Answer
(a) negative.
(b) negative.
(c) positive.
Work Step by Step
(a) The number of moles of gas decreases and therefore entropy decreases. $\Delta S^{\circ}$ is negative.
(b) Two moles of gas is converted to one mole solid and therefore entropy decreases. $\Delta S^{\circ}$ is negative.
(c) Solid-state is changed to aqueous state which results in more mobility and randomness. Entropy increases and $\Delta S^{\circ}$ is positive.
