1) Concentrated Chloroform Water BP contains 1 part of chloroform diluted to 10 parts with aqueous diluent. How should this be further diluted to give Chloroform Water BP?
A) Concentrated Chloroform Water BP diluted 1 in 4 with aqueous diluent
B) Concentrated Chloroform Water BP diluted 1 in 8 with aqueous diluent
C) Concentrated Chloroform Water BP diluted 1 in 20 with aqueous diluent
D) Concentrated Chloroform Water BP diluted 1 in 40 with aqueous diluent
E) Concentrated Chloroform Water BP diluted 1 in 80 with aqueous diluent
Question from student: I know concentrated chloroform water is 10% and Chloroform water bp is single strength i.e 2.5%, what do they mean by "Concentrated Chloroform Water BP contains 1 part of chloroform diluted to 10 parts with aqueous diluent". My understanding tells me that 10 in 100 so 1 in 10, so if 2.5 is in 100 then 1 should be in 40, please tell me if i am wrong
Answer from tutor: You are correct to say the 1 in 10 is a 10% solution. i.e. for every 1g there is 10ml of solution, similarly for every 10g there are 100ml of solution which is as you say the definition on 10% w/v. The question here is how do you get from a 1 in 10 (10%) to a 1 in 40 (2.5%). The answer you have given 1 in 40, means you would get 1ml of the 10% solution and dilute it with 40ml of diluent. To work this out: 1ml of a 10% solution contains 0.1g, you take this 0.1g and put it in 40ml this would give you a 0.25% solution which is 1 in 400, NOT 1 in 40!
A better way to look at this is to think what is the difference between a 1 in 10 (10%) and a 1 in 40 (2.5%). Well first off the former is a stronger solution, how much stronger? Well, exactly 4 times stronger. So how do you move from one to another well you get one piece, say 1ml and add 4ml of something else, that will make it a quarter the strength of the original. This is also called dilution factor.
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