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Homework Help: Identifying Substance Z

  1. Oct 4, 2009 #1

    I have been set the following problem. Z2CO3(aq)+2HCl(aq) -> 2ZCl(aq) + CO2(g) + H2o(l)

    We did the experiment in class to find the amount of HCl it would take to titrate the soloution. First by measuring an amount of Z2CO3 and dissolving it into 250cm^3 of water and using methl orange indicator to know when the reaction had taken place. We did this three times using 25cm^3 samples and i got a mean titre of 26.8cm^3
    We are told that it is a group one element and the concentration of the HCl is 0.1
    So using the formula M=number of moles/volume in dm^3 i rearranged and got 0.1*0.0268=2.68*10^-3 meaning that it would be 2.68*10^-2 for the entire soloution.
    Knowing it will take 2 moles of HCl to react one mole of substance Z it would mean there was 1.134*10^-2 moles of substance Z in the 250ml soloution and knowing the original mass that was dissolved (1.5g) i can use the formula weight(g)/number of moles=molar weight which gave me 132.275g. Then knowing that the rest of substance Z is CO3 i can subtract their weights (12+16+16+16) which gives 72.275 and then halfing to find the molar weight of Z (because in the formula we have Z2) we get 36.1375. which is relatively close to potassium.

    Does my method follow correctly?
    And is the 3g that i am off from potassium enough to say there was a significant error in working or is it ok?

    Thanks in advance

  2. jcsd
  3. Oct 4, 2009 #2


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    Staff: Mentor

    Approach is correct, error is too large for my liking (but it was very likely potassium as you think).

  4. Oct 5, 2009 #3
    Ok thanks Borek
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