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Higher 2001 Past Paper Q4

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  • Message 1. 

    Posted by MissChemTeacher_Bitesize (U15172316) on Sunday, 18th November 2012

    Hey guys!

    A wee multiple choice question to get you thinking about Chemistry:

    A mixture of sodium bromide and sodium sulphate is known to contain 5 mol of sodium and 2 mol of bromide ions.

    How many moles of sulphate ions are present?

    A: 1.5
    B: 2.0
    C: 2.5
    D: 3.0

    If you need help with the working post a message on the board.

    I'll put a solution up in a few days.

    Cheers Miss_Chem smiley - cool

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  • Message 2

    , in reply to message 1.

    Posted by MissChemTeacher_Bitesize (U15172316) on Sunday, 25th November 2012

    You need to use the valency cross over method to write the formula for each of the compounds:
    sodium bromide is NaBr
    sodium sulphate is Na₂SO₄

    This information can then be used to work out how many moles of ions there are in each of the compounds.

    Na : Br
    1 -->1
    2 <-- 2 (2 moles of bromide ions (from the Q) will give us two moles of sodium ions from the sodium bromide in our container.)

    Na₂SO₄
    2 -->1
    3 -->1.5 (we have 3 moles of sodium ions left over in the mixture (we had 5 to start off with) as the ratio of sodium to sulphate ions is 2 ---> 1 we can conclude that the number of moles of sulphate ions is 1.5).

    The answer is A.

    Hope this helps
    Miss_Chem smiley - cool

    Report message2

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