How do I calculate the concentration of an undiluted sample?

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Me_Me_Big_B0y
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So I have to figure out the concentration of Fe3+ in Irn-bru from an experiment and I'm struggling a bit as to where to start with the calculations for the concentrations so I can make a graph.

Basically in this experiment there are five 50 cm^3 flasks that were used.
All of the flasks have 20 cm^3 Irn-Bru inside.

During the middle of the experiment I had this:

Flask 1) Has the 20 cm^3 of Irn-Bru and is filled up to 50 cm^3 with water.

Flask 2) Has 2cm^3 of Nh3 solution added and 4 drops of ammonium citrate and 6 drops of ammonium thioglycolate

Flask 3) Has 2cm^3 of Iron Standard, and has 2cm^3 of Nh3 solution added and 4 drops of ammonium citrate and 6 drops of ammonium thioglycolate

Flask 4) Has 5cm^3 of Iron Standard, and has 2cm^3 of Nh3 solution added and 4 drops of ammonium citrate and 6 drops of ammonium thioglycolate

Flask 5) Has 10cm^3 of Iron Standard, and has 2cm^3 of Nh3 solution added and 4 drops of ammonium citrate and 6 drops of ammonium thioglycolate

After this I calculated absorbances for all of the flasks at 535 nm
(The first flask was set as the standard)
And for that I ended up with some values, I don't want to put actual values down because I want to work out the problem for myself, or at least know the steps in order to DO the problem myself- but:

Flask 1) 0.000 A
Flask 2) 0.100 A
Flask 3) 0.200 A
Flask 4) 0.300 A
Flask 5) 0.400 A

And the standard Fe3+ I used had 50 mg/L of Fe3+


I'm supposed to do a graph of the absorbance against the concentration but I'm struggling a bit with calculating the concentrations for the graph and then doing the concentrations for the undiluted Fe3+
I know undiluted concentration will be a negative number at least.

I don't really need anything to do with the absorbances here, I suppose.
I'm just curious as to how to find the concentrations of Fe3+ in various solutions that all have 20cm^3 of Irn bru in there, various amounts of the iron standard solution (which all have 50 mg/L of Fe3+), then filled up with water to have a total volume of 50 cm^3

I know if I convert my cm^3 units to dm 3 I'd have

So my amounts of iron standard for three of the flasks would be:

2 cm^3 = 0.002 dm^3
5 cm^3 = 0.005 dm^3
10 cm^3 = 0.010 dm^3

do I multiply these by the standard Fe3+? It's the one that has 50 mg/L of Fe3+
Or well when I calculated that, it would just give me units of mg since the dm^3 would cancel out
Last edited by Me_Me_Big_B0y; 2 months ago
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