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IGCSE Chemistry May/June 2015 Paper 32

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    Question 6 c iii asks for the number of moles of Cu(NO3)2H2O in 7.26g.

    I work out the molecular mass of Cu(NO3)2H2O to be 206 g mol.

    Therefore moles = 7.26 / 206 = 0.035 mol which rounds to 0.04 mol.

    The mark scheme gives 0.03. I'm baffled. The rest of the question doesn't work if this bit isn't right. Any ideas?

    You are told that the ration between Cu(NO3)2.xH2O and CuO is 1:1 in the previous part of the question.
    Therefore, you first find the number of moles of CuO which is 2.4/80 = 0.03
    There are also 0.03 moles of Cu(NO3)2.xH2O because of the 1:1 ratio.
    You can't find the molecular mass of Cu(NO3)2.xH2O because you don't know the value of x, so you don't know how many H2O (water) molecules the hydrated salt contains.
    Hope that helped.
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