A student collected the following data : Mass of gas = 0.800 g, temperature of t

Question

A student collected the following data : Mass of gas = 0.800 g, temperature of the water bath = 95.0 C , barometric pressure 760.0 torr, and the volume of the flask 250.00 mL, Later the student was told one drop (0.050g) of water had been present in the flask during the experiment and it completely vaporized ,contaminating the gas .(a) calculate the apparent molar mass,that is ,the molar mass the student would get if he did not know that there were any impurities present .(b) show how the student could still calculate the true molar mass of the gas. (c) calculate the percentage of error in part (a).

Explanation / Answer

For ideal gasses, the following relation holds true: PV = nRT where P and V are pressure and volume respectively. n is the number of moles, R universal gas constant, and T absolute temperature. P = 760 torr = 1 atmosphere = 1.01325 X 10^5 Pa, Where 1 Pa = 1N/m^2. T= 95 deg C = 368 K. V = 250 ml = 0.00025 m^3

a) From the above relation we have n = PV/RT =(1.01325X10^5X0.00025)/ (8.314*368) = 0.00828 moles. Accordingly the apparent molecular weight = 0.8/0.00828 = 96.6 gm/mol

b) Accurate molecular weight = 0.75/0.00828 = 90.6 gm/mol

c) Error % = 100*(96.6 - 90.6)/90.6 = 6.6%

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