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titration with potassium manganate and iron (II)

i'm stuck in understanding the dilution of potassium manganate for a titration.

I know that when diluted, the volume of MnO4- is 24.73 so roughly 25. my teacher suggested, because it is a 1:4 ratio (i have had to multiply the concentration of money by 1/2*1/4) that 25 goes in the flask and 100 goes in the flask. and that all of this solution should be added to the burette. 25:100

but i do not understand this because i thought the standard solution goes in the pipette and into the flask. i don't know. could someone please explain please.
Original post by esmeralda123
i'm stuck in understanding the dilution of potassium manganate for a titration.

I know that when diluted, the volume of MnO4- is 24.73 so roughly 25. my teacher suggested, because it is a 1:4 ratio (i have had to multiply the concentration of money by 1/2*1/4) that 25 goes in the flask and 100 goes in the flask. and that all of this solution should be added to the burette. 25:100

but i do not understand this because i thought the standard solution goes in the pipette and into the flask. i don't know. could someone please explain please.


I see that your confusion is with dilutions and concentrations, but you explanation is very difficult to understand. It would help if you were to give more detail about what you're describing (putting units on numbers for example)

ie '25 goes in the flask and 100 goes in the flask' is not much good is you dont tell us which chemicals are going in the flask.

Hopefully you can be more informative and help us help you!
Original post by esmeralda123
i'm stuck in understanding the dilution of potassium manganate for a titration.

I know that when diluted, the volume of MnO4- is 24.73 so roughly 25. my teacher suggested, because it is a 1:4 ratio (i have had to multiply the concentration of money by 1/2*1/4) that 25 goes in the flask and 100 goes in the flask. and that all of this solution should be added to the burette. 25:100

but i do not understand this because i thought the standard solution goes in the pipette and into the flask. i don't know. could someone please explain please.


Could you start at the begining and work through what you did. That way we can help you better.
Reply 3
Avatar for debbie394
debbie394
OP
11
Original post by MexicanKeith
I see that your confusion is with dilutions and concentrations, but you explanation is very difficult to understand. It would help if you were to give more detail about what you're describing (putting units on numbers for example)

ie '25 goes in the flask and 100 goes in the flask' is not much good is you dont tell us which chemicals are going in the flask.

Hopefully you can be more informative and help us help you!


Okay so first I need to make a standard solution with 5 iron tablets and sulphuric acid. This needs to go in the volumetric flask I think. I am using potassium manganate and this will go in the burette. From my dilution calculation,w which I will show you below, I have worked out that I need to put 25cm3 in the burette for every titration I think. Because of this, I can conduct 10 titrations from one standard solution.

These are my calculations but I don't know how to put them into a practical:

So first I worked out the volume of potassium manganate when one iron tablet was used, and found that the volume was too small. My teacher told me to increase the number of iron tablets used or decrease the concentration of MnO4- to increase volume of MnO4-

Mr of C4H2FeO4 (the iron tablet is ferrous fumurate (iron)): 169.8
I am given that the mass of one iron tablet is: 210g but I convert this to mg as told by my teacher so 210*10-3 mg
Moles of Fe2+: 1.236749*10-3 (this would be in 250cm3 because that is how much can fit in the volumetric flask, which I have in my notes)
Moles of Fe2+ in 25cm3 (i think this is done for the standard solution so this should be in the pipette): so i divided the moles of fe2+ by 10: 1.2367494*10-4
Moles of MnO4- (this is the molar ratio which is *5 ): 2.47398*10-5
Volume of MnO4- (I am given that the concentration of MnO4- is 0.02): 1.23674*10-3 so this would be 1.23674 dm3

so then i increased the number of tablets (the maximum number of tablets per titration i can use is 5):
moles of fe2+: (0.21*5)/169.8=6.183745583*10-3
moles of fe2+ in 25cm3: 6.183745583*10-4
moles of MnO4-:1.236749117*10-4 cm3
volume of MnO4-:6.183745585*10-3=6.18374558 dm3 so this is still too small.

then I did the dilution calculation:
Dilution of MnO4-: 1/2 * 1/2 * 0.02 (the concentration given this is)= 2.5 *10-3 mol dm3
=24.734 cm3

then my teacher gave me the ratio
1:4
?:250
25:100
my teacher said 25 (i suppose cm3) in the pipette and 100(cm3) in the flask
and then add of the solution to the burette.

