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

Question

a. 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 using this technique finds 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. (i) From the above information, how many moles (n) of gas was present in this bulb? (ii) If the weighed condensate has a mass of 1.335 grams, what is the molecular weight of the liquid? (iii) If the empirical formula of the unknown liquid is CH, what is the molecular formula?

Explanation / Answer

given pressure = 755.3 mm Hg= 755.3/760 atm =0.99 atm

volume = 500.0 cm3 = 500.0 ml( 1 cm3 = 1ml) = 0.5L

temperature = 80.1 0C = 80.1 +273 K =353.1

EQUATION : PV = nRT

R = universal gas constant =0.0821 L atm mol-1 K-1

no.of moles = PV/RT

= 0.99 X 0.5/0.0821 X 353.1

=0.017 moles

no.of moles = mass of substance/ molar mass of substance

molar mass of substance = mass of substance/no.of moles

= 1.335/0.017

=78.5 g/mol

emperical formula =CH , emperical formula mass = 12+ 1 =13

molecular mass/ emperical mass = 78.5/13 =6

there fore molecular formula is( CH)X6 =C6H6

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