Please Help with my chem lab

DETERMINATION OF THE GAS CONSTANT, R, EXPERIMENT NOTES

In this experiment, you will build an apparatus using glass tubing, a test tube, a flask, and a beaker that will allow you to collect a sample of oxygen gas produced in a reaction. From your measurements you will calculate the gas constant, R.

Procedure for Handling Glass Tubing Glass tubing has long been a very useful item in the chemistry laboratory. Much of the apparatus for many experiments has been fabricated from glass tubing. Soda-lime glass (sometimes called "soft glass") begins to soften and melt between 300-400 °C. This type of glass can be softened and bent or blown using a normal gas-air burner. However, this type of glass has a fairly large thermal expansion coefficient, which means that the glass expands and contracts considerably during heating and cooling. Rapid expansion and contraction often leads to cracking, so this must be avoiding by using techniques which result in gradual heating and cooling.

Borosilicate glass (such as Pyrex or Kimax) has a much lower thermal expansion coefficient and can be used in many applications where the glass experiences a rapid temperature change. Borosilicate glass, however, doesn't begin to soften until between 700-800 °C. This higher temperature requires a burner or blow-torch that combines pure oxygen (instead of air) and natural gas to make the flame.

We will be using soft glass tubing in our construction of a specialized lab apparatus, while almost all of your other glassware is fabricated from borosilicate glass.

Locations: You will be provided with a single piece of glass tubing about a yard long. If you are careful, this piece of tubing will be enough for the three experiments that require glass tubing. Tubing with an outside diameter (OD) of 6 mm is normally used because it fits the standard size of holes in rubber stoppers. The Fischer burners, that are used to heat the glass tubing, are located in the hood (and should not be removed from the hood) along with the glass scratchers, that are used to etch the glass tubing before breaking them. Rubber safety grips and bottles of the lubricant, glycerol, should also be found in the hood.

Determining lengths and bends of tubing: Carefully study the experimental diagrams and note relative lengths and where I APPARArus FoR EXPERIMENT 1s pieces of tubing are to be joined by rubber connectors. Notice that two rubber stoppers are never connected with one piece of glass tubing. Plan ahead so that beakers and flask are sitting flat on the counter, the burner will be uses at the proper height, and that the glassware is near the sink, as indicated in the diagram. To obtain the correct length of glass tubing, hold the tubing next to the piece of glassware, place your thumb where the bend would be, rotate the tubing and mark where it should be cut with your thumb.

GAS CONSTANT EXPERIMENT NOTES

test tuba clamp at very top of t.t.

-------,.,,

44

glass tubing 15·20 om In total length

t rubber connectors ,,·""1, ~ (Read "Rubber Connectors".)

600 ml

The length of the glass tubing connected to the test tube is important because it must be weighed, and therefore cannot touch anything in the balance weighing compartment except for the pan. If it touches the ring around the pan, or the floor of the compartment, then the balance reading will be smaller than actual. This is why the diagram indicates a length of 15-20 cm.

The diagram to the To the right is left 111ustrates a illustrated the end reasonable length of the gloss tubing of the tubing and · at the upper right how it would fit resting on the on the balance pan. floor of the

comoprtment or on the ring around the _eon,

Scratching tubing: Glass tubing of the appropriate length is obtained by making a scratch on the side of the tube and then breaking it along the scratch. The scratch is done with a scratching wheel, which is held firmly while the tubing is rotated just 1 /4 turn. Do not make a scratch that goes further around the tube. Do not go over the scratch again. Turn the tube so that the scratch is away from your body and place your thumbs close together on the tubing on the opposite side from the scratch. Exert a gentle pressure outward with your thumbs to break the tubing along the scratch. If the tubing doesn't break easily, check to make sure the scratch is opposite your thumbs, and try again. If that doesn't work, then wet the scratch with water and try again.

Avoiding burns: Remember that hot glass looks the same as cool glass. Always lay hot tubing on a wire gauze square to cool. Provide adequate time for the tubing to cool, then check it carefully by picking up an area that was not held in the flame and slowly moving your hand toward the region that may be hot. Always make sure there is ice available in the lab before starting, so that you can immerse a burn immediately.

