Thought i should add my idea. Since Sodium sulfate and benzoic acid are both soluble in hot water, add the mixture to hot water. As the water cools down, the solubility of benzoic acid becomes poor, and would thus separate, while the sodium sulfate would remain in solution. My main confusion is that since all 3 things, the 2 solutes and the solvent, are liquids, how would you go about separating?