How much solvent do you add for a 1:20 dilution, and why is it called 1 to 20? [closed]








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> 
> A 1:20 dilution implies that you take 1 part of stock solution and add 19 parts of water to get a total volume of diluted solution equal to 20 times that of the stock solution
> 
> 
> 


So is `1:20` the way it is because the ratio is `solute:solvent/diluent`, right? So the solvent/diluent is just `n-1` of the solvent?




Dilution ratio 1 : N has 2 different, conflicting meanings:


* (1) The ratio of the 2 mixed volumes ( typical for 1 : 1 to 1 : 3, for everyday/household chemistry )
* (2) The ratio of the final and initial concentrations (typical for 1 : 5 and more, for using calibrated volumes and exact concentrations ).


Consult the context of your question, but the quote obviously considers the latter case. 1 : 20 means the solute concentration is 20 times lower than before dilution ( the second row below).


1 : 20(1) means 1 : 21(2) ( approximately(\*) )  

1 : 20(2) means 1 : 19(1) ( approximately )


(\*) - **There is one trap:** It is better to say you take 1 volume of the stock solution and while mixing, you fill up by solvent the final volume N. As volumes are not generally additive. By other words, by mixing $\pu{10 mL}$ of solution and $\pu{190 mL}$ of solvent, you may not get $\pu{200 mL}$.


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