Tiglath M. answered • 05/15/16
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UC Berkeley Grad for Chemistry, Organic Chemistry and Biology Tutoring
The enthalpy change you're given refers to the combustion of methanol. Since the enthalpy change is given in KJ/mol and you know how much heat is released you can find how many moles of methanol are combusted. Then using the periodic table you need to calculate the molecular weight of methanol (g/mol) and use it to find the mass of methanol.
Tiglath M.
The first part of your calculation is not correct. 1KJ = 1000 J so
delta H = - 7.265 X 10^5 J/mol
Then I'm not sure why you're dividing by 6.90 x 10^-4. You need to do the opposite.
mol CH3OH = (6.90*10^-4)/(7.265*10^-5) = 9.49759
You've got the correct MW for methanol and it's correct that you need the multiply by the MW to get the mass of methanol but I'm not sure why you're dividing by 0.850 and multiplying by 4.18. All you need to find is the mass of methanol. I think that you're confusing this question with calorimetry.
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05/16/16
Pia R.
Oh okay i dont need to use the reaction like the coefficients in the reaction, i dont multiply them In the computation?
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05/16/16
Pia R.
-726.5 kJ x (1000J/1kJ)
then since J is on top, I have to divide by 6.90x10^4 J to cancel out J
then now im unitless so ill multiply it by MW?
am i right? I dont get why 6.90 HAS TO BE AT THE TOP. please help thanks
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05/16/16
Pia R.
or would it be
6.90 x 10^4 X 1/-726.5 kJ x 1kJ/1000 J x 32.04/1 mol
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05/16/16
Pia R.
SORRY FOR FLOODING!
I got -3.04 g is it possible to have a negative mass?
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05/16/16
Tiglath M.
I don't understand what you mean about " like the coefficients in the reaction, i dont multiply them In the computation?". The value for the enthalpy change refers to the amount of heat released when 1 mol of methanol is combusted. The answer to your third question is yes. The solution you've shown is correct, expect that you first need to convert KJ to J. This how find the mass of methanol. The answer to your fourth question is that it is physically impossible to have negative mass. The negative sign in front of the enthalpy value will get cancelled by 6.90 x 10^4 J since it also has a negative magnitude. Heat is being created when methanol reacts with oxygen.
Finally whenever you find the final answer check that it makes physical sense. For instance you cannot have negative mass.
Hope this answers your questions.
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05/17/16
Pia R.
05/15/16