An aqueous solution is 18.0% by mass ammonia, nh3. the mole fraction of ammonia in the solution is:

0.189

Explanation:

Aqueous solution only has ammonia and water in it. Assume total mass of 100 g

Meaning mass of ammonia is 18% of 100 g = 18 g

Mass of water is 82% of 100 g = 82g

Number of moles of ammonia  = mass / molar mass ammonia =18/17 = 1.059

Number of moles of water = mass / molar mass water = 82/18 =4.556

Mole fraction ammonia = 1.059 / (1.059+4.556) = 0.189

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