[cortes118]

Hi this is my first post, not sure if this is against rules or if it's in the wrong forum, but here I go

So I'm doing a standardisation of KIO3 against Ascorbic acid(1g/1L)

the problem is when I do the calculations I get 0.00607M when instead what I should be getting is around 0.002M

Here are the calculations I'm currenty doing

C1*V1=C2*V2

C1=n/1

n=g/mm

n=1/176.12

n=0.005 mol

C1=0.005/1

sub values into  equation (KIO3 added was 18.7 ml)(in method 20 mls of ascorbic acid was pippeted into 150ml of distilled water)

0.005 * 20 = C2 *18.7

C2 = 0.05*20/18.7 = 0.006 M

I'm thinking that the extra 150ml of water dilutes it further, I'm not too sure however. if anyone knows what is going wrong with the calculations any help would be greatly appreciated.


Method

Solutions Needed
Potassium iodate solution: (0.002 mol L−1).
Starch indicator solution: 0.5%. Potassium iodide solution: (0.6 mol L−1)
Dilute hydrochloric acid: (1 mol L−1).
Sodium Thiosulfate 0.1 M solution

Titration
1. Pipette 20 mL of Ascorbic acid(1g/1000ml) into a 250 mL conical flask and add about 150 mL of distilled water, 5 mL of 0.6 mol L−1 potassium iodide, 5 mL of 1 mol L−1 hydrochloric acid and 1 mL of starch indicator solution.
2. Titrate the sample with the 0.002 mol L−1 potassium iodate solution. The endpoint of the titration is the first permanent trace of a dark blue-black colour due to the starch-iodine complex.
3. Repeat the titration with further aliquots of sample solution until you obtain concordant results (titres agreeing within 0.1 mL).
Result Calculations
1. Calculate the average volume of iodate solution used from your concordant titres.
2. Calculate the moles of iodate that reacted forming iodine.
3. Using the equation of the reaction between the iodate ions and iodide ions (below) calculate the moles of iodine formed.
      2 IO3− + 10 I− + 12 H+ → 6 I2 + 6 H2O
4. From the titration equation (below) determine the moles of ascorbic acid reacting.
ascorbic acid + I2 → 2 I− + dehydroascorbic acid
5. Calculate the concentration in mol L−1, of ascorbic acid in the solution obtained from fruit/vegetable/ juice. Also, calculate the concentration in mg/100mL or mg/100g of ascorbic acid in the sample.

[Borek]

What you wrote is highly cryptic. Why don't you list what you mean by C1, V1, C2 and V2, and what their values are?

[cortes118]

Certainly
C1 is the concentration of the Ascorbic Acid 0.005 Molarity
V1 is the volume of the ascorbic acid          20 ml
C2 is the concentration of the KIO3            Unknown
V2 is the volume of the KIO3 added            18.7 ml

[Borek]

And what their known values are? Sorry, you want me to read everything and distill the information. I am ready to help, but I don't plan on reading everything to find all the necessary bits of data. Up to/Back to you.

[cortes118]

the values are to the right on my reply

if your wondering what n is it is the moles of vitamin C

mm is molar mass

and I forgot to say in equation C1 = n/1, 1 is 1 L (from equation C=n/v)

[Borek]

They react 1:1 and volumes are comparable, so concentrations must be very similar too - without even switching on my calculator I can tell that concentration of iodate should be by about 5% higher than 0.005M. 0.006 looks too large for me, but 0.002 is definitely out of the question (with the numbers you listed).

OK, with calculator now: 20*0.005=18.7*x, so

[tex]x = \frac {20} {18.7} 0.005 = 0.00535[/tex]