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1.05 g MgO is added to 110.15ml HCL with an initial temperature

  1. Feb 14, 2010 #1
    1.05 g MgO is added to 110.15ml HCL with an initial temperature of 21.5 degrees Celsius and a final temperature of 34.5 degrees Celsius. calculate the KJ of energy released for the reaction. assume that 4.18 J of energy is required to change the temperature of one mL of solution to one Celsius degree.


    my thinking. q=mcdeltat

    q=?
    m=110.15ml
    c=4.18kj
    delta t=13

    =(110.15)(4.18)(13)
    =5985.55Kj
    =6.0x10^3
     
    Last edited: Feb 14, 2010
  2. jcsd
  3. Feb 15, 2010 #2

    Borek

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    Staff: Mentor

    Re: reaction

    Your thinking is not incorrect, but you are incredibly cruel to your units. Mass is not in ml, and your use of k is completely off.
     
    Last edited by a moderator: Aug 13, 2013
  4. Feb 15, 2010 #3
    Re: reaction

    thanks, so how about i transfer the 110.15ml to grams which is 110.15g and then do q=mc[tex]\delta[/tex]t
    q=(110.15g)(4.18J/g^oC)(13^oC)
    q=5985.551 J = 598 KJ
    q=5.9x10^3 J

    so the answer is q= 598 KJ or q=5.985x10^3 J ??

    better?

    also how can I calculate KJ/mol of MgO from this? has to do with the molar mass in guessing? which for Mg is 24.31 and oxygen 16.0 so 40.31 so 5985/40.31=148.47 Kj/mol of MgO. did i do this correctly?
     
    Last edited by a moderator: Aug 13, 2013
  5. Feb 15, 2010 #4

    Borek

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    Re: reaction

    Very nice and very correct.

    Nice try - but wrong. kJ is 1000 Joules.

    No. You need molar mass, but you have to use it to calculate number of moles of MgO. Again - if you would pay attention to units it would be obvious something is wrong.
     
    Last edited by a moderator: Aug 13, 2013
  6. Feb 15, 2010 #5
    Re: reaction

    ok, thanks, i see so it must be 5.98KJ, makes sense

    for the second part, hmmm how about if i did the amount of MgO which is given

    (1.05g MgO)(1mol MgO)/(40.31 g MgO)=.0260

    im not sure about this one
     
    Last edited by a moderator: Aug 13, 2013
  7. Feb 15, 2010 #6

    Borek

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    Staff: Mentor

    Re: reaction

    As usual you omitted units in your answer, but I guess you mean 0.0260 moles of MgO?

    You know 5.98 kJ (beware: small k, not capital K) was produced by reaction of 0.0260 moles of MgO. Now just combine these numbers so that the answer is in kJ/mol.
     
    Last edited by a moderator: Aug 13, 2013
  8. Feb 15, 2010 #7
    Re: reaction

    yes that is what i meant, my apologies

    by combine do you just mean say that there is 5.98 kJ per 0.0260 moles of MgO

    or 5.98kJ/0.0260 mol MgO= 230kJ/mol
     
    Last edited by a moderator: Aug 13, 2013
  9. Feb 15, 2010 #8

    Borek

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    Re: reaction

    Looks OK :smile:

    Note: I have not checked numbers, but they look reasonable.
     
    Last edited by a moderator: Aug 13, 2013
  10. Feb 15, 2010 #9
    Re: reaction

    i appreciate the help, thanks
     
    Last edited by a moderator: Aug 13, 2013
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