Practice question
Question
A solution is prepared with 46 g of ethanol (molar mass = 46 g/mol) and 54 g of water. If the mole fraction of ethanol becomes 0.4 after adding more ethanol, what mass of ethanol was added?
Explanation
Initial moles of ethanol = (46/46) = 1 . Moles of water = (54/18) = 3 . Let additional moles of ethanol = x . New mole fraction = (1 + x/1 + x + 3) = 0.4 . 1 + x = 0.4 (4 + x) , 1 + x = 1.6 + 0.4x , 0.6x = 0.6 , x = 1 . Mass added = 1 × 46 = 46 g .
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