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# Chemistry AS Edexcel moles help?

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1. What volume of 0.200 mol/dm3 potassium sulphate solution is required to make, by dilution with water, 1.00 dm3 of a solution with a POTASSIUM ion concentration of 0.100 mol/dm3?

A 100 cm3
B 250 cm3
C 400 cm3
D 500 cm3

Please include all the relevant steps in the calculation.
2. I just want to check before I explain - is the answer B?
3. (Original post by thechineeseone)
I just want to check before I explain - is the answer B?
Yes, it is! But why?
4. I hope this is helpful

The only equation used was n = cv and the re-arranged form v = n/c
The number of moles needed of K+ = 1dm3 x 0.100M = 0.100 moles
Volume of potassium sulphate needed = 0.100moles / 0.200M = 0.5 dm3 = 500 cm3
However bearing in mind that the chemical formula for potassium sulfate is K2SO4 (SO4 has 2- charge so requires 2 K+ ions to form a salt!). Therefore there is a stoichiometry of 1 K2SO4 : 2 K+ ions being produced

Therefore you need to divide 500 by 2 --> 250 cm3

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Updated: June 8, 2016
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