Pia R.

# Enthalpy problem need help asap

CH3OH (l) + 3/2 O2 (g) -> CO2 (g) + 2H2O (l)

delta h = -726.5 kJ

calculate the mass of methanol needed to create 6.90 x 10^4 J of heat

By: Tutor
4.6 (26)

UC Berkeley Grad for Chemistry, Organic Chemistry and Biology Tutoring

Pia R.

Will it be

-726.5 kj * (1/1000 kJ) * (1/ (6.90 * 10^4 J)) * 32.04 g * (1/0.850 g) * 4.18 J
Report

05/15/16 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.
Report

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?
Report

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
Report

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
Report

05/16/16

Pia R.

SORRY FOR FLOODING!

I got -3.04 g is it possible to have a negative mass?

Report

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.