I'm having a debate with a friend. He says you cannot weight gases. I
say you can.
Hot air rises because it's liter. Helium balloons rise for the same
Reason. If a gas is denser molecularly then another, you must be able
to weight it. if I put a heavy gas in a balloon and drop it on a
sensitive scale it should register some weight (minus balloon weight).
Who is right?

Hello tonymast and thank you for your question.

Good news! You are correct! Gases can be weighed.

"Air has weight, and so has every gas and body. You can prove this
with a balance. A hermetically lockable vessel without air weighs less
than when it contains air or any other gas. Hence you can compare the
masses of gases by their weight. The amount of mass, that is, how many
grams are contained in a vessel filled with a gas, depends essentially
on its pressure and temperature. For a comparison ofweights, you must
relate them to a normal temperature and normal pressure. At 0ºC and
760 mm mercury you have:"
http://kr.cs.ait.ac.th/~radok/physics/e4.htm

"We have means of weighing gases or air; but, without stopping to
explain that, let me just tell you what their respective weights are.
The weight of a pint of hydrogen is three-quarters of a grain; the
weight of the same quantity of oxygen is nearly twelve grains. This is
a very great difference. The weight of a cubic foot of hydrogen is
one-twelfth of an ounce; and the weight of a cubic foot of oxygen is
one ounce and a third. And so on we might come to masses of matter
which may be weighed in the balance, and which we can take account of
as to hundred-weights and as to tons, as you will see almost
immediately."
http://www.rod.beavon.clara.net/Faraday_lecture_4.htm

A simple experiment:

"But how can we actually prove that air has weight? Isn?t it just an
invisible mixture of gases that we need to breathe? Let me show you.
 
The air in the atmosphere is kept close to the Earth by the pull of
gravity, the force that pulls everything ? including you and I ? down
to the ground. Without gravity, we would be weightless and would float
above the ground, as we see with astronauts in space. As well as
giving humans weight, gravity also does the same for air. We can
illustrate this with a simple experiment using two balloons.
We want to test whether the balloon is heavier or lighter (or the same
weight) after you blow air into it. To do this, tie a piece of string
around the middle of a stick of piece of cane so that it balances.
Then tie an empty balloon to each end of the cane. What happens? The
two balloons should balance evenly at each end.
Now remove one balloon and blow air into it. When you have done that,
tie it back onto the end of the cane. Is there any change? That?s
right, the end with the blown-up balloon on it should dip downwards.
This is because the air in the balloon is making it heavier."
http://www.rcn27.dial.pipex.com/cloudsrus/pressure.html#Weighing%20air


Thank you for your question, and if you neeed any clarification of my
answer do not hesitate to ask.

Very best regards

THX1138

Search strategy included:
"gas weights"
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&as_qdr=all&q=%22gas+weights%22

A simple Yes/No for 2 bucks would have sufficed. But you went above
and beyond and showed me how to prove it.  excellent answer thank you.
 (of course i enjoyed the a little more being right!!)  ;)

Hello again tonymast,

Just a note to say, I'm glad you won the debate! and thank you for the
excellent rating and generous tip.

Very best regards

THX1138

In the last experiment (the cane and ballons) I am skeptical. The
inflated balloon would would have warmer air (on a cold day, with low
humidity) so it might pull up the balance instead dip downward = a hot
air balloon. This effect is partially (or completely) offset by the
fact that the elastic of the balloon compresses the air increasing
it's density, plus a bit of spit that may have entered the balloon or
remain on the neck of the balloon. Also the exhailed air has more
carbon dioxide which is about twice as heavy as air; and more water
vapor which is about half as heavy as air. The cane experiment does
not have sufficient control of random errors. I do agree that air can
be weighed.   Neil

Neil.  You are correct that there is a source of error but this lab is
mearly qualitative anyway.  So I don't know if skeptical is an
appropriate response.  Any water vapor that enters with your breath
takes up little space, and if they are droplets, the humidity may even
increase the mass of the gas.  Any saliva on the neck could dry.  By
letting the balloon rest for anything more than a couple minutes would
ensure that the temperature changes to room temperature.  Carbon
dioxide is actually a small proportion of an exhaled breath
(especially when compared to the mass of the balloon).  Yes, the
elastic does compress the air, but that just helps to prove the point
which is really...does this invisible stuff have mass, whether it be
CO2, H20 vapor or whatever.