Bleach Titration

zam

Was just doing the question on the sample paper there and have no idea whether or not some things are right. Seeing as it's tipped to come up I want to be sure! Hopefully someone will be able to help me...

What colour change was observed in the conical flask when the sulfuric acid and potassium iodide were added to the bleach?

A 25cm^3 sample of the bleach was diluted to 250cm^3. A 25cm^3 portion of this diluted solution was added to an excess of acidified potassium iodide solution and titrated against a standard 0.21M sodium thiosulfate solution. The average titration figure was 20.7cm^3.
The equations:
ClO- + 2I- + 2H+ = Cl- + I2 + H2O
2S2O3(2-) + I2 = S4O6(2-) + 2I-

Calculate the concentration of sodium hypochlorite in the household bleach in moles per litre. Express this concentration in terms of %(w/v).

thanks

Burswood

Colour change: Goes dark red because of the liberated iodine

Ratio is 1:2 between hypochlorite & thiosulfate. So.. to get Moles per Litre of hypochlorite do this:

0.21x20.7 divided by 2 multiplied by 25xMoles divided by 1

Its that VxM/n = VxM/n formula

get the moles figure by cross multiplying


Then to get %w/v, you need to find the hypochlorite in grams first. multiply the moles figure you just got by the relative molecular mass of NaClO (23+35.5+16)

Then you'll have a grams per litre figure. You need a grams per 100cm cubed figure so divide your g/L figure by 10.

This new figure is the %w/v

zam

Burswood wrote: »

Colour change: Goes dark red because of the liberated iodine

Thanks a million! Does anyone know what colour it is originally that goes red?

Burswood

zam wrote: »

Thanks a million! Does anyone know what colour it is originally that goes red?

The original colour is the colour of the diluted bleach. So i dunno.. White?..

Captain-America

Colourless.

Burswood

Captain-America wrote: »

Colourless.

Ah. Thanks.

Mazda

Burswood wrote: »

Colour change: Goes dark red because of the liberated iodine

Ratio is 1:2 between hypochlorite & thiosulfate. So.. to get Moles per Litre of hypochlorite do this:

0.21x20.7 divided by 2 multiplied by 25xMoles divided by 1

Its that VxM/n = VxM/n formula

get the moles figure by cross multiplying


Then to get %w/v, you need to find the hypochlorite in grams first. multiply the moles figure you just got by the relative molecular mass of NaClO (23+35.5+16)

Then you'll have a grams per litre figure. You need a grams per 100cm cubed figure so divide your g/L figure by 10.

This new figure is the %w/v

After you use V1M1n2 = V2M2n1 are you not supposed to multiply the molarity by 10 to take into account that the bleach was diluted from 25cm to 250cm?
And then carry on the way you did?

Killio9

Mazda wrote: »

After you use V1M1n2 = V2M2n1 are you not supposed to multiply the molarity by 10 to take into account that the bleach was diluted from 25cm to 250cm?
And then carry on the way you did?

only if they ask for the concentration in the original bleach!

Mazda

Killio9 wrote: »

only if they ask for the concentration in the original bleach!

Is that not what it meant by asking for the % in the household bleach? Because the student had diluted the household bleach so I presumed that they were referring to the original bleach...?

Killio9

Mazda wrote: »

Is that not what it meant by asking for the % in the household bleach? Because the student had diluted the household bleach so I presumed that they were referring to the original bleach...?

oh yeah sorry didnt see that bit my bad!

Mazda

oh okay was going to cry if i had learned the wrong method :P fingers crossed for a nice paper today good luck!

H2student

Thanks for the bleach answer, I can't find my bleach notes :P

Can't believe that I'm cramming so much hours before this exam.

I only looked at:

Chem equilibrium, Q5, Fuels & heats of rtn. in the last 2 hours. Only started experiments just now. This is gonna be a fun 3 hours, and worse part is I'm still on boards.ie

jawileth

Burswood wrote: »

Colour change: Goes dark red because of the liberated iodine

colour change is strawyellow - blue/black - colourless!
while you let the sodium thiosulfate solution into the dilute bleach it will eventually turn light/strawyellow. then you add the starch indicator it canges to blue/black. you then open the tap again and the titration is over when a colour change from blue black to colourless is seen.

Captain-America

Yes, but before you start the titration it is red because the iodine is liberated.