Teacher Notes



Massing GasesDemonstration Kit
Publication No. 13842
IntroductionMake Avogadro proud by using his law to determine the molar mass of several gases. In this demonstration, the mass of oxygen and various other gases will be determined, and their mass ratios will be used to calculate the molar mass of each gas. Concepts
MaterialsBalance, milligram (0.001g precision), 100g capacity
Gas sources, several Latex tubing and pinch clamp, 2* Luerlock plastic syringe, 140mL, with nail* Medicine dropper, glass piece only, 2* Plastic freezer bags, quart size, 2* Rubber stopper, size #2, 2* Rubber stopper, size #10, 2* Syringe tip cap* *Materials included in kit. Safety PrecautionsWear chemical splash goggles, chemicalresistant gloves and a chemicalresistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. DisposalPlease consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. Carefully release the samples of air and oxygen into the atmosphere. Burner gas or poisonous gases should be released under an efficiently operating fume hood. Prelab PreparationGas Delivery Apparatus
Procedure
Student Worksheet PDFTeacher Tips
Correlation to Next Generation Science Standards (NGSS)^{†}Science & Engineering PracticesUsing mathematics and computational thinkingDeveloping and using models Disciplinary Core IdeasMSPS1.A: Structure and Properties of MatterMSPS3.D: Energy in Chemical Processes and Everyday Life HSPS1.A: Structure and Properties of Matter Crosscutting ConceptsCause and effectEnergy and matter Stability and change Scale, proportion, and quantity Performance ExpectationsMSLS24: Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations. DiscussionAir, like water, exerts a positive or upward buoyant force on all objects. This force is compensated for in balances when “massing” liquids and solids. When massing gases, however, this force is not negligible. The apparent mass of gas will be less than the actual mass of the gas. True mass of gas = apparent mass of gas + mass of air displaced. By evacuating the syringe volume of all gas (steps 1–3), the last term, mass of air, is eliminated and the true mass of the gas can be directly determined.The molar mass of any gas can be estimated using the Ideal Gas Law. {13842_Discussion_Equation_1}
{13842_Discussion_Equation_2}
{13842_Discussion_Equation_3}
However, this determination requires accurate values for temperature (T), pressure (P) and syringe volume (V).The principles of Avogadro’s law can be used to eliminate the need for these values. Avogadro’s law states that the number of moles of a gas is directly proportional to its volume, when pressure and temperature are held constant. {13842_Discussion_Equation_4}
k = P/RT If two gases are at the same temperature, pressure and volume, it follows that they have the same number of moles.{13842_Discussion_Equation_5}
The moles of any gas are equal to the mass of the gas divided by its molar mass. Substituting into Equation 5
{13842_Discussion_Equation_6}
If the molar mass of one gas is known, the molar mass of the other gas can be determined from Equation 6.In this demonstration, pure oxygen or air can be used for the known gas. The molar mass of oxygen is 32.0 g/mol. Air is a homogeneous mixture of gases and has an apparent molar mass of 28.9 g/mol. A sample calculation using oxygen as the reference gas and air as the “unknown” gas is summarized. If the mass of air is determined to be 0.168 g and the mass of oxygen is 0.176 g, the experimental molar mass of air is calculated as follows: {13842_Discussion_Equation_7}
ReferencesSpecial thanks to DeWayne Lieneman, retired chemistry teacher, Glenbard South High School, Glenn Ellyn, IL, for providing the idea and procedure for this demonstration. Recommended Products