:=: ..... ,.._.:: ..... :.:.-.~- -:- v- .:.., , , ........... :: 1 .... : l'-'11 f-" v...:~~.,;, .... ~ ~;Q,;io,;;,, ; , .... ;~ ~; IV VI l\,A VI LI l'Ci ~UIJII ·~ ~VYVl1VVC11 u, en about a 45 ° angle, into the hottest part of a Fischei buinei flame (2-3 mm above the rnetal) for about 10-15 sec. Heat only until the sharp edges have melted some and become more rounded. Avoid heating for too long as the opening may become closed. All cut ends of tubing should be fire-polished.

Bending: To bend glass tubing, hold the region which is to be bent over the hottest part of a Fischer burner flame, and rotate the tubing so that it is heated evenly. When the tubing begins to sag, stop rotating and quickly remove the tubing from the flame. Allow the tubing to bend of its own weight. Adjust the angle of the bend by moving your hand. A properly made bend will lie in one plane and will have a rounded inside and outside curve so that there is little constriction to the flow of fluids through the tube.

There is one sharp (U-turn) bend that you will need to make, at the end of a piece of tubing. When it is completed, it will need to fit in a test tube, both open ends must be pointing in the same direction, and the tube must remain open enough to allow liquids to pass through. To make this bend, hold the tubing with just one hand, and hold it over the Fischer burner flame so that about 1 cm of tubing extends beyond the flame. Heat, while rotating the tubing, until the tubing begins to sag. Take the tubing out of the flame and push it against the side of the burner to make the sharp turn. A sharp bend made in this manner is generally much more constricted, but is still useful for experiments where only a slow passage of fluids is required.

GAS CONSTANT EXPERIMENT NOTES

45

Choosing stoppers: The proper rubber stopper is generally determined by trial fitting to the glassware being used. Stoppers are usually marked on the top with a number that corresponds to the size, and run from very small (#000) to quite large (#16). Most work done in this lab require sizes #2-6. The proper sized rubber stopper will be one in which 1/3 to 1/2 extends into the mouth of the glassware. The stopper should show no cracks when squeezed, and the surface should be pliable with a glazed appearance.

Inserting tubing into stoppers: Glycerol (glycerin) is normally used as a lubricant to allow for smooth insertion (and easier removal) of glass tubing into rubber stoppers. Glycerol is water soluble and thus easy to clean up. Water can be used when glycerol cannot be tolerated. Lubricate the inside surface of the hole of the stopper by using the rod attached to the stopper of the glycerol bottle.

When inserting glass tubing into a lubricated (or unlubricated, if needed) stopper, always use a rubber safety grip to hold the tubing. Pry open the slot along the length of the safety grip and drop the glass tubing into place. Position the tubing so that no more than 2 inches sticks out of the wide, flared end of the safety grip. If a bent tube is being used, the holes in the safety grips that are perpendicular to the slot that the tubing is in, can be used to accommodate the bend in the tubing, as needed. Hold the rubber safety grips (with the glass tubing properly inserted) as close as possible to the site of insertion, and push the tubing into the stopper hole with a slight twisting motion. Reposition the tubing in the gripper as needed so that no more than 2 inches is sticking out of the wide end of the safety grips. Do not allow your fingers to wrap around any portion of the tubing, or the hole in the stopper that the tubing is being inserted through, while you are pushing it into the stopper. When you need to insert two pieces of tubing into the same stopper, insert the shorter one first.

Using rubber connectors: To connect two pieces of dry glass tubing, use a short piece (-1.5 cm) of unlubricated rubber tubing. Make certain that the two ends of the glass tubing butt against each other at the middle of the connector, and that the glass tubing is in a straight line, without any stress at the connector.