Have I even done the dilution calculation correctly and what exactly does the ratios mean. please help
actually now that i have done the dilution calculation i think have multiplied the concentration of 0.02 by 1/8 so 1/2*1/2*1/2*0.02 which gives me 2.5*10-3
does this mean the ratio is 1:8 and what does this mean for the volumes. i haven't replaced this with my original calculation because I don't even know if this is even correct. this is so much harder!!
(edited 6 years ago)
Reply 4
Avatar for debbie394
debbie394
OP
11
Original post by TutorsChemistry
Could you start at the begining and work through what you did. That way we can help you better.



Okay so first I need to make a standard solution with 5 iron tablets and sulphuric acid. This needs to go in the volumetric flask I think. I am using potassium manganate and this will go in the burette. From my dilution calculation,w which I will show you below, I have worked out that I need to put 25cm3 in the burette for every titration I think. Because of this, I can conduct 10 titrations from one standard solution.

These are my calculations but I don't know how to put them into a practical:

So first I worked out the volume of potassium manganate when one iron tablet was used, and found that the volume of potassium manganate that would be put in burette was too small. My teacher told me to increase the number of iron tablets used or decrease the concentration of MnO4- to increase volume of MnO4-

Mr of C4H2FeO4 (the iron tablet is ferrous fumurate (iron)): 169.8
I am given that the mass of one iron tablet is: 210g but I convert this to mg as told by my teacher so 210*10-3 mg
Moles of Fe2+: 1.236749*10-3 (this would be in 250cm3 because that is how much can fit in the volumetric flask, which I have in my notes)
Moles of Fe2+ in 25cm3 (i think this is done for the standard solution so this should be in the pipette): so i divided the moles of fe2+ by 10: 1.2367494*10-4
Moles of MnO4- (this is the molar ratio which is *5 ): 2.47398*10-5
Volume of MnO4- (I am given that the concentration of MnO4- is 0.02): 1.23674*10-3 so this would be 1.23674 dm3

so then i increased the number of tablets (the maximum number of tablets per titration i can use is 5):
moles of fe2+: (0.21*5)/169.8=6.183745583*10-3
moles of fe2+ in 25cm3: 6.183745583*10-4
moles of MnO4-:1.236749117*10-4 cm3
volume of MnO4-:6.183745585*10-3=6.18374558 dm3 so this is still too small.

then I did the dilution calculation:
Dilution of MnO4-: 1/2 * 1/2 * 0.02 (the concentration given this is)= 2.5 *10-3 mol dm3
=24.734 cm3

then my teacher gave me the ratio
1:4
?:250
25:100
my teacher said 25 (i suppose cm3) in the pipette and 100(cm3) in the flask
and then add of the solution to the burette.

Have I even done the dilution calculation correctly and what exactly does the ratios mean. please help
actually now that i have done the dilution calculation i think have multiplied the concentration of 0.02 by 1/8 so 1/2*1/2*1/2*0.02 which gives me 2.5*10-3
does this mean the ratio is 1:8 and what does this mean for the volumes. i haven't replaced this with my original calculation because I don't even know if this is even correct. this is so much harder!!
(edited 6 years ago)
Original post by esmeralda123
Okay so first I need to make a standard solution with 5 iron tablets and sulphuric acid. This needs to go in the volumetric flask I think. I am using potassium manganate and this will go in the burette. From my dilution calculation,w which I will show you below, I have worked out that I need to put 25cm3 in the burette for every titration I think. Because of this, I can conduct 10 titrations from one standard solution.

These are my calculations but I don't know how to put them into a practical:

So first I worked out the volume of potassium manganate when one iron tablet was used, and found that the volume of potassium manganate that would be put in burette was too small. My teacher told me to increase the number of iron tablets used or decrease the concentration of MnO4- to increase volume of MnO4-

Mr of C4H2FeO4 (the iron tablet is ferrous fumurate (iron)): 169.8
I am given that the mass of one iron tablet is: 210g but I convert this to mg as told by my teacher so 210*10-3 mg
Moles of Fe2+: 1.236749*10-3 (this would be in 250cm3 because that is how much can fit in the volumetric flask, which I have in my notes)
Moles of Fe2+ in 25cm3 (i think this is done for the standard solution so this should be in the pipette): so i divided the moles of fe2+ by 10: 1.2367494*10-4
Moles of MnO4- (this is the molar ratio which is *5 ): 2.47398*10-5
Volume of MnO4- (I am given that the concentration of MnO4- is 0.02): 1.23674*10-3 so this would be 1.23674 dm3

so then i increased the number of tablets (the maximum number of tablets per titration i can use is 5):
moles of fe2+: (0.21*5)/169.8=6.183745583*10-3
moles of fe2+ in 25cm3: 6.183745583*10-4
moles of MnO4-:1.236749117*10-4 cm3
volume of MnO4-:6.183745585*10-3=6.18374558 dm3 so this is still too small.

