[Twickel]

Hello
Today I did a titration using sodium thiosulfate as the standard to determine concentration of copper, using starch as indicator.

So I made my standard and copper solution and added KI to my copper solution that I was titrating. So I added the sodium thiosulfate until the dark brown colour was pale yellow, I then added starch indicator and added standard until the blue was gone. BUT for 2 runs I forgot to add potassium thiosulfate to the mixture as the pale blue colour was nearly gone. For the two runs in which I did not add postassium thiosulfate I got around 3.7ml of standard added. When I added the postassium thiosulfate I got 23.9 ml of standard added. When the potassium thiosulfate was added, the endpoint was never white it was weird white with a grey tinge to it.
 
Does that seem reasonable on a theoretical base. The computer marks the calculations, so I will porbably get 2/10 and fail the course now because of this.

How annoying and stupid of me

Now the unknown solution of copper hydrate was pre made to 20mL, I had to dilute it 100mL. From my titrations I had a mean titration volume of 23.89mL of NaSa2O3 hydrate added.
so, n=m/M = 1.5597/ ( molar mass of 248.18). From that I got n= 6.28 x10^-3
so c= 6.28x10^-3/0.02389 = 0.2629 M
My demo said you multiply the number of moles by 5 ( to account for the dilution) to determine moles of copper ion. then use m= Mxn.

I am very confused. I weighed 1.5597gm of sodium thiosulfate to make a roughly 0.025M of 250mL solution.
So why is it that when I use my mean titration volume in the c=n/v formula do I get a concentration of 0.2629 M, was I that of in my titration?

If so please let me know so I can tie my noose.

[Borek]

I would love to help, but it is so chaotic I can't understand what you did.

[Twickel]

ok. Thanks btw, I am so anxious I am literally shaking.

Some points about my titration - made standard to 250mL, copper solution was made ready as 20mL i diluted it to 100mL.

I weighed 1.5597 grams of sodium thiosulfate hydrate. So the number of moles = 1.5597/248.18 = 6.3x10^-3

My mean titration volume was 23.89mL.  so c( S2O3-) = 6.3x10^-3/0.02389 = 0.26M

n( 6.3x10^-3) = n ( Cu2+) x 5 = 0.0314 mol
m= 0.0314 x  249.7 =7.84 gm. That cannot be correct, what have I done wrong in my calculations?

I do not know how in the prac I ended up getting 0.7497 gm. 

Was my titration so off that the concentration using the volume of titration was 0.008M higher then what it was when I calculate it using n=m/M

[Borek]

I weighed 1.5597 grams of sodium thiosulfate hydrate. So the number of moles = 1.5597/248.18 = 6.3x10^-3

My mean titration volume was 23.89mL.  so c( S2O3-) = 6.3x10^-3/0.02389 = 0.26M

In what volume have you dissolved the thiosulfate? In 23.89 mL, or in 250 mL?

[Twickel]

250mL

[sjb]

250mL

So, if you have 1.5597g of thiosulfate in 250 mL, what is the concentration?

[Twickel]

N= 1.5597/248.2
n= 6.28x10^-4 mol of thiosulfate ion

c= 6.28x10^-4/0.25
c=0.02514M

Overall eqn isCu2+ + S2O32- + I2I- ---> 2CuI + S4O6-.

Now how do I figure out the mass of copper sulfate penthydrate from this information. My mean titration volume was 23.89mL
somehow in the prac and this got checked by 2 demos, I got 0.74xx g of Copper sulfate penthyrate, now i cannot reproduce that number.

[Twickel]

I meant n= 6.28x10^-3 mol

[Borek]

At least you have titrant concentration now.

http://www.titrations.info/titration-calculation

[Twickel]

Ok, so n= 1.5597/248.2
6.2840x10^-3 mol of S2O3 2-

c= 6.2840x10^-3/0.250
 = 0.02514 M

number of moles of thiosulfate used to reach end point = 0.02514M x0.02389
=6.005946x10^-4 mol

number of moles of cupper ion = 6.005946x10^-4 x 5 = 3.002973 x10^-3

mass of copper sulphate hydrate = 3.002973x10^-3 x 249.7 = 0.7498g

Is that about right?

[Borek]

Looks OK to me.

[Twickel]

If only my titrations were done properly, may I ask, why do some methods say add the starch indicator and potassium thiosulfate. But my method said add starch then when the blue colour is almost gone ( did not know if that meant when it turns white but the blue re appears or when its pale blue), then add the potassium thiosulfate?

Thank you so much for your help.

[Borek]

Potassium thiosulfate? Sodium thiosulfate is the titrant, amount of thiosulfate added has to be precisely controlled, adding some random amount of potassium thiosulfate is going to ruin the result. Its not making any sense to me.

[Twickel]

Potassium thiocyanate is added I mean.