Drying glass tubing with an aspirator: Glass tubing can be dried, by passing air through, it as described in the following procedure:

In the hood, or in a drawer at the front of the lab, you will find some plastic tubing connected to an aspirator that is attached to the faucet. Attach one end of the tubing that is to be dried to the aspirator and turn on the cold water. The flow of water through the aspirator will dissolve air in the tubing, decreasing the pressure in the tubing, and therefore allowing the atmospheric air to push through the tubing. Because this air is drier than the air in the tubing, the water in the tubing will evaporate. When the tubing appears dry, disconnect it from the plastic tubing before turning off the flow of water. 1 APPARArus '°"EXPERIMENT,,

Apparatus Guidelines: Build the apparatus consistent with the following guidelines: The Test Tube and its Stopper: 1) The test tube clamp should be at the very

top of the tt, so that it is far from the flame. 2) The glass tubing should extend about 2 cm

into the test tube. 3) Note the angle of the test tube. 4) The stopper should be about 1/2 way into

the test tube.

GAS CONSTANT EXPERIMENT NOTES

46

tesl tube clamp at very top of t.t.

...........

rod £ ,ertlcal

.. .,,.nt I

glass tubing 15-20 cm In total length

t rubber conneotors 1 w,~·l\ / (Rsad"FlubberConneotora".)

r~..._ pinch clamp

H,O I I

\..,,.____ rubber tubing ls at

fQ laaS115cmfong

' W'J/it''"'iW'"'1 f.:,-, __ , ... ""'t"l ml

5) Notice the distance from the stopper of the test tube to the bend in the glass tubing. 6) NO LUBRICATION in the stopper in the test tube. This is the ONLY place where lubricant

will not be used. 7) Notice the distance between the Bunsen burner and the test tube. 8) A rubber connector above the flask connects the tubing from the test tube to the tubing

from the flask. 9) The straight piece of tubing in the flask extends above the bend in the other piece of tubing. 1 0)The water level in the flask is below the end of the straight piece of glass tubing. 11 )The flask is a 500 ml Florence flask. 12)The stopper in the flask goes about 1/2 way into the flask. 13)The bent piece of tubing has a smooth 90 °bend. 14)The bent tubing extends into the flask, with only a very small distance between the flask

bottom and the tube. It should not be touching the bottom. 15)The other end of the bent piece of tubing extends about 1 cm beyond the edge of the flask. 16) Note the length of the rubber tubing between

the flask and the beaker. 17)There is a pinch clamp on the long piece of

rubber tubing. 18)The glass tubing is long enough to allow

some slack in the rubber tubing. 19)The apparatus is right next to the sink, with

the beaker closest. 20)The angle of the short piece of glass tubing

from the flask towards the beaker is about 45 ° from the edge of the sink. (This is so that the beaker can be lowered into the sink without moving the flask.)

TOP VJ<)// OF THE APPARA llJS

sink

600 ml beaker·'-------~

:..,:..;..:,_ ...,,.,...~-: •• ~:•""-~ Y""'WI :,i~~IU~~u,1 v:l"-''v~-.11 JVMi ll,;llt't'CIIICi~U~ a;~'C;,I f\JU ;IQVCi Gli'.>;:)CJll~;C::u ll 1 ULil J..,,,....,1:.,...-,... ,,..,,,, ...,4,,.. ..... 1,.,,...,,.,..,4,;..,...,. nL-.+-:.,.. ••-••"' : .... _,1._.,._,A,_ .. 1_ =-:L:-1- f"ltf""f"'-i"'fl,F" 1~-L.L~--- ------- ------ __ _ U'OIVl'I;; yvu ,:nc:11 L I ICCILII I~. VIJlctll I yuu, 11 l~llU\,.;lUI ~ II lllli:::11::i ccrvn.c 11gnung your cunsen burner. Failure to do so will result in a significant point loss. If assembled incorrectly, this apparatus can be VERY DANGEROUS.

Disassembling tubing and stoppers: Rubber connectors can be removed from glass tubing by rolling the rubber back onto itself. Glass tubing should be removed from rubber stoppers using a rubber safety grip, keeping the grip close to the point that the tubing is being pulled from the stopper. Application of glycerol to portions of glass tubing that are to be pulled through the stopper may help. Return rubber connectors and clean, dry rubber stoppers to the proper drawers, and discard all of the used glass tubing into the glass waste box in the hood.

Protect yourself from cuts due to broken glass by always treating broken glass and freshly cut tubing with care and by following these guidelines: 1) Always fire-polish the ends of freshly cut tubing. 2) Always lubricate the tubing before insertion into stoppers (unless specifically told not to.) 3) Always use a rubber gripper to hold tubing being inserted into a stopper, and hold the

tubing close to the working region as possible. 4) Always work with glass at a safe distance from your body.

