Working out the concentration of a sample solution..

[chloeukc]

I have this question to do.. and I've been stuck on it for AGES! Please help!

 

Calcium in a sample solution was determined by atomic absorption spectrophotometry. A stock solution of calcium was prepared by dissolving 3.668 g CaCl2.2H2O in water and diluting to 1000 cm3. This was diluted 1:10. Standards were prepared by diluting the second solution, respectively, 1:20, 1:10 and 1:5. The sample solution was diluted 1:25. Strontium chloride was added to all solutions before dilution. Absorbance signals, when the solutions were aspirated into an air-acetylene flame, were as follows: standards, 9.1, 18.6, 37.0; sample 20.9. What was the concentration of calcium in the sample in parts per million?

[John Cuthber]

Work out the concentration of calcium in all the standard solutions. Plot a graph of the absorbance vs concentration. Look on the graph to find what concentration corresponds to 20.8 then remember that it had been diluted and calculate the original concentration.