Challengeproblem CHEMISTRY GAS－dvhtlbz的部落格

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Challengeproblem CHEMISTRY GAS

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1) An organic chemistry student isolates a hydrocarbon (a compound containing only hydrogen and carbon) that is found to be 87.73% carbon by mass. The student finds that 0.67 grams of the substance when vaporized in a 500-mL bulb exerts a pressure of 306.5 torr at 28 °C. What is the chemical formula of... 顯示更多 1) An organic chemistry student isolates a hydrocarbon (a compound containing only hydrogen and carbon) that is found to be 87.73% carbon by mass. The student finds that 0.67 grams of the substance when vaporized in a 500-mL bulb exerts a pressure of 306.5 torr at 28 °C. What is the chemical formula of the hydrocarbon? (enter the chemical formula, for example C2H4) ---------- 2) A student performs this lab using 0.106 grams of magnesium with excess acid. She collects 118 mL of product gas (over a water bath) at 23.0°C and an atmospheric pressure of 760.5 mmHg. Water vapor pressure 21.1mm Hg P(atm) = P(gas) + P(water vapor) A)Based on the data above what is P(gas)? B)Using these values of P(gas), V(gas), n(gas) and T(gas), calculate the student's experimental value of R (the gas constant). Watch the units! R = L·atm·mol-1·K-1 更新: the first answer is right but the both the 2a and b are wrong, can anyone help me?thanks

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(1) Assume there's 100 g of the hydrobarbon, then there will be 87.73 g of C and 12.27 g of H Giving a mole ratio of: C : H = 87.73/12 : 12.27 = 7.311 : 12.27 which is about 3 : 5 Therefore the empirical formula of the hydrocarbon is C3H5 with emprical mass = 41. Now, for the 0.67 g of the vapurized substance, applying: n = PV/RT where P = 306.5 x 133.322 = 40863 Pa V = 5 x 10-4 m3 T = 301 K R = 8.314 We have n = 0.008164 So molar mass = 0.67/0.008164 = 82 g/mol Therefore the chemical formula is C6H10. (2a) P(gas) = 760.5 - 21.1 = 739.4 mm Hg = 739.4 x 133.322 = 98.6 kPa (b) Volume in L = 0.118 Pressure in atm = 98.6/101.325 = 0.973 Temperature in K = 296 No. of moles of gas = 0.106/24.1 = 0.00440 since 1 mole of Mg can produce 1 mole of hydrogen gas. So expt. value of R is: R = PV/nT = 0.08818 L atm mol-1 K-1 2009-05-01 11:16:18 補充： Any ans can be provided for 2a and 2b ?

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