How many hydrogen-filled balloons (of any size you can find) would it
take to lift aproximatly 170lbs?  In other words, I'm looking for
something like, "It would take ___ balloons of ____ size, filled with
hydrogen, to lift 170lbs."

I've found the following resources that pertain to helium, but not
hydrogen:
http://www.howstuffworks.com/question185.htm
http://www.wolflink.net/~kiyote/liftcalc.html

Thanks in advance!

Hello.

According to the table "Lift of Hydrogen Balloons" on a web page
called "Balloon Lift," it would take one balloon with a diameter of 17
feet filled with 72847.88 liters of hydrogen to lift 179.07 lbs.

source: "Lift Tables: Lift of Hydrogen Balloons"
Balloon Lift - University of Hawaii
http://www.chem.hawaii.edu/uham/lift.html
Note that these tables assume "standard temperatures and pressures" 
(Standard Temperature and Pressure = 20 degrees C and 760 mm Mercury).

Now, you asked for 170 pounds.  Thus, the volume of hydrogen would be
proportionately less. Using the chart, it takes appears that it takes
approximately 406.8 liters of hydrogen to lift one pound.
Thus, 72847.88 liters/ 179.07 lbs = 406.8 liters/lb.  
406.8 liters/lb x 170 lbs = 69156 liters

The diameter of the balloon would be slightly less than 17 feet.

Note that if you planned to use multiple balloons, you could use the
University of Hawaii's lift table to figure how many of each size
you'd need to lift 170 lbs.  For example, if you wanted to use
balloons with diameter of 7-feet, you'd need about 14 balloons. (Since
a 7 ft balloon can lift 12.5 pounds, simply divide: 170 / 12.5 = 13.6
).

Also note that this answer is for informational purposes only. 
Hydrogen is a highly flammable gas... So, as the saying goes, don't
try this at home! :-)
 
search strategy:
"hydrogen balloons" lift

I hope this helps.

This is exactly what I was looking for.  The page you found also
contains a great deal of other helpful information. Thanks!

Thank you for the tip.
-juggler

One thing to keep in mind is that the hydrogen must also lift the
weight of the balloons.  Although for a small children's party balloon
this is not much, when you are talking about enough balloons to hold
that much hydrogen that can add up.  And balloons that can hold a
significant amount of hydrogen without either leaking or
bursting/tearing are probably a lot more substantial than your typical
balloon.  Using smaller balloons, they may not have to be so rugged,
and can thus be lighter, but then you have to use more of them and get
a larger surface to volume ratio, so more weight.

I recall an o-level discussion which might be relevant to any
application you may have in mind. The gist of it was that since
hydrogen has a very low molecular weight, it diffuses across a
membrane at a relatively fast rate, sufficient for a balloon to
visibly 'sag' and lose a proportion of its lifting ability after only
a week or two.

Take a single 3-foot balloon.  Fill it with two parts hydrogen, one
part oxygen.  Put a 170-lb. weight on top.  Light a match under it. 
Boom!!!  The 170-lb weight will be lifted several feet into the air.

if you actually wanted to calculate things out you would need to
include the weight of the hydrogen otherwise you aren't going to lift
the 170 lbs...which makes this problem a little more difficult (ie a
little nonlinear) than pure ratos (which would work fairly accurate if
the change was small but larger changes will have a larger(in terms of
percentage) error than small ones...so, be carefull if you actually
were doing something where people could get hurt

kmc

also if there were a difference in temperature of the hydrogen and
ambient air would cause some errors...but for the sake of a homework
problem ya I would just use ratios to solve this problem