[P]

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.

Alright then  -  THIS time, weigh the acetone FIRST while COLD.  THEN heat it up!  You will see the mass decrease this time AGAIN due to evaporation through your pin holes.

Last time, you heated it up, weighed it and let it cool then weighed it again.  The stuff evaporated while it was cooling. Acetone is very volatile.

To do the experiment properly you need to enclose the system in test tube with an impermeable rubber stopper (as someone said above, although a balloon will dissolve in the acetone).

In answer to your carefully read questions:
1)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)
ANSWER  -  some evaporated during cooling.

2)So why the mass of the hot vapor is higher than the cool one?
ANSWER  -  because some of it evaporated during cooling.

3)If we explain it because of the hole, it is just a tiny little hole, does it really matter?
ANSWER  -  Yes  -  the acetone can evaporate through the holes, however small - especially if warmed up.

4)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?
ANSWER  -  yes that's right  -  it's called evaporation.


5)P
When HOT there was more mass not less.
By the time it cooled the rest evaporated?
ANSWER  -  I think you should know the answer to this one by now???