This problem is providing the mass of potassium nitrate at 70. °C and asks for the amount of water necessary to completely dissolve it. At the end, the answer turns out to be 26 g of water.
In chemistry, we define solubility as the maximum amount of solute one can dissolve is an specific amount of solvent, at a determined temperature, without getting any trace of precipitate.
In such a way, with a solubility chart, one can see that potassium nitrate has a solubility of about 138 g in 100 g of water at 70. °C, which means we can compute the amount of water dissolving 36 g of this salt with the following relationship:
[tex]x=\frac{36gKNO_3*100gH_2O}{138gKNO_3} \\\\x=26gH_2O[/tex]
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