[Anood]

Can you please help me in understanding the answer of the following question:

An empty latex balloon has a mass of 4.00g. Under a pressure of 1 atmosphere the balloon is inflated  to  24.0 liters with methane [ CH4] gas. The temperature is determined to be 298.15K.



What reading would you expect to register if the balloon was adhered to a piece of Scotch tape stuck to the pan of a tared electronic digital balance ?
the solution is -8.8 g but i don't get how is that.

i know the downward arrow is 4+16
but how come the upward arrow is 28.8

which will make it -8.8

[Yggdrasil]

Look up Buoyancy in a physics text book or in wikipedia.

[enahs]

There are many ways to approach this.

First, start with your Ideal Gas Equation.

PV = nRT
You know P,V,T and R, solve for n.

Therefore:
n =0.98 mol


Convert moles to mass of methane
(0.98mol CH₄) *  ( 16.04 g CH₄)        =    15.737 g CH₄
                            1 mol CH₄


Now, do the same for air*, assume the same conditions.

n = 0.98 mol

(0.98 mol air) * ( 28.97 g air)    = 28.39 g air
                         1 mol air


Now, you can view it like this.
Given the same quantity (number of moles) and volume as air, methane "weighs" less. The amount of "lift" given is the "weight" of air that would be occupied in that space minus the weight of the substance giving "lift".

In this case, if the balloon was filed with air it would have 28.39 g air, but it is instead filled with 15.737 g of methane, a difference of 12.7 g (and since it is lighter, due to "bouyancey" it is lifting and causes the negative reading on the balance). Now, also the original balloon weighed 4 g so you must account for that, or 12.7-4 = 8.7g difference if the balloon. Now, to get 8.8 it just depends on how accurate you keep your numbers, and what you use for your constants and such and how/if/when you round, etc.


* For info on how to determine the molecular weight of air: (http://www.engineeringtoolbox.com/molecular-mass-air-d_679.html)


I chose to explain it that way using the ideal gas law, because that is the most important one, in your stage, to work with.

But it can be done equally as easy with densities of the gases.

[Borek]

i know the downward arrow is 4+16
but how come the upward arrow is 28.8

You better call them vectors/forces