I've added 20 ml methanol with 1 g powder then it was evaporated by a rotary evaporator and re-dissolved in 4 ml methanol and then it was diluted with 10 ml water. So, How can I calculate the final concentration? What is the actual dilution factor?
Short answer is - you can't, unless you weighed the flask with 1g powder+20 ml methanol before and after rotary evaporation.
If you know the weight before and weight after, then you know how much methanol you removed and can work out how concentrated your remaining solution was (then you can calculate the concentration you've made after addition of 4 ml methanol and 10 ml water)
Fiona Smail - Thanks for your answer but after getting the concentration of remaining solution should I have to multiply 4 and 10 with the remaining concentration of methanol (final result*remaining conc. of MeOH*4*10)?
Depending on the level of accuracy you need, you will have to check something called "mixing volumes". 4 ml methanol + 10 ml water will not equal 14 ml of liquid - so you need to work out what the actual volume of the solvent mixture will be (google "mixing volume of methanol and water").
Do you expect that your have lost some of your 1g of powder (have some components from that mixed powder been removed via distillation?)
a) If you expect that ALL of your 1 g remains, then you need to add your water and methanol, measure the final volume and calculate a g/ml concentration for your new solution
b) If you have lost some of your material due to distillation, but can measure the concentration by some other method (gas chromatography?) then you can (again) calculate the mass remaining, measure the final volume of your solution after you've added your solvents and calculate a g/ml concentration
So.... you need
1) an accurate, small volume measuring cylinder or graduated pipette
2) Possibly a way to check the concentration you have left after your rotary evaporation (gas or liquid chromatography, but you'd need to compare with a standard of known concentration)
3) Information on mixing volumes (depending on how accurate you want this to be - reduction in volume will probably be ~0.2 ml maximum)
I hope I've understood you correctly - from concentration values you should be able to convert to dilution factors.
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