Week 5/6 Sapling Question 7

[Sofia Lucido 3L]

I am having some trouble with question 7 for week 5/6 sapling. I thought I carried out the same steps the hint suggested, but I am still missing something.

To start, I converted 450 W of energy supplied by the heater to 450 J s-1. Then I used dimensional analysis to find how much energy was supplied in 3.42 minutes and got 92.34 kJ. For the first sample I subtracted the initial mass from the final mass, got -104.85 g and then divided it by the molar mass to get -2.28 moles. Then I divided the amount of heat supplied by the amount in moles: 92.34/-2.28 to get -40.5 kJ/mol. I checked this answer to see if I was on the right track to starting the problem and it was incorrect so now I am puzzled. Can someone help me figure out how to start this problem off?

Thanks!

[IsaacLaw1E]

You shouldn't have a negative number for the mass. You want to have a positive number that represents how much was vaporized, which will be the initial amount minus the final amount. The final amount is what wasn't vaporized, so the difference is what was vaporized.

[darchen3G]

You should do final - initial, not initial - final. You want the change in the mass. The answers for this question are all positive. Otherwise, your procedure is correct.

[Joshua Eidam 2A]

Your problem is coming from the calculated difference in the masses. Switch the order of subtraction so that your value is positive and you should get the right answer.

[Lilly Catarozoli 1B]

Everything is right except it is final - initial. Then your calculations should work!

[Katelynn Shaheen 2C]

The amount that was vaporized is represented by a positive number not a negative number.