A chem student named A. Hayes determines the mass of anunknown gas to be 6.84 gr

Question

A chem student named A. Hayes determines the mass of anunknown gas to be 6.84 grams at 25 degrees celsius and 740 mm Hg ina 3.0 L container. What is the molecular mass of the unknowngas? The "unknown" gas (among the following) is mostprobably: a) methane, CH4 b) propane, C3H8 c) butane, C4H10 d) pentane, C5H12 A chem student named A. Hayes determines the mass of anunknown gas to be 6.84 grams at 25 degrees celsius and 740 mm Hg ina 3.0 L container. What is the molecular mass of the unknowngas? The "unknown" gas (among the following) is mostprobably: a) methane, CH4 b) propane, C3H8 c) butane, C4H10 d) pentane, C5H12

Explanation / Answer

We know that  ideal gas equation PV = nRT Where P = Pressure = 740 mmHg                                     = 740 / 760atm              Since 1 atm = 760 mm Hg                               = 0.9736 atm T = Temperature = 25 o C = 25 + 273 = 298 K R = gas constant = 0.0821 L atm / mol - K V = Volume = 3 L n = No . of moles = mass /Molar mass = m / M = 6.84 g /M Plug the values in the above Equation we have PV = ( m / M )RT                                                                   m / M = PV / RT                                                                        M = mRT / PV                                                                             = 57.2946 g                                                                              ~58 g Molar mass of C4H10 = 4 * 12 + 10 * 1 = 58 g So the answer is c) butane, C4H10

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