Is there a mathematical conversion for volume to weight? Specifically for liters to pounds?

What you need is density, which is mass per unit volume, for the material you’re interested in. A liter of air and a liter of water would have much different mass (and therefore weight) because they have different densities.

Of what substance?

Not all liquids have the same weight /volume ratio.

Weight = volume * density

You’ll probably need some conversion factors to get the weight to come out in the right units depending on the volume and density units you have to work with. For example, for water (density approx 1 g / mL or 1 kg / L):

Weight (pounds) = N liters * ( 1 kg / L ) * ( 2.205 pounds / kg )

Weight (pounds) = N * 2.205

Other materials have different densities, but the general procedure would be the same with different numbers. Just keep the units on all values to make sure stuff cancels out properly.

And before I get jumped on, I’m well aware that mass does not equal weight, but I figure the OP is doing his/her work on Earth, so I played a little fast-and-loose with terminology.

If the conversion is one to one for water, why would I double it to 2.205? The liquid I am using is a slimfast type product. Would you still suggest the 2.205 measurement?

The conversion for water is 1:1 in the metric system, because that’s the way the metric system was designed. To convert kilograms to pounds you must multiply by 2.205