Dilution Calculation

[LikeA'13OSS]

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?

Thanks for your time

[LMaxwell]

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 April 13, 2012 by brofessional

[Sandwich]

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.

[ILM]

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

[Survival Robot]

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