Where are your numbers? Ir;s been more than 1 hour...
There still crunching the numbers.
got distracted...
The molecular weight of N2 is 28g/mole.
For O2 it's 32g/mole.
The atmosphere is 20% oxygen and since it's Thursday, the rest of the minor components are on vacation, so...
0.8*28+0.2*32="molecular weight" of air = 28.4g/22.4l = 1.29g/l
The balloon contains He, with an atomic weight of 4.
4g/22.4l = 0.179g/l
The An object in a fluid is bouyed up by a force equal to the weight of the fluid displaced.
The balloon has a radius of 10ft and a height of 5ft. The volume is:
3.14*10*10*5 = 1570ft3.
1"=2.54cm, so...
12"/ft=12/ft*2.54cm = 30.5cm/
1ft3=30.53 = 28317cm3/ft3
28317cm3/ft3 / 1000cm3/l = 28.3 l/ft3
28.3 l/ft3 * 1570ft3 =44431 l
44431*l * 1.29g = 57316g/(air displaced by balloon)
57316g/ 454g/lb = 126lbs of air displaced by the balloon.
The weight of the helium is:
0.179g/l * 44431*l / 454g/l = 17.5lbs
So...
126 - 17.5 = 109lbs of lift.