then I did the dilution calculation:
Dilution of MnO4-: 1/2 * 1/2 * 0.02 (the concentration given this is)= 2.5 *10-3 mol dm3
=24.734 cm3

then my teacher gave me the ratio
1:4
?:250
25:100
my teacher said 25 (i suppose cm3) in the pipette and 100(cm3) in the flask
and then add of the solution to the burette.

Have I even done the dilution calculation correctly and what exactly does the ratios mean. please help
actually now that i have done the dilution calculation i think have multiplied the concentration of 0.02 by 1/8 so 1/2*1/2*1/2*0.02 which gives me 2.5*10-3
does this mean the ratio is 1:8 and what does this mean for the volumes. i haven't replaced this with my original calculation because I don't even know if this is even correct. this is so much harder!!



We may need a rethink of what's going on here.

The tablet is really 210g? Please check that. 210g is the weight of about of 4 mars bars. 210mg perhaps?

You teacher said put 25cm3 (of the tablet solution?) into the pipette and 100cm3 (of what? water?) into the flask. If you are adding 100 cm3 of water then this "dilution" is irrelevant - the 25cm3 of tablet solution you put into the flask contains all the Fe2+, and this is what you will titrate with the permanganate no matter how much water you added. The extra water would be there just to help see the endpoint better perhaps.
Maybe it was dilute acid you added? Afain, it isn't changing the amount of Fe2+ being titrated.

What did you use to measure the 100cm3 of added whatever?

Why would you only put about 25cm3 of permanganate solution into the burette? Always fill the burette initially.
Reply 6
Avatar for debbie394
debbie394
OP
11
Original post by TutorsChemistry
We may need a rethink of what's going on here.

The tablet is really 210g? Please check that. 210g is the weight of about of 4 mars bars. 210mg perhaps?

You teacher said put 25cm3 (of the tablet solution?) into the pipette and 100cm3 (of what? water?) into the flask. If you are adding 100 cm3 of water then this "dilution" is irrelevant - the 25cm3 of tablet solution you put into the flask contains all the Fe2+, and this is what you will titrate with the permanganate no matter how much water you added. The extra water would be there just to help see the endpoint better perhaps.
Maybe it was dilute acid you added? Afain, it isn't changing the amount of Fe2+ being titrated.

What did you use to measure the 100cm3 of added whatever?

Why would you only put about 25cm3 of permanganate solution into the burette? Always fill the burette initially.


here's the method i have put together to make this clear. i just have to change to measurements obviously. also it is 210mg but i have used 210*10-3 g. away here's the method, does this make the dilution more clearer?

1.

1. Find the mass of five iron tablets. Crush the weighed tablets in a mortar and pestle to give a large surface area to volume ratio. This will mean that the rate of reaction will be increased because the more particles are exposed to the other reactant, which will be sulfuric acid.





1.

2. Transfer all the ground material to a beaker where it will be dissolved in approximately 100cm3 of the 1 mol dm-3 of aqueous sulfuric acid. Sulfuric acid is used instead of deionised water because the titration needs to be carried out under acidic conditions. Iron (II) is less likely to be oxidised to iron (III) by atmospheric oxygen in acidic solution than in neutral solution and for an accurate result the iron should not be converted to iron (III) before the titration is conducted.





1.

3. All of this solution (including washings) is transferred to a 250cm3 volumetric flask and the solution made up to the mark with deionised water. Deionised (distilled) water is used as it does not contain ions or impurities that may affect the results of the titration, usually by reacting with acid/alkali themselves or by changing the pH of the solution. As water is pH 7, it is neutral and will not change the pH. A pipette should be used when the solution is very close to the gradation line. The solution should be brought up to the graduation line so that the bottom of the meniscus lines up with the flask’s graduation mark. The pipette can be used to remove the excess solution. The volumetric flask should be stoppered and inverted several times. This is done to ensure that the solution is thoroughly mixed so that the reactants can react together. This is the standard solution containing iron (II) ions.





1.

