Answer
See the answer below.
Work Step by Step
a) From Henry's law:
$S=k_HP$
$S=0.0011\ mol/kg.bar\times0.21\ atm\times1.013\ bar/atm$
$S=0.0023\ mol/kg$
b) By Henry's law, if the partial pressure of butane is increased, its solubility in water will increase and so will its dissolved concentration.