[Borek]

I would start converting all these volume numbers into hydrogen peroxide and sodium iodide concentrations.

[Marcus Soutlo]

So since

n = m / M

is the volume (in ml) a mass?

For example, I am assuming that 8.0 mL of 6% H2O2 is actually 8.0 grams?

[Frederick95]

Wouldnt I use the formula

molarity = moles of solute / liter of solution

in order to convert all these volume numbers into hydrogen peroxide and sodium iodide concentrations?

If this is what I must do, how would I find the moles of solute? For reaction 2, for instance should I assume that 8.0 mL of 6% H2O2 is actually 8.0 grams, and from this mass I would find the moles of solute by using the formula n=m/M? Would the 2.0 mL of distilled water be 0.002 L for litre of solution? Please help for this assignment was due yesterday and I need to finish it

[Marcus Soutlo]

This is regarding reaction 2

Reaction 2
                                 H2O2                                                                          NaI   
Reaction   Volume of 6% H2O2 (mL)  Volume of distilled water (mL)  Volume of 1.0 M NaI (mL)  Volume of distilled water (mL)  O2 production in 60 s 
                                                                                                                                                                                    (mL)
     2                    8.0                               2.0                                    10.0                               0.0                                    93.0

My teacher told me If I find the number of moles of peroxide decomposed per second(0.00006470 mol O2/second), it is only a single calculation to convert that to change in concentration by using volume.  What is this single calculation?

Ive tried n = c/v (mols = concentration / volume) but I do not think this is it. Can anyone tell me this calculation and I will sub in my values to see if it is correct.

[Borek]

Wouldnt I use the formula

molarity = moles of solute / liter of solution

Yes.

should I assume that 8.0 mL of 6% H2O2 is actually 8.0 grams

Yes.

and from this mass I would find the moles of solute by using the formula n=m/M?

No. You should use definition of percent concentration, solved for mass of the solute.

Would the 2.0 mL of distilled water be 0.002 L

Yes.

[Borek]

My teacher told me If I find the number of moles of peroxide decomposed per second(0.00006470 mol O2/second), it is only a single calculation to convert that to change in concentration by using volume.  What is this single calculation?

You have to find out how many moles of hydrogen peroxide were decomposed to produce given amount of oxygen. Concentration is amount of solute per volume of solution - assume that volume doesn't change, calculate how much solute was left (you know initial amount, you know how much was decomoposed) - and you have everything needed.

[Marcus Soutlo]

C%w/w = msubstance / msubstance + msolvent x 100%

            = 8.0 / 8.0 + 2.0 x 100%

            = 8.0 / 10 x 100%

            = 80%

Therefore reaction 2 is made of 80% hydrogen peroxide?


But how do I get mass from this?

[Marcus Soutlo]

How would I get this mass and is this mass the change in hydrogen peroxide?

Thanks again

[Borek]

C%w/w = msubstance / msubstance + msolvent x 100%

Solve for msubstance, simple algebra. Note, it is even easier than it looks, as you don't have to treat mass of solution as (msubstance + msolvent) - you already know total volume and you can assume density of the solution to be 1 g/mL. That's not exactly true, but will do.

[Frederick95]

2 moles were decomposed to produce 1 mol of oxygen.

Concentration = amount of solute per volume of solution

= 8 g / 10 g

Therefore mass of hydrogen peroxide must be 8 grams

If this is right, then would I use 8 grams to find mols and then plug the number of mols into the value

moles of solute / liter of solution to get M?

------------------------------------------------------------------------------------------------------------------------------------------
OR

How do I rearrange

C%w/w = msubstance / msubstance + msolvent x 100% to find  m substance?

[Borek]

http://en.wikipedia.org/wiki/Elementary_algebra

[Frederick95]

I think I understand

C%w/w = msubstance / msubstance + msolvent x 100%

rearranging gives

msubstance = C%w/w divided by msolution x 100%

[Borek]

Cp = msubst/msol x 100%

How do you "move" msolution to the LHS?

[Frederick95]

To move msolution to the left hand side, I would rearrange the formula to get

msol = Cp / msubst x 100%

[Borek]

You are doing it wrong. Slow down. Try to multiply BOTH sides of the equation by msolution. If you do some operation to BOTH sides of the equation, it still holds.

Geez, you really were not taught how to do such things?