## Recommended Posts

please can i have help on part iii of this question. thanks!

##### Share on other sites

Just now, Anonymous32 said:

please can i have help on part iii of this question. thanks!

You will need to paste the relevant section into a post. No one is going to open unknown files. Also, please give an idea of your thoughts on tackling it, so we can see where to help. We will not just give you the answer, as you don’t learn anything from that.

##### Share on other sites

b) Breakfast cereals frequently have elemental iron added to them as a dietary supplement. A method for making a quantitative measurement of the amount of iron is to use the reaction between Fe3+(aq) and thiocyanate, SCN– (aq), which gives the deep red complex FeSCN2+(aq). Fe3+(aq) ൅ SCN– (aq) ื FeSCN2+(aq) The depth of the colour can be measured using a spectrophotometer which gives a value for the absorbance that is proportional to the concentration of the complex: absorbance ൌ constant ൈ [FeSCN2+] Equation 1 The constant can be found by measuring the absorbance of a solution of known concentration.

(i) The absorbance of a solution of the complex with concentration 2.5 ൈ 10–4 moldm–3 was measured to be 1.85; determine the value of the constant in Equation 1.

100 g of breakfast cereal was mixed with sufficient dilute acid to dissolve all of the iron. The solution was carefully filtered and mixed with sufficient oxidising agent to convert all of the iron to Fe3+. The solution was made up to a total volume of 250cm3 . 10.0cm3 of this solution was mixed with 10.0cm3 of a solution of thiocyanate; you may assume that all of the iron is converted to the complex. The absorbance of the resulting solution was measured as 0.519.

(ii) Using the value of the constant found in (i), calculate the concentration of Fe3+ in the solution for which the absorbance was measured. [2 marks]

(iii) Hence calculate the concentration of Fe3+ in the solution prepared from the cereal.

I have done part 1 and part 2 however for part 3 shouldn't the concentration of fe3+ in the solution be 25 greater than the concentration calculated in part 2 since the volume of solution is 25 times greater?

##### Share on other sites

15 minutes ago, Anonymous32 said:

b) Breakfast cereals frequently have elemental iron added to them as a dietary supplement. A method for making a quantitative measurement of the amount of iron is to use the reaction between Fe3+(aq) and thiocyanate, SCN– (aq), which gives the deep red complex FeSCN2+(aq). Fe3+(aq) ൅ SCN– (aq) ื FeSCN2+(aq) The depth of the colour can be measured using a spectrophotometer which gives a value for the absorbance that is proportional to the concentration of the complex: absorbance ൌ constant ൈ [FeSCN2+] Equation 1 The constant can be found by measuring the absorbance of a solution of known concentration.

(i) The absorbance of a solution of the complex with concentration 2.5 ൈ 10–4 moldm–3 was measured to be 1.85; determine the value of the constant in Equation 1.

100 g of breakfast cereal was mixed with sufficient dilute acid to dissolve all of the iron. The solution was carefully filtered and mixed with sufficient oxidising agent to convert all of the iron to Fe3+. The solution was made up to a total volume of 250cm3 . 10.0cm3 of this solution was mixed with 10.0cm3 of a solution of thiocyanate; you may assume that all of the iron is converted to the complex. The absorbance of the resulting solution was measured as 0.519.

(ii) Using the value of the constant found in (i), calculate the concentration of Fe3+ in the solution for which the absorbance was measured. [2 marks]

(iii) Hence calculate the concentration of Fe3+ in the solution prepared from the cereal.

I have done part 1 and part 2 however for part 3 shouldn't the concentration of fe3+ in the solution be 25 greater than the concentration calculated in part 2 since the volume of solution is 25 times greater?

I'm not sure why you would think that. What concentration of Fe3+ did you calculate in (ii), i.e. for the solution made up from the breakfast cereal?

##### Share on other sites

for part ii, i did 0.519/7400 which gives a concentration of 7.0135*10-5 mol/dm3

##### Share on other sites

1 hour ago, Anonymous32 said:

for part ii, i did 0.519/7400 which gives a concentration of 7.0135*10-5 mol/dm3

OK that's what I got too.

I suppose the problem is that question in (iii) is not very clearly expressed. I had to read it several times to work out what was going on. I assume what they want is the concentration of the solution from the breakfast cereal after making it up to 250ml.

Since you take 10ml of this and add the same quantity of thiocyanate solution to it, that operation dilutes it by half, doesn't it? So I'd have thought to get the concentration of the solution, you just double the result from (ii), don't you?

But it's a funny question, since I'd have thought the interesting thing to work out would be how much iron there is in the original breakfast cereal, which you can now work out, since you know from the concentration of this solution and its volume how much iron it contains, and you also know it comes from 100g of cereal. But maybe you will do that in class later or something.

(Please check my logic though, as I may have misunderstood the question.)

Edited by exchemist
##### Share on other sites

thank you, i understand my mistake now!

##### Share on other sites

40 minutes ago, Anonymous32 said:

thank you, i understand my mistake now!

That's a result, then!

## Create an account

Register a new account