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Topic: Help with setting up an equation/did I get the right solution?  (Read 694 times)

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Offline lauremn

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Help with setting up an equation/did I get the right solution?
« on: January 31, 2019, 09:15:59 PM »
I am an utter newb in chemistry and I haven’t done as well as I'd like so far. My class is an online hybrid so I don’t actually have a lot of time with my instructor. I have the following question on a take home quiz and I’m struggling with it.

How much heat is required to raise the temp of 105.70 kg of iron from 350 C to 2500 C? The specific heat capacity (s) of iron is 0.449 J/g •C.

Here is the equation I have set up but idk if I should use exponents to convert kg to g?

H = 0.449 J/g •C x 105.70 kg x (2500 C - 350 C)

The answer I got is 1.020 x 10^8 J

I think I lost points on my first quiz for not showing my work properly or labeling the units correctly.

Offline whitefloss

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Re: Help with setting up an equation/did I get the right solution?
« Reply #1 on: January 31, 2019, 09:35:27 PM »
You should convert the kg of Iron into grams because the specific heat capacity of Iron is also in grams

Offline lauremn

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Re: Help with setting up an equation/did I get the right solution?
« Reply #2 on: January 31, 2019, 11:00:47 PM »
Yes, I did that but I didn't type out that step.
1.057 x 10^2 kg = 1.057 x 10^5 g  correct?
Scientific notation confuses me a bit still.

Offline Borek

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Re: Help with setting up an equation/did I get the right solution?
« Reply #3 on: February 01, 2019, 02:56:23 AM »
Yes, I did that but I didn't type out that step.

You should write it out, otherwise what you wrote doesn't look correct (even if your thinking was OK and the result is OK).

Quote
1.057 x 10^2 kg = 1.057 x 10^5 g  correct?

Yes. You can also write it just as 105.7×103 (103 being a conversion factor, number of grams in kilogram).
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Offline Enthalpy

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Re: Help with setting up an equation/did I get the right solution?
« Reply #4 on: February 04, 2019, 12:37:18 PM »
A constant heat capacity up to 2500°C makes no sense. Iron melts around 1500°C.

The very absolute minimum would be to add the heat of fusion, and use a liquid's heat capacity above the fusion temperature. The result is already quite different.

Even between 350°C and 1500°C, the heat capacity changes a lot, so a constant value isn't good enough. The best way would use an enthalpy table for iron, which is available in the Handbook of Chemistry and Physics for many elements. That would be even simpler, as the given enthalpy includes all events like melting.

Or is a "degree" misread as "zero"? Does the question rather tell 35°C and 250°C?

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