The Dumas method was one of the first reliable techniques for determining the mo

Question

The Dumas method was one of the first reliable techniques for determining the molar mass of a volatile liquid. In this method, a liquid is boiled inside an open glass bulb so that its vapor completely fills the bulb, displacing any air present. The vapor inside the bulb is then cooled and the condensate weighed From this mass, the boiling point of the liquid, and the atmospheric pressure, one can determine the molar mass of the liquid. A chemist used this technique found that a pure volatile liquid with a boiling point of 80.1 degree C fills a 500.0 cm^3 bulb at an atmospheric pressure of 755.3 mm Hg. a. Assuming that all of the liquid has been converted to vapor, how many moles of this compound are in the bulb? b. If the weighed condensate has a mass of 1.335 grams, what is the molecular weight of the liquid? c. If the empirical formula of the unknown liquid is CH, what is the molecular formula?

Explanation / Answer

(a)

n = P V / R T = [(755.3/760)(500.0/1000.)] / [0.0821 * (273.15+80.1)] = 0.01713 mol

(b) Molar mass= mass / moles = 1.335 / 0.01713 = 77.93 g/mol = 78 g/mol

(c) n = molecular weight / emperical formula weight

n = 78 / (12+1) = 6

MOlecular formula = (CH)6 = C6H6, benzene

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