they are hardly better.

You don't have to convert everything to L, you can do your calculations using mL and mmol/mL - numbers are the same as for mol/L.

You don't have to calculate concentration in the intermediate solution, looking at volumes you can calculate that final solution will have 2/15 of the initilal amount of H₂O₂, which is 30 mmol, so final solution contains 30*2/15 = 4 mmol. 4 mmol divided by 80 mL gives 0.05M.

Is it better? I doubt.

[MitchTwitchita]

Thanks Borek.