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OCR chemistry past questions confusion AS

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
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    A student mixes 100 cm3 of 0.200 mol dm–3 NaCl(aq) with 100 cm3 of 0.200 mol dm–3 Na2CO3(aq).
    What is the total concentration of Na+ ions in the mixture formed?
    A 0.100 mol dm–3
    B 0.200 mol dm–3
    C 0.300 mol dm–3
    D 0.400 mol dm–3
    How would I go about working this out ? The answer is C
    Which mass of substance contains the greatest number of atoms?
    A 3.00 g of ammonia, NH3
    B 3.00 g of chloromethane, CHCl3
    C 4.00 g of hydrogen sulfide, H2S
    D 4.00 g of hydrogen chloride, HCl
    The answer is A , would you use Avogadro ?
    Which volume of oxygen gas at room temperature and pressure is required for complete combustion of 1.25x10-3 mol of propan-1-ol ?
    The answer is C ?
    Thank you in advance

    In the 1st question, NaCl releases 1mol of Na+ per mole of NaCl, so 0.2moldm^-3 of NaCl will release the same concentration of Na+. However, Na2CO3 releases 2 moles of Na+ per mole of compound, so the concentration of Na+ is double that of Na2CO3, so it has concentration 0.4 moldm^-3 of Na+
    As the 2 solutions are of equal volume, the mean concentration of Na+ should be found, which is (0.2+0.4)/2 =0.3
    For the second question, yes you would use avogadro's constant
    For the 3rd question, write out the balanced equation for the combustion of propan-1-ol, and multiply the number of moles of oxygen in the equation by the number of moles of propan-1-ol, which is 1.25x10^-3 . Multiply this value by 24000cm^3, as this is the volume that 1 mole of gas occupies at RTP. This should give an answer of 135cm^3
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