[horo1988]

Here is wat I did:

I pour about 10ml of Acetone into a flask, then I cover the top of the flask by a piece of aluminum foil, then I tie it up with a rubber band. Next, I use a needle to make a little tiny hole on the foil After heating about 10 mins at around 80 to 90 degree Celsius, I record the mass of the vapor inside.

Questions:

There is a significant difference between the mass of the cooled vapor and the mass of the hot vapor. Why? Mass never changes even if it is hot or cold (i.e a glass of hot water or cold water has a same mass)

So why the mass of the hot vapor is higher than the cool one?

If we explain it because of the hole, it is just a tiny little hole, does it really matter? u can say because the molecules of the hot vapor get excited and move out the flask (through the hole), so the mass gets lower? But in this experiment, we r supposed to record the mass of the cooled vapor. So after the class, I just wonder why we have to wait until the vapor cools down.

[sondakem]

Did you weigh the empty flask before adding the acetone?
Does all the acetone evaporate?  The volume needed to contain 10 mL of acetone is 22.4x10xdensity/58 Liters.  Most of it should escape.
The hole is huge compared to molecules

[AWK]

The volume needed to contain 10 mL of acetone is 22.4x10xdensity/58 Liters. 

At STP. At boiling point od acetone the volume will be ~30 % greater

[lemonoman]

If we explain it because of the hole, it is just a tiny little hole, does it really matter?

Ever open a door and have a huge breeze blow by you?  That's because there's a different pressure inside the building than outside.  This difference in pressure causes a flow from one to the other, making the wind.

Heating the liquid will make the pressure inside the flask (on one side of the hole) greater than on the other side (the room).  This difference in pressure is what would cause most of the flow from inside your flask to outside your flask.

[agrobert]

Why don't you do this experiment with a balloon?

[lemonoman]

Why don't you do this experiment with a balloon?

How would that work?

[sondakem]

So far I don't see any answer to your original question.  AWK correctly supplemented my calculation but that still doesn't account for weight loss on cooling.  Lemonman described what happens on heating.

On cooling, air rushes in.  I seem to have a vague memory of the "buoyancy effect" of air in analytical weighing.  (The course was in 1963 and we used swing balances).

A balloon would at least be a closed system.  It better have a large expansion capacity and be impervious and nonreactive to acetone.

[Mitch]

I've never seen so much discussion on such a straight forward problem.

Anyways, what everyone should of picked up on is the fact they were heating acetone at 90 C! Well above its flash point and without starting a fire!

[sondakem]

OK. So safety precautions were lacking.  Let's just say that the gas escaping cooled or diluted itself so that it didn't flash.  How does that answer the question?

[Mitch]

It obviously boiled away. Is there as much water in a teapot if you let it boil for longer than you should? Of course there is less water and thusly less mass. The question should of been in regards to the concept of evaporation, and in a non-closed system it is irreversible.

[The Tao]

Here is wat I did:

I pour about 10ml of Acetone into a flask, then I cover the top of the flask by a piece of aluminum foil, then I tie it up with a rubber band. Next, I use a needle to make a little tiny hole on the foil After heating about 10 mins at around 80 to 90 degree Celsius, I record the mass of the vapor inside.

Questions:

There is a significant difference between the mass of the cooled vapor and the mass of the hot vapor. Why? Mass never changes even if it is hot or cold (i.e a glass of hot water or cold water has a same mass)

So why the mass of the hot vapor is higher than the cool one?

If we explain it because of the hole, it is just a tiny little hole, does it really matter? u can say because the molecules of the hot vapor get excited and move out the flask (through the hole), so the mass gets lower? But in this experiment, we r supposed to record the mass of the cooled vapor. So after the class, I just wonder why we have to wait until the vapor cools down.

If you heat up a substance in a "closed" container, the vapor pressure will increase. It is not to say that the particles have gained mass, but the the equilibrium between gaseous molecules escaping and those condensing has changed. The pressure increases and the scale interprets it as in an increase of mass, when it is only an increase in force.

[Borek]

The pressure increases and the scale interprets it as in an increase of mass, when it is only an increase in force.

Huh?

[sondakem]

The only way the problem makes sense to me is if the pinhole is facing upward and not enough time was given to allow for complete evaporation of the acetone.  The effect of the escaping gas would be like igniting a rocket upside-down.  It would press downward.  In our case when the acetone cools this pressure stops.

If the acetone evaporates completely then the internal pressure equals the external pressure and there is no difference in weight.

How many times did you run this?
And as I asked before, did all the acetone evaporate?

[P]

I have to agree with Mitch  -  the stuff has obviously just evapourated. It's a no brainer. The temperature of the acetone will not effect its mass.   

[sondakem]

Of course mass does not change.

P
When HOT there was more mass not less.
By the time it cooled the rest evaporated?

We're all trying to interpret results of what may be only 1 expt.
Some of us aren't even answering the question asked.
Some of us haven't read the question carefully.