UV vis calculation

[Martoman]

I think NEAP are smoking crack.

A 3.5 g sample is dissolved in 20.0mL of acid. It is then diulted to 100mL.

Now you work out the amount of sample in the undiluted sample, then the dilution factor is 5, and the concentration in the diluted is 2.5ppm, so its just 2.5 * 5 = 12.5ppm.

Here is where I think its wrong. It asks to calculate the mass present in the sample.

What I did was that we know there is 12.5mg/L (as that's ppm). Then we want to know how much of that is in the 20mL sample.



This means that in the 20mL sample, there is 0.25mg.

They go at it a whole different way.They say xg of sample per 3.5 grams and 12.5 grams of sample per g of sample. (which is valid)

Then ratio it up to get
They seem to completely ignore the fact that it is in a 20mL sample.

But... i'm confused.

[m@tty]

What did you do in your calculation?

I would have done it like this:



Ah, turns out the same as yours. Wait, I see now, it is exactly the same.. Anyway..

I don't know what they've done...

Shouldn't it work out the same?

I now realise that I have not helped you at all. Fail.

[happyhappyland]

isnt ppm ug per L not mg per L?

[stonecold]

You don't want to know how much is in the diluted sample, you want to know how much there there is in the original sample.
We are talking about how much is in a certain mass, not how much is in the solution.

Remember your goal is to find out how much of whatever you are analysing for is present in the in the original sample, which in this question is a SOLID.

You are supposed to get w/w.  Instead you have done w/v.

SO basically there is x mg in 3.5g sample and there is 12.5mg in 1000g sample.

Do your calculations now and you should get 0.04mg.






That is how they got there answer, but it is wrong!

[stonecold]

isnt ppm ug per L not mg per L?

No.

It is either mg/L  or mg/kg (or 1000g)

[fady_22]

isnt ppm ug per L not mg per L?

No.

It is either mg/L  or mg/kg (or 1000g)

I think what he meant was ug per ml. That is equivalent to mg per litre.



I got a similar question on Insight, 2009. I really don't understand how they got to their answer.

[Mao]

@Martoman, can you give the exact wording of the question please?

It seems like there's two different interpretation of what 'ppm' refers to. You are thinking it is ppm (compound per solvent) [w/v], and they are thinking ppm (compound per sample) [w/w].

[Martoman]

Brazil nuts are an important source of the metal selenium, which is essential in trace quantities in virtually all animals. A 3.50 g sample of Brazil nuts was finely chopped and dissolved in 20.0mL of acid. The mixture was filtered and diluted to 100.0mL with distilled water. A small quantity of this solution was then passed through an AAS along with a number of solutions of known selenium concentration.

Question: Calculate the mass of selenium (in mg) present in the sample of Brazil nuts.

[Mao]

Yes, that seems fairly standard. What about the labels of the graph?

[Martoman]

absorbance vs solution concentration (ppm)

[Blakhitman]

Lol I remember the last part of that question asked something like, "suggest another way to determine the presence of selenium", I thought it'd be a qualitative technique they're looking for...the answer was gravimetric? I reckon that's pretty bad wording on their part.

I actually hate Neap.

[Martoman]

It said suggest another alternate technique to AAS which could analyse selenium. A flame test is what I put. That worked.

Their SOLUTION WAS AAS!!!!!!!!!!!!!!!!!!!!!!!

[Blakhitman]

O.o...was I looking at the right solution?

[m@tty]

Lol, if I remember correctly I put HPLC.... And didn't get the mark

[Blakhitman]

I put colorimetry...but I got it wrong...is that wrong?