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    How do you calculate the mass lost when 0.50 moles of pink cobalt (II) chloride is turned completely into blue cobalt (II) chloride.
    CoCl2 + 2H2O + 4H2O (blue) CoCl2. 6H2O (pink)
    I know that mass = number of moles x relative atomic mass
    Mass = n x Mr
    Mass = 0.50mol x 237.9Mass = 118.95g
    The relative atomic mass of pink cobalt (II) chloride is 237.9.
    How do I go on from this to find the mass lost?

    You can deduce from the equation that for each mole of cobalt chloride you would "lose" 6 moles of water.
    Clearly you would lose 3 moles of water for half a mole of the salt.
    You need the mass of 3 moles of water- this is your answer.

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