# Dilution Calculation

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I have 50 cm3 of sodium thiosulfate at 0.2 mol/dm3. If i used 30 cm3 of it and added 20 cm3 of water, whats the new concentration?

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50cm3 sodium thio. with 0cm3 water = 0.2 mol/dm3

30cm3 sodium thio. with 20cm3 water = 0.12 mol/dm3

Calculation: 50cm3 = 100%, therefore 30cm3 = 60%.

(60/100) x 0.2 mol/dm3 = 0.12 mol/dm3.

You can use the same method to calculate the concentrations at other volumes.

Hope this helped,

brofessional

p.s. I always doubt my maths, but I think this is correct.

Edited by brofessional
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That's not quite right - for example if you had a sip out of a glass of salt water, you wouldn't expect the remaining water to become less salty. In a similar way, using some of a solution isn't the same as diluting it. If you use 30cm3 of 0.2mol/dm3 then the remaining stuff would still be 0.2mol/dm3. You've not diluted it in any way, and as the solution is of an equal concentration throughout to begin with, removing some of it won't change the overall concentration, it just means you have less of it. You'll have less mols of sodium thiosulphate if you dried it out and looked at how much you had, but PER DM3 it's the same concentration.

So the only equation is to work out what would happen if you diluted 30cm3 of 0.2mol/dm3 with 20cm3 water. THIS time you're diluting it, so the concentration will change.

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To end the confusion, you are taking 30 cm3 of 0.2 thiosulfate and add 20 cm3 of water to it, so:

C1V1= C2V2

0.2*30= C2*50

0.6*0.2= C2

C2= 0.12

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