Germall Plus is a preservative for lotion making. It’s used at 0.1-0.5% of total weight. I know that 1 gallon of Germall Plus weighs 9.6 pounds. How much lotion will 2 LIQUID ounces of Germall Plus preserve for me?
(Bonus points for anyone who can convincingly explain why suppliers aren’t using friggin’ milliliters and grams to sell supplies. Ounces (weight) and ounces (volume) is just stupid.)
I know I learned the Dimensional Analysis for this, but my brain is malfunctioning at the moment. 
Until you can tell us how many weight ounces or grams equals 2 liquid ounces (fluid ounces) of your particular lotion we can’t help you.
To be sure I could go weigh some lotion I have sitting around here. But in general weight vs volume is going to vary a bunch based on how viscous any given lotion is.
My advice would be to weigh a larger volume of lotion and tell us how much volume & weight it is. That way both figures can be more precise and the end result more accurate. e.g. 1 carefully measured quart volume weighs 48.2 carefully weighed ounces.
Second issue:
Assuming you got the dilution factor correct, that’s 1 to 5 parts per thousand by weight. Said another way, your pail of roughly 10 pounds of preservative will be enough to preserve 2,000 to 10,000 lbs of lotion. Or 1 to 5 *tons *of lotion.
So we’re talking a pretty big batch of preserved lotion. I hope your side business is really really successful! Or else the preservative itself better have a long shelf life. 'cuz you’ll be using that one pail for many many years. 
1 gallon is 128 fluid ounces, so 2 fluid ounces is (9.6 * 16 / 128) * 2 = 2.4 ounces of weight. Divide that by 0.1% (or multiply it by 1000) and you get 2400 ounces, or 150 pounds. For 0.5%, divide that by 5 to get 30 pounds.
That, of course, is assuming the lotion has the same density as the preservative.
2 fl.oz =0,015625 gal. Multiply by density: 0,015625 gal * 9,6 lb/gal = 0,15 lb.
At 0,1 % you get 0,15 lb / 0,001 = 150 lb of the preserved product.
Thanks! I don’t know the density of the final lotion, but I know it won’t be anywhere near 150 pounds of lotion, so one 2oz bottle will do me fine. 
(This is why they make you do word problems, kids. Real life is one big word problem!)
If you want to be really picky, the calculations above aren’t quite correct. In the example with 0.1% concentration, 2400 ounces is the total weight, so you need to subtract the 2.4 ounces to get the weight of only the lotion, 2397.6 oz.
Obviously it doesn’t make any difference given the required accuracy here, but this is the SD, land of pedants.
And I love you for it. 