1-The following procedure provides a crude method of determining the molar mass

Question

1-The following procedure provides a crude method of determining the molar mass of a volatile liquid. A liquid of mass 0.0175 g is introduced into a syringe and the end is capped (sealed). The syringe is transferred to a temperature bath maintained at 46.8 oC, and the liquid vaporizes. As the liquid vaporizes the plunger is pushed out. At equilibrium, the plunger reads 5.55 mL of gas. Atmospheric pressure is 740. mmHg. What is the approximate molar mass of the compound (in g/mol)?

2-The reaction of magnesium metal with HCl yields hydrogen gas and magnesium chloride. What is the volume, in liters, of the gas formed at 724 torr and 23 oC from 3.59 g of Mg in excess HCl? (Hint, first write the balanced equation.)

Explanation / Answer

m = 0.0175 g liquid mass

T = 46.8 °C

liq vaporizses...

V final = 5.55 mL

P = 740 mm Hg

so

P V= nRT

n = PV/(RT)

n = (740*55.55/1000)/(62.4*(46.8+273))

n = 0.002059931 mol

MW = mass/mol = 0.0175/0.002059931 = 8.49543 g/mol

Q2.

V in liters

Mg(s) + 2HCl(aq) = MgCl2(aq) + H2(g)

PV = nRT

mol of Mg = mass/MW = 3.59/24.3 = 0.1477

V = nrT/P = 0.1477*0.082*(23+273)/(724/760) =

3.7632 Liters

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