Answer
Peat moss: $0.132g/cm^3$
Topsoil: $2.522g/cm^3$
No, it would not be correct to say that peat moss is "lighter" than the topsoil because they have the same weight, 40lbs.
Work Step by Step
Peat moss: $40lb \times \frac{453.5g}{1lb} \times \frac{1}{14in\times20in\times30in}\times \frac{1in^3}{16.39 cm^3}=0.132g/cm^3$
Topsoil: $40lb \times \frac{453.5g}{1lb} \times \frac{1}{1.9gal}\times \frac{1gal}{3785.4 cm^3}=2.522g/cm^3$
The term "lighter" refers to the objects weights, not their densities. Thus, because both the peat moss and topsoil weigh 40 lbs, neither one is lighter.
