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# Chemistry - Iodine/thiosulphate titration

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1. Hi! Any help with this question would be much appreciated!

1.52g of impure sodium chromate is dissolved in distilled water, transferred into volumetric flask, and made up to a volume of 250cm3. 25cm3 portions of this solution are pipetted into a conical flask, acidified, and then reacted with excess potassium iodide. The resulting mixture is titrated with 0.1moldm-3 sodium thiosulphate solution, 25.4cm3 of the thiosulphate being required.
Calculate the percentage by mass of sodium chromate in the impure solid.

2. (Original post by cassie05)
Hi! Any help with this question would be much appreciated!

1.52g of impure sodium chromate is dissolved in distilled water, transferred into volumetric flask, and made up to a volume of 250cm3. 25cm3 portions of this solution are pipetted into a conical flask, acidified, and then reacted with excess potassium iodide. The resulting mixture is titrated with 0.1moldm-3 sodium thiosulphate solution, 25.4cm3 of the thiosulphate being required.
Calculate the percentage by mass of sodium chromate in the impure solid.

What have you tried?
3. (Original post by alow)
What have you tried?
Na2CrO4 volume: 25cm3 = 0.025dm3
Na2S2O3 volume: 25.4cm3 = 0.0254dm3
Na2S2O3 conc: 0.1moldm-3
Na2S2O3 mol: 2.54mol

Then I guess I need to work out the molar ratio of Na2S2O3 to Na2CrO4 to work out how many moles of Na2CrO4 there are. But I'm not sure how to.

Once I know how many moles there are, I can use that and the molar mass to find out the mass of sodium chromate.
4. (Original post by cassie05)
Na2CrO4 volume: 25cm3 = 0.025dm3
Na2S2O3 volume: 25.4cm3 = 0.0254dm3
Na2S2O3 conc: 0.1moldm-3
Na2S2O3 mol: 2.54mol

Then I guess I need to work out the molar ratio of Na2S2O3 to Na2CrO4 to work out how many moles of Na2CrO4 there are. But I'm not sure how to.

Once I know how many moles there are, I can use that and the molar mass to find out the mass of sodium chromate.
It is the excess KI which is reacting with the thiosulfate in the titration ...

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