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Chemistry AS Edexcel moles help?

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    • Thread Starter

    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.

    I just want to check before I explain - is the answer B?
    • Thread Starter

    (Original post by thechineeseone)
    I just want to check before I explain - is the answer B?
    Yes, it is! But why?

    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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