# density and inertial mass

Water's density at 60-degree Fahrenheit is 62.366 pounds/ft^3. Is it inertial or gravitational mass? Actually, I am trying to calculate Potential Energy (PE) of 1 gallon of water at 1' height. Is PE = mgh = 8.35 x 32.17 x 1? or is PE = 8.35 x 1? (Here I am assuming 1 gallon of water weighs 8.35 pounds and gravity as 32.17 feet/second^2) Essentially my question is when we say 1 gallon of water weighs 8.35 pounds, is it inertial or gravitational mass?

It depends how you measured the weight. If you used Newton ($F=ma$) to determine the mass you would call it inertial mass. If you used the the gravitational law ($F = G \frac{Mm}{r^2}$) by measuring the force between the unknown mass $m$ and the known one $M$ you would call $m$ the gravitational mass (that's what a scale does). Luckily it turns out that, as far as we know, those two are the same.