|By Vivian on Sunday, October 13, 2013 - 11:28 am: Edit|
I am having trouble with #115 on the chapter 6 homework. I started by using the ideal gas equation to find the total number of moles. Then, using the percentages, I found the moles for each compound. Next I looked up the heats of formation and cancelled out the moles, added them, but got a wrong answer.
|By Bob Neilson on Sunday, October 13, 2013 - 05:34 pm: Edit|
Vivian.....#6.115: Yes, you need moles of each hydrocarbon and also the balanced combustion reactions for each. Use the heats of formation to find deltaH for each combustion reactions, prorate each to moles, then add.
|By MerielL on Monday, October 14, 2013 - 06:18 pm: Edit|
I'm having trouble with number 17 on the online homework. I've gotten as far as calculating delta E and the E associated with the given wavelength but I'm not sure how these two numbers fit together.
|By Bob Neilson on Monday, October 14, 2013 - 06:36 pm: Edit|
Meriel....From the given wavelength, determine the corresponding energy. This is the low energy end of the range. Your calculated deltaE is the energy difference between the low and high energy extremes. Thus you also know the high energy from which you can get the corresponding wavelength.
|By Brianna on Tuesday, October 15, 2013 - 06:49 am: Edit|
Hi I'm having trouble with number 99 and 105 and 109 from ch. 6. With number 99 I got q for the body, but I'm not quite sure where to go from there. And with 105 and 109 I'm not quite sure how to get calories, like with 109 I found qcal but I'm once again not sure of where to go from there
|By Bob Neilson on Tuesday, October 15, 2013 - 09:04 am: Edit|
Brianna.....#6.99: It takes 44 kJ of energy to evaporate 1 mole of liquid H2O. Find heat lost by the body in kJ and multiply by 1 mole / 44 kJ, then convert moles to grams.
#6.105: Write the balanced combustion reactions and use standard heats of formation (products - reactants) to find the heat of combustion for each substance in kJ/mole. Convert to kcal/g, remembering that a food Calorie is actually a kcal.
#6.109: The calorie content of the food equals the heat gained by the calorimeter when the food is combusted:
Q = (120 kJ/deg)(3.2 deg) = 384 kJ
That's the amount of heat from 16 g of food. Now just convert to kcal/g
|By Brianna on Tuesday, October 15, 2013 - 12:25 pm: Edit|
Thanks so much, although I'm still slightly confused as to how you are suppose to get from kJ/ mol to kCal/g on number 105 if you found the heat for the entire equation not just one element therefore you do not have a molar mass to help convert from moles to grams.
|By Bob Neilson on Tuesday, October 15, 2013 - 12:51 pm: Edit|
Brianna.....The deltaH for any reaction represents the amount of heat that is gained or lost for the number of moles of reactants and products as specified by the coefficients in the balanced equation. e.g., If a reaction produces 3 moles of SO2 and deltaH = -900 kJ, it means that the reaction is exothermic and 300 kJ are produced per each mole of SO2 (because it's 900 kJ per 3 moles).
|By Kirkland Polk on Tuesday, October 15, 2013 - 02:15 pm: Edit|
Hey Dr. B, I'm having trouble with #89 on the Chapter 7 homework. I used the given information and plugged it into PV=nRT to find the number of moles and used the given wavelength to find the Energy in joules. At this point, I didn't know whether to convert the number of moles to molecules using Avogadro's number, or if I was missing a step.
|By Bob Neilson on Tuesday, October 15, 2013 - 03:28 pm: Edit|
Kirkland.....#7.89: You're on the right track. One photon of light dissociates one I2 molecule, so find the number of I2 molecules from moles of I2, noting that only 15% of them react. From the wavelength, find E which is the energy of one photon, i.e., one I2 molecule. Multiply molecules by energy/molecule to get total energy.
|By Hunter Vaccaro on Tuesday, October 15, 2013 - 04:07 pm: Edit|
Hi Dr. Bob, I can't figure out what I'm doing wrong on #6.123. I set up the equation saying the quantity of heat lost by the coffee will equal the heat needed to melt the ice plus the heat the water gains (Q-lost by coffee=Q-melting ice+ Q-water gains). I plugged in all the values in the equation and end up with a final temperature of 83.7 degrees Celsius but the book says the final temperature should be 78.2 degrees Celsius. Any advice on where to recheck my work and see where I went wrong?
|By Bob Neilson on Tuesday, October 15, 2013 - 06:51 pm: Edit|
Hunter.....#6.123: It sounds like your method is OK -- probably just a minor math error. It takes 3,000 J to melt 9.0 g of ice so the overall heat transfer looks like the following.
(4.184 J/g.degC)(120 g)(90 degC - T) = 3,000 J + (4.184J/g.degC)(9.0 g)(T - 0 degC)
Solve for T = 78 degC
|By Mikaela Miller on Friday, October 25, 2013 - 09:30 am: Edit|
I'm having troubles with 9.108 on Mastering Chemistry. I don't understand what it's asking for. Here's what I put:
Is the problem asking for one lewis structure or three? What does it mean by "contributing structure"?
|By Bob Neilson on Friday, October 25, 2013 - 10:16 am: Edit|
Mikaela.....It's asking for three contributing resonance forms. As we discussed in class, one form may be preferred because it distributes the formal charges more evenly. However, it is still possible to consider other reasonable resonance forms. To be "reasonable" they should at least obey the octet rule for each nitrogen. One of yours does not.
|By Vivian on Friday, October 25, 2013 - 05:36 pm: Edit|
Dr. Bob I am having trouble with #113 from the chapter 8 homework problems. Could you please help me approach the problem?
|By Bob Neilson on Friday, October 25, 2013 - 07:52 pm: Edit|
Vivian.....Make plots of density vs atomic radius and density vs atomic number. The former is curved upward but the latter is almost linear. Estimate the missing data points by reading the graphs. It's quite approximate but the point is that such plots illustrate the periodic trends in properties like density and the others we discussed in class. The general concept here is more important that the details of the particular problem.
|By kaitlynupton on Saturday, October 26, 2013 - 11:00 am: Edit|
Dr. Bob, I am having trouble with chapter 8 book problem number 125. I understand how to get the 120 but not the 170
|By Bob Neilson on Saturday, October 26, 2013 - 02:22 pm: Edit|
Kaitlyn.....#8.125: In row 8, after element 120, you'll be filling the subshells: 5g^18, 6f^14, 7d^10, and 8p^6. Then, row 9 begins with 9s^2 subshell, making the next one in Group 2, element 170.