CHEM 121L General Chemistry Laboratory Revision 1 1 Determination of the Percentage Oxygen in Air In this laboratory exercise we will determine the percentage by volume of Oxygen in Air We will do this by allowing the Oxygen in the Air to react with Iron in the form of Steel Wool This will convert the gaseous Oxygen into solid Iron Oxide This will cause the volume of the Air to decrease as it becomes devoid of Oxygen Once the reaction is complete the decrease in the Air s volume will be equal to the volume the gaseous Oxygen occupied in the Air One of the earliest observations concerning combustion was that it stops when the supply of Air is removed from the combustible material Various theories were put forth to explain this observation The first of these theories to take hold was the Theory of Phlogiston This theory put forth in 1667 by Johann Joachim Becher postulated that bodies were composed of three earths terra lapidea vitreous terra mercurialis mercurial terra pinguis fatty Becher considered combustible substances to be rich in terra pinguis which was lost during burning Even metals that were calcinable contained some of the fatty earth These ideas were later developed into an elaborate chemical system in which the term phlogiston replaced terra pinguis The Development of Modern Chemistry Aaron J Ihde Page 2 According to this theory during combustion a hypothetical substance called Phlogiston is released Air was believed to absorb the Phlogiston until a point of saturation It is at this saturation point that combustion ceases unless fresh Air is introduced Material Dephlogisticated Material Phlogisticated Air This theory was capable of explaining not only combustion and calcination but was also capable of explaining the smelting of metal ores and respiration Although inconvenient facts kept pointing to inconsistencies in the Theory of Phlogiston it was not until the discovery of the gas Oxygen in the late 1700 s that it was finally abandoned Gaseous Oxygen was first produced by Carl Wilhelm Steele in 1774 when as a result of the strong heating of the Red Calx of Mercury Mercuric Oxide the Calx decomposed into elemental Mercury and a gaseous substance now known as Oxygen Antoine Lavoisier and Joseph Priestley were then able to show that Oxygen is a component of Air and is required for combustion and respiration With these discoveries it was realized that Air supports combustion because of the presence of Oxygen When Air is depleted of Oxygen it no longer supports combustion Material Oxygen Oxide of the Material Although it seems as though the Theory of Phlogiston has merely been inverted Oxygen is an isolatable substance whereas Phlogiston is not Additionally all those inconvenient facts unexplainable by the Theory of Phlogiston suddenly become explainable by the presence and reactivity of Oxygen Thus as Ihde points out The phlogiston concept gradually disappeared as its elder adherents were removed from the scene by death Lavoisier was able to show that Air that combined with elemental Mercury lost about 1 6 of its volume In turn he was able to regenerate this volume of gas by decomposing the Red Calx that formed Thus he reasoned Air is approximately 1 6 Oxygen We will do something similar in our determination of the Oxygen content of Air We will combine Air with elemental Iron In the presence of Water and an acid catalyst the Oxygen in Air reacts with Iron to form a complex Hydrate 2 Fe s 3 O2 g x H2O Fe2O3 xH2O s Eq 1 We must be careful not to use too much acid however because if the system is too acidic the Iron will react directly with the acid to form Hydrogen gas Fe s 2 H aq Fe2 aq H2 a Eq 2 Thus we will soak Steel Wool in a dilute aqueous solution of Acetic Acid and allow it to react with the Oxygen in Air As it does this the volume of the Air will contract When complete the Page 3 volume lost by the Air will be equal to the volume Oxygen initially present We will measure this volume lost by the Air by carrying out this reaction in an inverted test tube placed in a bath of Water As the Oxygen is depleted from the Air and its volume contracts Water will rush into the test tube to replace it By measuring the volume of this Water we will be measuring the volume of Oxygen in the Air This method of measuring the volume of Oxygen in Air has one draw back The Air above the Water is Moist due to some Water Vapor that will be present in the gas We can however correct for this by subtracting out the volume of the Air occupied by the Water Vapor Thus by applying an appropriate correction factor we can determine the Volume Percentage Oxygen in Dry Air We can then compare this determination with the accepted literature result Page 4 Pre Lab Questions 1 Dry Air is reported to be 20 94 Oxygen What is the Percentage Error in Lavoisier s determination 2 Write a balanced chemical reaction for Scheele s method for producing Oxygen 3 The word Oxygen is derived from the Greek word What is the meaning of this word and why was it selected as the name for Scheele s new gas Page 5 Procedure 1 Obtain two 20 x 150 test tubes without lips Mark each tube from the open end using a wax pencil every 0 5 cm for a total length of 6 cm You will use these markings to judge the rate of reaction by observing how fast Water fills the tube Arrange to clamp these tubes to a ring stand in an inverted position in a 1 L beaker filled with tap Water 2 On an Analytical Balance weigh out two 0 5g pieces of fine Steel Wool size 00 Do not compress the Wool 3 In a fume hood obtain 50mL Acetone 50 mL of 1 0 M Acetic Acid and 50 mL 0 1 M Acetic Acid in 100 mL beakers 4 Using forceps in a fume hood rinse one of the pieces of Wool in Acetone for about 30 seconds This will remove any oil from the surface of the Steel Wool Shake off the excess Acetone and drain it briefly on a paper towel Do not compress the Steel Wool 5 Transfer the Wool to the 1 M Acetic Acid Agitate the Wool in the Acid for about 1 minute This will help remove any Oxide coating from the Steel Wool and prepare the Wool s surface for reaction with Oxygen Shake off the excess acid and drain the Wool briefly on a paper towel Do not compress the Steel Wool 6 Transfer the Wool to the 0 1 M Acetic Acid Agitate the Wool in the Acid for about 30 seconds Remove the Wool and shake it vigorously in the sink to remove as much of the acidic solution as possible A small amount of this solution will remain adhered to the Wool s surface This will provide the