\textbf{220.} The solubility of potassium nitrate at $42^\circ\text{C}$ is 61 grams per 100 grams of water. To approximately dissolve this many moles of salt, how many liters of water must be added to obtain a saturated solution at this temperature? (Density of water is $1\ \text{g.mL}^{-1}$.)
$(K = 39,\ O = 16,\ N = 14 : \text{g.mol}^{-1})$
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(1) $9.04$ \hfill (2) $12.08$ \hfill (3) $18$ \hfill (4) $24$
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