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Homework Help: Two Tricky Problems

  1. Mar 7, 2006 #1
    A Tricky Problem

    1. If 4.74g of magnesium is reacted with 4.000L of 2.500mol/L hydrochloric acid, calculate the change in temperature for the hydrochloric acid.
    I think the equation is Mg(s) + HCl(l) ------> MgCl(2)(l) + H(2)(g)

    I know these two equations will be in use.
    1. Q=mTc
    2. H=Q/n

    I suppose I find the mol of magnesium.
    4.74g x 1mol/24.305g

    That's all I know about this, I don't know the next steps.

    Thank You.
    Last edited: Mar 7, 2006
  2. jcsd
  3. Mar 8, 2006 #2
    The change in temporature in this case is caused by the change in mol left after and used during the reaction.
    From the ratio 1:2:1:1 you calculate the HCl left. mol_left=given_mol - used_mol, then find the grams of HCl used.
    Use the table given in the textbook you are reading to check for c, reread the chapter to understand Q and also note the starting temperature that you need to compare with I think already stated in the problem itself. dQ=mcdT
  4. Mar 9, 2006 #3


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    Science Advisor
    Homework Helper

    In addition to what Emieno said, you should have been given the relevant enthalpy of reaction (or simply standard change in the energy of the reaction).
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