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.