# chem lab exam 2.docx - Chem Lab 1 Exam 2 study guide...

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Chem Lab 1 Exam 2 study guide Experiment 4: Molar Mass- Dumas Method -Objectives oUsing vacuum trap to prevent liquids from getting into the vacuum pump oUsing the ideal gas law to determine the moles of gas (PV=nRT) oUse the mass of a liquid and its density to determine the volume of an irregularly shaped object oConnect microscopic pictures of gases and liquids and macroscopic observations in the lab oUse Van der Waals equation of state to determine how the parameters of non-ideality affect macroscopic observations P+an2v2(Vnb)=nRTP=nRTVnban2v2a related to intermolecular forces omolecules with strong intermolecular forces will have larger values of a osmall molecules (e.g. hydrogen) and inert gases have small a values attractive forces hold the molecules together reduces the numbers of collisions with the walls makes pressure lowerb actual volume taken up by 1 moles of molecules odecreasing volume will increase pressure oUsing error in different parts of an experiment and how affects the overall error -Molar mass the mass of one mole of a substance oWe are finding the molar mass of a volatile liquid which has a high vapor pressure -Because it is hard to measure the mass of a gas use the dumas tube oNarrow neck prevents the exchange of molecules between the inside and outside of the tube BUT, there is still some exchange because the pressure in the tube equilibrates to the room pressure -What’s inside the Dumas Tube? oRoom temperature: air @room temp oWhen volatile liquid is added: liquid, air, some of liquid’s vapor Total pressure is still room pressure Everything is in equilibrium o100(boiling point and above): just unknown vapor oWhen cooled to room temperature: air, unknown vapor, unknown liquid Most of the vapor should have condensed -To find the mass of unknown subtract the mass of the tube originally at room temperature from the tube that had cooled to room temperature after the experiment oWill give you the mass of unknown vapor and liquid Phase doesn’t matter! – mass will be the same -Experimental tips or sources of error oDry the outside before weighing the tube oAllow the tube to return to room temperature, otherwise the tube won’t have enough air -Avogadro’s hypothesis equal volumes of gases at equal temperatures and pressure hold the same number of particles -Ideal gas law: PV=nRT n= PV/RT oPressure – room pressure
oVolume – fill the dumas tube with water and weigh it, then use density to figure out volume Use trap to avoid filling the pumps with water oTemperature - 100When all of the particles are unknown gas -Molar mass = massof compoundnumberof moles-Error and uncertainty oDirection how the error affects the mass (numerator) or number of moles (denominator) oMagnitude determined by the size of the uncertainty or error in the numerator or denominator oUnknown vapor is not ideal – it’s a real gas o