## Question 8.19 units and equation

$\Delta U=q+w$

marinah
Posts: 29
Joined: Fri Sep 25, 2015 3:00 am

### Question 8.19 units and equation

For number 19, it asks to calculate the required heat that must be supplied to a 500.0-g stainless steel vessel containing 400.0g of water to raise its temp from 22.0 C to 100.0 C. I started out by using the equation q= gCpΔT I'm just confused on why we have to add (400g)(4.184)(100-22) + (500g)(0.38)(100-22) together; is it because qsys + qsurr=0?
Also why don't we convert the temp C to K in this problem?
Is the standard unit for q KJ?
Sorry these are a bunch of questions together, I'm just trying to understand the conceptual network behind this problem.

Kelsey Ouyang 3H
Posts: 27
Joined: Fri Sep 25, 2015 3:00 am

### Re: Question 8.19 units and equation

We need to add the two together because we need to supply enough heat to raise the temperature of both the vessel and the water. We know that heat moves from a hot object to a cold object so therefore not only one can be heated because if just the vessel is heated, the heat will move to the water and the vessel's temperature will lower and vice versa. Therefore both the vessel and the water have to be heated to the same desired temperature so that heat transfer will not lower one and raise the other so that the temperature of both end up settling at a temperature that is not the desired temperature value.
We do not need to convert the temperature from Celsius to Kelvin because we are looking at delta T. Though Kelvin and Celcius are different temperature scales, the differences in increments are the same. K = °C + 273 so if we are looking at delta T of 1ºC and 6ºC, delta T will be 5. Now if we converted the celcius to Kelvin, 1ºC= 274 K and 6ºC = 279 K so the delta T will still be 5.
The standard unit of q is Joules.

Hope this answered your questions! sorry if my explanation is a little confusing...

Bryan Lau 3H
Posts: 53
Joined: Fri Sep 25, 2015 3:00 am

### Re: Question 8.19 units and equation

I got 1.5 x 10^2 kJ for part (a) of this problem. The solution manual & textbook get 1.4 x 10^2 kJ, but it rounds twice, for q(water) and q(Cu). As a result, it gets 1.45, which is rightfully rounded to 1.4 for 2 sig figs. However, I got 1.5 x 10^2 kJ because I rounded (apply sig figs) only once, at the end of the problem. Therefore, I got 1.453..., so I rounded to 1.5 for 2 sig figs. Can anyone explain to me my misunderstanding of sig figs?

SubparChemist
Posts: 27
Joined: Fri Sep 25, 2015 3:00 am

### Re: Question 8.19 units and equation

Could just be an error in the solution's manual, I was wondering the same thing Brian. The sig figs in the manual seem sorta wonky at time.

Bryan Lau 3H
Posts: 53
Joined: Fri Sep 25, 2015 3:00 am

### Re: Question 8.19 units and equation

SubparChemist wrote:Could just be an error in the solution's manual, I was wondering the same thing Brian. The sig figs in the manual seem sorta wonky at time.

Okay, Thanks !

Cristian Yanes 1L
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

### Re: Question 8.19 units and equation

I've noticed that for a few problems the solutions manual/textbook round multiple times, leading to a bit of a difference in answers.

Chris Huerta 2D
Posts: 10
Joined: Fri Sep 25, 2015 3:00 am

### Re: Question 8.19 units and equation

To the question about rounding, in our workbook there are rules for rounding off and one of them is to round at the end of the calculation in one step. I think it's safe to assume that the solutions manual was incorrect here.