kaplan thermochem q pg 96

[inaccensa]

A 50g sample of metal was heated to 100c and then dropped into a beaker containing 50g water at 25c. If the specific heat of the metal is 0.25cal/gc, what is the final temp of water?

I think the answer should be 50c, but the correct answer is 40c

The explanation that is given is correct, except at one point where the change is temp = final temp - initial temp is interchanged to initial -final. can some one please explain this discrepancy?

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[GoodmanBrown]

Does the change make sense with the explanation? I mean, could it just be a typo? If the explanation makes more sense with the term being final - initial, it's probably just a typo.

 

[RogueUnicorn]

A 50g sample of metal was heated to 100c and then dropped into a beaker containing 50g water at 25c. If the specific heat of the metal is 0.25cal/gc, what is the final temp of water?

I think the answer should be 50c, but the correct answer is 40c

The explanation that is given is correct, except at one point where the change is temp = final temp - initial temp is interchanged to initial -final. can some one please explain this discrepancy?

the final temp of water would be 50 if the specific heat were the same as the metal. in reality, the specific heat of water is 4 times greater than that of the metal. thus, whille the metal will emit 0.25 cal of energy per gram as it drops one degree celsius in temperature, the water will only rise by a quarter degree due to its much larger heat capacity.

also, if you are taking a kaplan class, your instructor should be more than happy to walk you through these kinds of questions

 

[inaccensa]

the final temp of water would be 50 if the specific heat were the same as the metal. in reality, the specific heat of water is 4 times greater than that of the metal. thus, whille the metal will emit 0.25 cal of energy per gram as it drops one degree celsius in temperature, the water will only rise by a quarter degree due to its much larger heat capacity.

also, if you are taking a kaplan class, your instructor should be more than happy to walk you through these kinds of questions

Tht does make sense. But wouldn't I need both the final and the initial temp of the metal to find the final temp of water. Can you solve this problem by using Qlost= Qgain

mcdt- water= mcdt -metal

1cal/gc * ( tf -50) = .25cal/gc *( 100-ti)

but then how do you get 2 unknowns with one equation.

 

[RogueUnicorn]

Tht does make sense. But wouldn't I need both the final and the initial temp of the metal to find the final temp of water. Can you solve this problem by using Qlost= Qgain

mcdt- water= mcdt -metal

1cal/gc * ( tf -50) = .25cal/gc *( 100-ti)

but then how do you get 2 unknowns with one equation.

ding ding ding! yes. now note there is only one unknown, Tf. Ti for both water and the metal is known and given.

 

[inaccensa]

ding ding ding! yes. now note there is only one unknown, Tf. Ti for both water and the metal is known and given.

i'm confused. can u please solve it

 

[RogueUnicorn]

i'm confused. can u please solve it

Remember, heat RELEASED is a negative sign

heat released by metal = heat absorbed by water

-[0.25 cal/(g.C) * 50g * (Tf-100)C] = 1 cal/(g.C) * 50g *(Tf-25)C

-[0.25 *(Tf-100)] = Tf-25

25+25 = 1.25Tf
50 = 1.25 Tf
Tf = 40

 

[inaccensa]

Remember, heat RELEASED is a negative sign

heat released by metal = heat absorbed by water

-[0.25 cal/(g.C) * 50g * (Tf-100)C] = 1 cal/(g.C) * 50g *(Tf-25)C

-[0.25 *(Tf-100)] = Tf-25

25+25 = 1.25Tf
50 = 1.25 Tf
Tf = 40

but i thought, Tf of the metal was 100c, since it said that the metal was heated to 100c. If I read this wrong, then your ans matches kaplan.

 

[RogueUnicorn]

the metal is dropped in the water. this is the only system we care about. we don't care how it got to 100C all it matters is that it is.

 

[inaccensa]

the metal is dropped in the water. this is the only system we care about. we don't care how it got to 100C all it matters is that it is.

I just think the q is poorly worded. I kept thinking that the final temp of the metal was 100c and the initial temp of water was 25c. When I solved it obviously there were 2 different unknowns.

 

[crazybob]

the initial temperature of the metal is 100 C because it is the starting temperature before it goes into the water.

 

[RogueUnicorn]

I just think the q is poorly worded. I kept thinking that the final temp of the metal was 100c and the initial temp of water was 25c. When I solved it obviously there were 2 different unknowns.

no the wording is quite fine. you're overthinking it.