[highschoolhelp16]

How do you calculate the amount of heat required to convert 100.0g of solid methanol at its normal freezing point to gaseous methanol at its normal boiling point? I have tried everything I can think of, and after 2hrs of ridiculous answers I need help.

[fledarmus]

So what does "everything you can think of" consist of?

[highschoolhelp16]

I have tried q=m*s*deltaT, and I have done multiple equations involving the boiling and vaporization point. I tried (100.0g)(0.424ca/gc)(337.2k-175.5k)=6,877.28cal, i think that is in the right direction but I am unsure. it think that is the heat required to bring the methanol to its boiling point.

[UG]

I think you are forgetting to include the enthalpy of fusion (energy needed for solid liquid at the melting temperature) and the enthalpy of vaporisation (energy needed for liquid gas at the boiling temperature) into your calculation. Your calculation of (100.0g)(0.424ca/gc)(337.2k-175.5k)=6,877.28cal only gives the energy needed to heat the liquid from 175.5 K to 337.2 K, it doesn't take into account the conversion of solid to liquid or liquid to gas.

[highschoolhelp16]

so how would i fit 3.16 KJ/mol into the equation?

[UG]

Well, how many moles of methanol do you have?

[highschoolhelp16]

0.062 mol nh4no3

[UG]

Methanol has molecular formula CH₃OH.

[highschoolhelp16]

sorry I was looking at the wrong problem, its 0.16 mol CH3OH

[UG]

I think you might like to check that again because I got a different answer. Remember you have 100 g of methanol. But anyways, once you have worked out the number of moles, you can find the energy needed for the phase transformation. I assume 3.16 kJ/mol is the enthalpy of fusion?

[highschoolhelp16]

its 3.12 mol, and yes the Hfusion is 3.16kJ/Mol

[UG]

Ok, so if you have 3.12 moles and the enthalpy change is 3.16 kJ PER mole, then the energy change needed for solid to liquid transformation is? It is a similar process for finding the energy needed to go from liquid to gas, but you'll be using a different enthalpy value obviously. Then you need to add these energy values onto your calculation of (100.0g)(0.424ca/gc)(337.2k-175.5k) but you will need to convert to the same units.

[highschoolhelp16]

so is the energy change needed 9.86kJ/Mol?

[UG]

It will just be 9.86 kJ, the 'moles' unit cancel out when you multiply.

[highschoolhelp16]

so the equation should be (100.0g)(0.424cal/gc)(337.2K-175.5K)(9.86kJ)?