[Amine]

"Sodium Hydroxide is often sold as a concentrated solution containing 15.0moldm^-3 of NaOH solution. Calculate the volume of water that should be added to 20.0cm³ of a 15.0moldm^-3 solution of NaOH, to make a 0.0250moldm^-3 solution.

I was using C₁V₁=C₂V₂

So:
(15.0)*(20/1000)=(0.0250)*V₂

Hence V₂=12dm³

However this is the total volume, so the volume of water needed to be added is 12-(20/1000)=11.8dm³.

But, I am unsure, because I looked at answers provided and this is their method:
Total Volume= (20*15)/0.25=1200cm³, therefore volume of water=1200-20=1180cm³. What I don't understand is why was (20*25) divided by 0.25 and not 0.025?..

Thanks for any help provided!

[Borek]

12-(20/1000)=11.8dm³

Check your math (you are close, but slightly wrong).

What I don't understand is why was (20*25) divided by 0.25 and not 0.025?

Must be a typo, your calculations look OK to me (ignoring the mistake mentioned).