4. Wash the pipette, burette and conical flask with deionised water. Rinse the burette with the potassium manganate (VII) solution and the pipette with the iron(II) solution. Using a pipette filler, fill the pipette with the iron(II) solution and transfer the contents of the pipette to the conical flask. Acidify this solution by adding 10cm3 of dilute sulfuric acid. If a brown precipitate appears during the titration this means that Mn(IV) is formed, because of incomplete reduction of the Mn(VII). This should only happen if there is insufficient sulfuric acid in the conical flask. The remedy is to add more dilute sulfuric acid to the flask, or, preferably, to repeat the experiment with sufficient acid present in the flask.





1.

5. Using a funnel, fill the burette with potassium manganate (VII) solution, making sure that the part below the tap is filled before adjusting to zero. Because of the intense colour of KMnO4 solution, readings are taken from the top of the meniscus.





1.

6. With the conical flask standing on a white tile, add the solution from the burette to the flask. Swirl the flask continuously and occasionally wash down the walls of the flask with deionised water using a wash bottle.





1.

7. The end point of the titration is detected by ‘the first persisting pink colour’. Note the burette reading.





1.

8. Repeat the procedure ten times, adding the potassium manganate (VII) dropwise approaching the end point. These accurate titres should agree to within 0.1cm3. Calculate the concentration of the iron (II) solution and from this calculate the mass of iron in an iron tablet.

Original post by esmeralda123
here's the method i have put together to make this clear. i just have to change to measurements obviously. also it is 210mg but i have used 210*10-3 g. away here's the method, does this make the dilution more clearer?

1.

1. Find the mass of five iron tablets. Crush the weighed tablets in a mortar and pestle to give a large surface area to volume ratio. This will mean that the rate of reaction will be increased because the more particles are exposed to the other reactant, which will be sulfuric acid.






1.

2. Transfer all the ground material to a beaker where it will be dissolved in approximately 100cm3 of the 1 mol dm-3 of aqueous sulfuric acid. Sulfuric acid is used instead of deionised water because the titration needs to be carried out under acidic conditions. Iron (II) is less likely to be oxidised to iron (III) by atmospheric oxygen in acidic solution than in neutral solution and for an accurate result the iron should not be converted to iron (III) before the titration is conducted.






1.

3. All of this solution (including washings) is transferred to a 250cm3 volumetric flask and the solution made up to the mark with deionised water. Deionised (distilled) water is used as it does not contain ions or impurities that may affect the results of the titration, usually by reacting with acid/alkali themselves or by changing the pH of the solution. As water is pH 7, it is neutral and will not change the pH. A pipette should be used when the solution is very close to the gradation line. The solution should be brought up to the graduation line so that the bottom of the meniscus lines up with the flask’s graduation mark. The pipette can be used to remove the excess solution. The volumetric flask should be stoppered and inverted several times. This is done to ensure that the solution is thoroughly mixed so that the reactants can react together. This is the standard solution containing iron (II) ions.






1.

4. Wash the pipette, burette and conical flask with deionised water. Rinse the burette with the potassium manganate (VII) solution and the pipette with the iron(II) solution. Using a pipette filler, fill the pipette with the iron(II) solution and transfer the contents of the pipette to the conical flask. Acidify this solution by adding 10cm3 of dilute sulfuric acid. If a brown precipitate appears during the titration this means that Mn(IV) is formed, because of incomplete reduction of the Mn(VII). This should only happen if there is insufficient sulfuric acid in the conical flask. The remedy is to add more dilute sulfuric acid to the flask, or, preferably, to repeat the experiment with sufficient acid present in the flask.






1.

5. Using a funnel, fill the burette with potassium manganate (VII) solution, making sure that the part below the tap is filled before adjusting to zero. Because of the intense colour of KMnO4 solution, readings are taken from the top of the meniscus.






1.

6. With the conical flask standing on a white tile, add the solution from the burette to the flask. Swirl the flask continuously and occasionally wash down the walls of the flask with deionised water using a wash bottle.






1.

7. The end point of the titration is detected by ‘the first persisting pink colour’. Note the burette reading.






1.

8. Repeat the procedure ten times, adding the potassium manganate (VII) dropwise approaching the end point. These accurate titres should agree to within 0.1cm3. Calculate the concentration of the iron (II) solution and from this calculate the mass of iron in an iron tablet.




Step 1.2 and 1.3?

You dissolve 5 tablets in a quantity of acid and make this solution up to 250cm3. The intermediate volume is not relevant to the dilution, only the weight of the tablets and the final volume.
I hope that is the dilution you are referring to, I don't really see another in your script :smile:

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