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Practice question

MCQ

Question

A solution contains 32 g of methanol (molar mass = 32 g/mol) and 90 g of water. If the mole fraction of methanol is to be increased to 0.3 by adding methanol, what mass of methanol is added?

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Explanation

Initial moles of methanol = (32/32) = 1 . Moles of water = (90/18) = 5 . Let additional moles of methanol = x . New mole fraction = (1 + x/1 + x + 5) = 0.3 . 1 + x = 0.3 (6 + x) , 1 + x = 1.8 + 0.3x , 0.7x = 0.8 , x ≈ 1.1429 . Mass added = 1.1429 × 32 ≈ 36.57 g .

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