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

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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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### CottonTheButton

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Updated on August 22, 2022

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?