(a) A light balloon is filled with $400 \mathrm{~m}^{3}$ of helium. At $0{ }^{\circ} \mathrm{C}$, what is the mass of the payload that the balloon can lift? (b) In Table 15.1, note that the density of hydrogen is nearly one-half the density of helium. What load can the balloon lift if it is filled with hydrogen?