Help with this thermodynamics and entropy question please

• takelight2
You can't tell that from what I wrote, but that's clearly what I was thinking about.Nope. I mistakenly meant the ratio of the MC's. You can't tell that from what I wrote, but that's clearly what I was thinking about.In summary, the conversation involves someone trying to solve a physics problem but getting one part wrong and not understanding why. The first question asked for the final temperature and the person got 42.06 Celsius. The second question asked for the total change in entropy and the person is using the equation Q=mcT but is getting different results. They are unsure of where they went wrong and ask for clarification on their process.

takelight2

Homework Statement
A 124.4 g insulated aluminum cup at 18.46 ∘C is filled with 130.0 g of water at 46.91∘C. After a few minutes, equilibrium is reached.

a. Determine the final temperature. (completed)

b. Determine the total change in entropy.
Relevant Equations
Q=mcT
So I've answered the first question and I got a final temp of 42.06 Celsius.

Now for this second one, I don't know why I am getting it wrong:

Im doing 0.215*ln(315.06/291.46) + 1*ln(315.06/319.91)

But it says I am wrong. What about my process is faulty?

takelight2 said:
Homework Statement:: A 124.4 g insulated aluminum cup at 18.46 ∘C is filled with 130.0 g of water at 46.91∘C. After a few minutes, equilibrium is reached.

a. Determine the final temperature. (completed)

b. Determine the total change in entropy.
Relevant Equations:: Q=mcT

So I've answered the first question and I got a final temp of 42.06 Celsius.

Now for this second one, I don't know why I am getting it wrong:

Im doing 0.215*ln(315.06/291.46) + 1*ln(315.06/319.91)

But it says I am wrong. What about my process is faulty?
Please fill in the details of how you get the factors 0.215 and 1.

Also, to be pedantic, if you are working to hundredths of a degree, 0°C is 273.15K, not 273.00, though doubtless it makes very little difference to your answer.

haruspex said:
Please fill in the details of how you get the factors 0.215 and 1.
Yes, I differ from these values (and more importantly, from their ratio). Instead of the 0.215, I get 112 J/K, and, instead of the 1, I get 544 J/K.

My numbers agree with @Chestermiller.
(But I don't get the 'ratio' part).

rude man said:
My numbers agree with @Chestermiller.
(But I don't get the 'ratio' part).
Thanks Rudy. I don't either. I was mistaken about that.

Chestermiller said:
Thanks Rudy. I don't either. I was mistaken about that.
Actually, I thought you meant the limits of integration which turn out to be ratios after the integration, but separately for the water and the cup.

rude man said:
Actually, I thought you meant the limits of integration which turn out to be ratios after the integration, but separately for the water and the cup.
Nope. I mistakenly meant the ratio of the MC's.