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Help on a-level chemistry question

  1. Apr 19, 2004 #1
    Hi I got given these 2 planning q's to do by my teacher but I dont have a clue how to do the actual experiment. If i knew that much then I would b able to write about it. Could n e 1 help me please

    Plan an experiment that would enable you to measure the molar enthalpy change of the reaction between magnesium metal in different concentrations of dilute hydrochloric acid. :eek:

    I thought about this question and I thought could it be the calorimeter with the zinc and the cup and the thermometer but then it says diff concentrations of HCl so i got a bit confused so could some1 please help me.

    Plan an experiment to purify solid H by recrystallisation.
    You are required to plan an experiment by which a pure sample of H could be prepared, from an impure sample of solid, ready for a melting temperature determination. Your method will involve recrystallising H from a suitable solvent. Assume that you have access to data giving the solubility of H in different solvents at different temperature.

    I looked at this question and the first thing that hit me was what is recrystalisation? and what has it got to do with the solubilities? Please help me!!!

    Thank you for anyone who may be able to help me.

  2. jcsd
  3. Apr 20, 2004 #2
    If you're measuring enthalpy, then yeah, do a calorimetry experiment. It's easy enough to just make up a variety of solutions with different concentrations and measure these. Then you can just plot enthalpy change versus concentration to work out molar enthalpy.

    I'm not sure what you mean by the second one. Do you want to prepare solid H2? I have no idea how to do that. I mean, H2 is a gas even at about 20 K.

    Recrystallisation is a method where you prepare a sample, then dissolve it and then let it crystallise out of solution. It increases the purity of your sample.
  4. Apr 20, 2004 #3
    Thanks for your help and H is not hydrogen but a letter that stands for an unknown compound.
  5. Apr 20, 2004 #4
    Alright, so say your sample is slightly soluble in water, and the solubility greatly increases with heat. Figure out from the table how much water will dissolve your sample at 100 degrees C. Add that much water and boil. When you get to one hundred degrees C, all of your sample will dissolve. Now take off the heat and cool to 0 degrees. Your sample will crystallize out of solution while the impurities remain in the solution. This is recrystallization. You can then just filter out the water. You'll lose a little bit of the sample, and it won't be exactly 100% pure, but you can get pretty close and always do another recrystallization.
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