# A simple pchem question involving calculations **URL:** \ **Category:** Factual Questions\ **Created:** [October 3, 2006, 4:04am UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074 "2006-10-03T04:04:54Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [October 3, 2006, 4:04am UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074/1 "2006-10-03T04:04:54Z")
Ok, so apparently, after getting through Cal II, I can’t do simple algebraic calculations. I’m looking for, in the end, the resonance energy of benzene, and I have a method to get there, but I’m stuck on a certain part. I have that ΔU of a process that I’m using to calculate this is 180.kJ/g=29214kJ/mol. (The compound has a molecular mass of 162.3, this isn’t where I’m worried about my math.) So, I need the enthalpy now. I have: ΔH=ΔU+Δ(PV) ΔH=ΔU+VΔP (constant volume bomb calorimeter) ΔH=ΔU+(VRT/V)Δ_n_ ΔH=ΔU+RTΔ_n_ ΔH=???+(.06214kJ/mol K)(298K)(-4.5mol) Why question marks? How do I get from either kJ/g or kJ/mol to just plain old kJ? And the reverse is true, because I need kJ/g or kJ/mol for the next step, when I reference a table for some information needed to calculate the resonance energy. (Homework disclaimer: This is _technically_ homework, but it’s for a lab, and I’ve hit a wall, and done the rest of the labwork and calculations myself) ---
**Author:** ![Ins\_Outs\_What-have-yous](https://avatars.discourse-cdn.com/v4/letter/i/8797f3/32.png) [@Ins\_Outs\_What-have-yous](https://boards.straightdope.com/u/Ins_Outs_What-have-yous)\ **Post date:** [October 3, 2006, 4:53am UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074/2 "2006-10-03T04:53:44Z")
In the equation ΔH=ΔU+RTΔn, the change in internal energy (i.e. ΔU) should be in terms of joules or kJ instead of kJ/mol or kJ/g. It appears as if you already know that. You need to provide more info. You say that “The compound has a molecular mass of 162.3.” I assume you meant 162.3 g/mol. Correct? So, how many grams of this compound are in the system? (180 kJ/g) \* (X g) = 180X kJ = ΔU It’s been a while since I’ve taken thermodynamics. I hope this helps. ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [October 3, 2006, 12:16pm UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074/3 "2006-10-03T12:16:17Z")
That’s really all I do? I thought about that last night, and I didn’t see how it was OK to arbitrarily multiply by the number of grams sample just so the units canceled. ---
**Author:** ![Dervorin](https://avatars.discourse-cdn.com/v4/letter/d/eb8c5e/32.png) [@Dervorin](https://boards.straightdope.com/u/Dervorin)\ **Post date:** [October 3, 2006, 4:14pm UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074/4 "2006-10-03T16:14:26Z")
[QUOTE=chaoticbear] That’s really all I do? I thought about that last night, and I didn’t see how it was OK to arbitrarily multiply by the number of grams sample just so the units canceled. [/QUOTE] Think about this logically - any reaction that gives off, say 120 kJ/g is going to give off 120 kJ if you react 1g of the substance, 240 kJ if you react 2g, and so on. So total energy of reaction = specific energy \* mass (assuming you have the same units for mass for both specific energy and mass). Alternatively, specific energy = total energy / mass. Rearranging, we get: total energy = specific energy \* mass. I’m just putting this out in point form because I think it’ll jog your memory. If you’d like more detail, that can be provided. ---
**Author:** ![sinjin](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@sinjin](https://boards.straightdope.com/u/sinjin)\ **Post date:** [October 3, 2006, 10:14pm UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074/5 "2006-10-03T22:14:30Z")
Or in more specific terms you are mixing Intensive and Extensive properties. You are looking for H on the left (an extensive property) while using u + v(delta-P) on the right side, (intensive properties). Units rule!!! BTW how do you get greek symbols on the dope? ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [October 4, 2006, 12:36pm UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074/6 "2006-10-04T12:36:49Z")
Point 1: After trying what you said, I still get a positive ΔH[sub]combustion[/sub], which is totally wrong for what I’m doing. This means I made a mistake a thousand steps back somewhere, or our results for the experiment put the resonance energy of benzene somewhere near 7000kJ/mol. I’m going to go talk to the professor today. I feel like such an idiot having to get help on the calculations for a lab, though! Point 2: Character map. The Alt code for it is supposedly in there, but I can’t get it to work in hex or in decimal. ---
**Author:** ![threemae](https://avatars.discourse-cdn.com/v4/letter/t/5f9b8f/32.png) [@threemae](https://boards.straightdope.com/u/threemae)\ **Post date:** [October 4, 2006, 1:28pm UTC](https://boards.straightdope.com/t/a-simple-pchem-question-involving-calculations/375074/7 "2006-10-04T13:28:08Z")
Wait, what’s the change in gas states? I think it would help if you could write out the balanced chemical reaction with states, or have I missed that? Okay, so once you have that, just plug in the difference via the ideal has law to your delta (PV) term and voila, correct?