Thermodynamics phase changes steam and water

[Megan20102011]

Homework Statement

In each case, the following are mixed in a perfect calorimeter with no heat lost or gained from the system. Find the final temp in each case.

12.5 g of steam at 100C with 344 g of water at 22.2C.

Homework Equations

Q=Mc(Tfinal-Tinitial)
M is the mass in kg
C is the specific heat given by whatever substance we are dealing with
And the parantheses is the change in temperature

The Attempt at a Solution

So I didn't really know where to start with this problem I realize that one of the substances is going to undergo a phase change but am unsure how to find which one does it. If I could help with that I believe I could calculate the final temperature.

 

[PeterO]

Homework Statement

In each case, the following are mixed in a perfect calorimeter with no heat lost or gained from the system. Find the final temp in each case.

12.5 g of steam at 100C with 344 g of water at 22.2C.

Homework Equations

Q=Mc(Tfinal-Tinitial)
M is the mass in kg
C is the specific heat given by whatever substance we are dealing with
And the parantheses is the change in temperature

The Attempt at a Solution

So I didn't really know where to start with this problem I realize that one of the substances is going to undergo a phase change but am unsure how to find which one does it. If I could help with that I believe I could calculate the final temperature.

Steam exists at 100^o or higher.
Water exists at 100^o or lower. [but not below 0^o]

Given that one quantity starts at 100^o and the other starts at 22.2^o, it is reasonable to assume that the final temperature will be somewhere in between those two - so which one do you now think will change phase.

 

[Megan20102011]

The steam should become water since water boils at 100C. So it should just be common sense? Or is there a calculation I should be making?

 

[Megan20102011]

So since the steam is undergoing a phase change to become water. I'm not sure what to multiply the mass for since it isn't fusion or vaporization.

 

[PeterO]

So since the steam is undergoing a phase change to become water. I'm not sure what to multiply the mass for since it isn't fusion or vaporization.

Have you heard of melting and condensation?

 

[Megan20102011]

Yes my physics book doesn't give those values though so I hadn't even thought I could use those.

 

[PeterO]

Yes my physics book doesn't give those values though so I hadn't even thought I could use those.

Your book does give that - you just didn't recognise it.

The energy input need to vaporise some water equals the energy returned when that (now) vapour condenses.

Same thing with fusion and melting.

 

[Megan20102011]

So it's just the sign that changes
. Thanks very much