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Exam question

  1. Jun 2, 2009 #1
    (a) An acid, HX, reacts with sodium hydroxide as shown in the equation
    below:

    H2X(aq) + 2NaOH(aq) -----> 2Na+(aq) + X2-(aq) + 2H2O

    A solution of this acid was prepared by dissolving 1.92 g of H2X in water and making the volume up to 250 cm3 in a volumetric flask.

    A 25.0 cm3 sample of this solution required 21.70 cm3 of 0.150 moldm-3 aqueous NaOH for complete reaction.

    (i) Calculate the number of moles of NaOH in 21.70 cm3 of 0.150 moldm-3 aqeous NaOH.


    (ii) Calculate the number of moles of H2X which reacted with this amount of NaOH. Hence, deduce the number of moles of H2X in the 1.92 g sample.

    Moles of H2X in 25.0 cm3 of solution .

    Moles of H2X in 1.92 g sample


    (iii) Calculate the relative molecular mass, Mr, of H2X.




    3. The attempt at a solution[/b





    part i) is 21.7 × 10-3 × 0.150 = 3.255 × 10-3 (mol)
    ii) is In 25 cm 3 = (3.255 × 10-3)/2 = 1.63 × 10-3 (mol)

    but i dont know how to get the second part, however i know the answer is 1.63 × 10-2

    iii) the anwer is 118, and again how do i get to it


    thanks.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
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