The solubility of a salt is a function of the solution temperature. At 50 degrees C, the solubility of sodium nitrate is about 114 g per 100 ml of water. Thus, a saturated solution at 50 degrees C will contain 114 g sodium nitrate per 100 ml of solution. As the solution cools down, the solubility of sodium nitrate reduces to about 90 g per 100 ml of water at 25 degrees C. This means that the solution can only hold 90 g of sodium nitrate and that 114 - 90 = 24 g of sodium nitrate per 100 ml of solution would be precipitated out. 

Thus, in cooling sodium nitrate solution from 50 C to 25 C, a saturated solution will result in precipitation of 24 g salt per 100 ml of solution volume.

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