# Question on thermochemistry problem

#### Swenis

##### Member
10+ Year Member
5+ Year Member
Hey guys, a fellow pre-pharm here. I'm in general chem 1 currently, and we have recently started thermochemistry. I've been doing some practice problems, but I've come across one that's left me stumped. If anyone would care to lend me some insight, it would be much appreciated!

An Al pan(c = 0.902J/gK), weighing 850.0 g, that was being used to boil water has just boiled dry. Assuming the Al pan is at 100.oC what is the final temperature if the pan is placed into 1.00L of water at 20.0oC?

Thanks a lot guys (and gals )

#### DrEaMeRcHiCk

##### Senior Member
7+ Year Member
15+ Year Member
Swenis said:
Hey guys, a fellow pre-pharm here. I'm in general chem 1 currently, and we have recently started thermochemistry. I've been doing some practice problems, but I've come across one that's left me stumped. If anyone would care to lend me some insight, it would be much appreciated!

An Al pan(c = 0.902J/gK), weighing 850.0 g, that was being used to boil water has just boiled dry. Assuming the Al pan is at 100.oC what is the final temperature if the pan is placed into 1.00L of water at 20.0oC?

Thanks a lot guys (and gals )
*Edit

I don't know if I'm right, but here's a shot at it because it's similar to a problem I recently did. You use the formula q=nC (Tf- Ti)...so when looking for the final temperature you have to take into account that you need to heat up the Al pan in order to boil the water so what you're looking for is Tf. The basic outline should be:

Also it says that the water has just boiled dry...so you might need to add a phase change in there i.e. liquid to gas...but I'm not completely sure.

-q(Al pan)=q(water)
so you get
-(850g)(0.902J/gK)(Tf- 373K)=(1.00L <--you have to convert this to grams)(heat capacity of water- this should be given but in my book it says 4.18 J/gC and that's the wrong units so look for it in the right units)(Tf-293)

Then from here it's just algebra, hope this helps and if I'm wrong I'm sorry...

##### Senior Member
10+ Year Member
yeah, i think the first poster is correct

-q_Al = q_w
(m_Al)(c_Al)(100-T_f) = (m_w)(c_w)(T_f - 20)

Solve for T_f

#### mahishrimp

##### Member
10+ Year Member
5+ Year Member
since the water desity is 1 g/ml. 1.00L = 1000mL= 1000g.

OP
S

#### Swenis

##### Member
10+ Year Member
5+ Year Member
I appreciate it, guys and gals! Thanks!

OP
S

#### Swenis

##### Member
10+ Year Member
5+ Year Member
Can someone explain the (100-T_f) and (T_f - 20)? I get the rest, but I don't understand this part. I can do the math, but I'm having comprehending why I do that.

Edit: Nevermind, I just got it! Thanks

#### Requiem

##### Senior Member
Moderator Emeritus
10+ Year Member
Swenis said:
Can someone explain the (100-T_f) and (T_f - 20)? I get the rest, but I don't understand this part. I can do the math, but I'm having comprehending why I do that.

Edit: Nevermind, I just got it! Thanks
Ok, well he switched the Ti-Tf to remove the negative sign
And T_f-20 means Temperature final minus inital temp(20 C ) of water.

OP
S

#### Swenis

##### Member
10+ Year Member
5+ Year Member
Ahh so I noticed. Thanks

OP
S

#### Swenis

##### Member
10+ Year Member
5+ Year Member

How do I find the number of joules in one Btu (Btu- amount of heat required to raise temp of 1 lb of water by 1o F)? I know the answer is 1054 Joules, but I can't figure out how to do the math.

This thermochemistry is confusing

##### Senior Member
10+ Year Member
Swenis said:

How do I find the number of joules in one Btu (Btu- amount of heat required to raise temp of 1 lb of water by 1o F)? I know the answer is 1054 Joules, but I can't figure out how to do the math.

This thermochemistry is confusing
use the formula dQ = mc(dT)
you know dT .. change 1oF to oC
you know m ... covert 1 lb to kg
you know c

OP
S

#### Swenis

##### Member
10+ Year Member
5+ Year Member
use the formula dQ = mc(dT)
you know dT .. change 1oF to oC
you know m ... covert 1 lb to kg
you know c
Wow, thank you. My chemistry professor has failed to inform me of some of these formulas. I appreciate it! What does dQ and dT stand for?

#### DHG

##### Senior Member
10+ Year Member
Swenis said:
Wow, thank you. My chemistry professor has failed to inform me of some of these formulas. I appreciate it! What does dQ and dT stand for?
"d" is Calculus notation for "change in."

dT = "change in T"

dQ = "change in Q"

The best way to do well in thermochemistry, physical chem, or physics is to take calculus and really try to understand it and do well. Everything after that will be cake.

I'm taking Pchem right now, and it's tough. I had calculus a loooong time ago, so I had to brush up on it a little, but it's helped a lot. Try taking your calculus before you do Pchem, even if your school offers "non-calc" pchem. It really helps to know the calc, or at least to be familiar with it.

good luck.