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Chemistry: Finding a temperature change using a temperature/gram ratio

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    In this investigation, you mix .5g of Mg(s) with 115mL HCl. You measure the temperature change which turns out to be 25.5 degrees C. The equation is as follows:

    Mg + 2HCL --> MgCl2 + H2

    Use the specific heat of water (c=4.184) to find the heat of formation.

    What is the temperature change of 1mol of MgCl2?

    2. Relevant equations
    Q=cm(t2-t1)
    Kind of basic algebra that I think I'm getting wrong. Or maybe I'm using the wrong variables.

    3. The attempt at a solution
    This problem uses the specific heat of water. This means that we must use the mass of water as well (1mol = 18g).
    Q=4.184(18g)(t2-t1)

    I am having trouble with the temperature change. Mg is the limiting reactant. Therefore:

    .5g Mg (mol/24.31g) = .021mol Mg (mol MgCl2/mol Mg) = .021mol MgCl2

    .021mol MgCl2 (95.21g MgCl2/mol MgCl2) = 1.99941 ~2.00g MgCl2

    Therefore:

    (25.5C/2.00g MgCl2) * 95.21g MgCl2 = 1214C

    This answer however, isn't correct. Am I supposed to use the masses of water instead?
     
  2. jcsd
  3. Feb 28, 2009 #2

    Borek

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    Staff: Mentor

    Temperature of _what_ has changed?
     
  4. Feb 28, 2009 #3
    Can anyone help?
     
  5. Mar 1, 2009 #4

    Borek

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    Staff: Mentor

    Can you answer the question asked in my previous post?

    You are not asked about temperature change of 1 mol of MgCl2, you must have misread the question. This is a classic experiment for determination of molar enthalpy of the reaction.
     
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