Answer
See the answer below.
Work Step by Step
a) $28\ ppb = 28\times10^{-9}\ kg_{solute}/kg_{solvent}$
To molality:
$28\times10^{-9}\ kg_{solute}/kg_{solvent}\div 107.87\times10^{-3}\ kg_{solute}/mol_{solute}=2.60\times 10^{-7}\ m$
b) $1.0\times10^2\ g_{Ag} \div28\times10^{-9}\ g_{Ag}/g_{H2O}\div 1.0\times10^{3}\ g_{H2O}/L=3.57\times10^{6}\ L$
