First, we need to convert the mass of the compound from pounds to grams:
1.28 pounds * 453.59 grams/pound = 580.61 grams
Next, we can use the formula for density:
Density = Mass / Volume
Density = 580.61 grams / 4.50 L
Density = 128.91 g/L
Therefore, the density of the unknown compound is 128.91 g/L or 0.12891 g/mL (since there are 1000 mL in 1 L).