# Homework Help: Beer lambert ques

1. Aug 29, 2009

### UKstudent

Hi, I'm an pharmacy student and I've forgotten how to answer questions on the beer lambert law. I would be grateful if anyone could show me how to answer these questions below.

(a)In pharmaceutics it is usual to use the specific absorption coefficient A (1%, 1 cm). What is this term?
(2 marks)

(b)The A(1%, 1cm) value for paracetamol in 0.1M sodium hydroxide is 715 at a wavelength of 257 nm. Calculate the concentration in μg/ml of a solution of paracetamol in 0.1M sodium hydroxide which gives an absorbance of 0.9 in a cell of pathlength 1 mm.
(6 marks)

(c)The concentration calculated in part (b) was as a result of a sample taken at 45 minutes from a dissolution test of a conventional release 500 mg paracetamol tablet. The volume of the dissolution medium was 900 ml. Calculate the amount of paracetamol (in mg) dissolved in the vessel and the percentage released from the 500 mg paracetamol tablet.
(4 marks)

Many thanks!

2. Aug 29, 2009

### kldickson

Beer-Lambert's law, if I remember correctly, is A=$$\epsilon$$lc, where A is absorbance, $$\epsilon$$ is absorption cross-section, l is path length, and c is concentration.

3. Aug 30, 2009

### UKstudent

Yep, you're correct. Any ideas on which numbers to plug in into that equation?

4. Aug 30, 2009

### Staff: Mentor

Those given in the questions.

5. Aug 30, 2009

### UKstudent

LOL. so tell me then, what would be my A, L,E and concentration values.

6. Aug 30, 2009

### Staff: Mentor

Have you actually read the question, or are you just hoping someone will do it for you? Some parameters are given explicit values and are called identical in both your post and kldickson's post. Other may require a little bit of tinkering, but they are all there.

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7. Aug 30, 2009

### UKstudent

See that’s the thing. I’ve forgotten completely how to answer questions like this. My book doesn’t explain it and neither is it in the lecture notes. So I would plug these values into the equ:
A= lc

0.9=0.1c

so 9=c….at which point I doubt my answer.

Moles=mass/conc

So 0.1= 151/c
So c=1510
And at this point I know I’ve missed out a step or am making stuff up. Just really confused.

8. Aug 30, 2009

### Staff: Mentor

9. Aug 30, 2009

### kldickson

What are you plugging in for equations B and C?