Chem Lab Final Study Guide - EXPERIMENT 4 MOLAR MASS DUMAS...

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EXPERIMENT 4: MOLAR MASS – DUMAS METHOD o At room temp, dumas tube contains air. Mass=mass of tube+mass of air o Volatile liquid is placed in tube at room temperature vapor is in equilibrium with liquid, tube also contains air o When tube is heated, liquid vaporizes, pushing air out of the tube. At temps above boiling point, tube only contains vapor at a pressure equal to room pressure. Temperature used in PV=nRT o When tube is brought back to room temperature, volatile substance goes back into equilibrium with vapor and liquid. Mass of tube=mass of liquid + mass of vapor + mass of air + mass of dumas tube. Mass of tube + mass of air can be subtracted (we found this in the beginning) to find mass of volatile substance molecules. Mass of liquid + vapor must be equal to mass of vapor at high temp. o We use Avagadro’s hypothesis that equal volumes of gases at the same temp and pressure contain equal numbers of molecules molar mass can be determined. o Condensed liquid is effectively trapped in Dumas tube (can evaporate very slowly through narrow neck. o Standard deviation of the molar mass is the standard deviation of the grams over the number of moles o Necessary equations: ```` PV=nRT (solve for n) ( P + an 2 V 2 ) ( V nb ) = nRT

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