GAS CONSTANT EXPERIMENT NOTES

47

The Experimental Procedure In this experiment oxygen will be generated, its pressure, volume, temperature and number of moles determined and the ideal gas law and van der Waals equation will be used to calculate the gas constant, R.

The oxygen will be produced by the decomposition of melted potassium chlorate, catalyzed by manganese dioxide. Heat will be used to maintain a constant rate of reaction (not a constant temperature.) Balance the following equation:

KCI03 (0 _, KCI (s) + 02 (g)

The sample of KCI03 will have some water adsorbed onto its surface, and must be heated to melting. As long as this is done without the catalyst, the above reaction will not occur. Once the sample has resolidified, you will weigh the test tube. Weighing again after the reaction is complete will indicate the amount of oxygen produced.

There are three stages to the appearance of the tt contents during the reaction.

1) Initially, the solid next to the hottest test tube wall will melt and bubbles will form in the liquid between the solid KCI03 and the glass. While bubbles are being formed, you will either remove the flame or heat another part of the solid. You will resume heating when the rate of bubbles formed decreases or nearly ceases. You do not want to overheat the KCI03, as you will produce a white smoke (vaporized KCI03, which has recondensed) that will result in errors in your data. At this point the liquid is almost all KCI03.

2) Eventually, all the solid will melt, and the liquid will contain some KCI, KCI02, and/or KCIO. The presence of these increases the melting point and decreases the likelihood of vaporizing KCI03 (but it is still possible). During this phase, heat enough to maintain two layers of bubbles next to the glass, When three layers develop, remove the burner. Resume heating when only one layer of bubbles forms.

3) During the third stage the liquid solidifies and the formation of smoke is not likely. It is possible to produce too much oxygen at this point Because there is no longer any liquid in the test tube, you will not see bubbles, but can anticipate the rate of oxygen produced by watching the rate that the water level in the beaker increases. You do not want to get the solid so hot that it evolves oxygen long after you stop heating it

PV = nRT: The pressure of the oxygen will be determined from barometric pressure (more in a minute). You will determine the volume of oxygen produced by measuring the volume of water pushed out from the flask. The mass decrease in the tt is related to the moles of oxygen produced. The temperature of the oxygen will be assumed to be the temperature of the water.

To get the pressure of oxygen from the barometric pressure, it will be necessary to adjust the apparatus so that the pressure in the flask is the same as atmospheric pressure. The beaker will need to be lowered until the upper level of water in the beaker is at the same height as the upper level of water in the flask.

The pressure in the flask is due only in part to the oxygen produced in the reaction. It is also due to the presence of water vapor, because you are collecting the oxygen over water. The barometric pressure represents the overall pressure, that is, the sum of the partial pressure of oxygen and the partial pressure of the water vapor. The water vapor partial pressure can be found in a reference book. It may be necessary to interpolate a vapor pressure at your temperature.

GAS CONSTANT EXPERIMENT NOTES

48

Reading a Barometer 1) Zero the barometer by adjusting the level of mercury in the cistern (glass cup at the bottom

of the barometer) by turning the brass knob at the very bottom of the barometer, until the mercury just touches the ivory pointer (when the pointer just touches its reflection, or the dimple in the mercury goes away). The scale measures the distance from the tip of the pointer to the top of the meniscus (notice that it is inverted from a water meniscus.) Your eye must be quite low to see the pointer.

2) Adjust the movable slide, by turning the brass knob on the right side of the barometer, until the bottom edges (front and back) are even with the top of the mercury meniscus. The lowest black line, on the right side of the movable slide, is the zero line. " .!=

3) Read the vernier scale by reading where the zero line 0 intersects the pressure scale on the right side of the barometer. :0 In the example shown, the zero line is between the 758 and 759 N lines. Rather than estimate the tenth digit, use the vernier scale to determine it. The tenth's digit is the value of the line on the movable scale that is lined up exactly with a line on the pressure scale. In the example shown, the 4 on the vernier scale lines up with the pressure scale, so the pressure is recorded as 758.4 mm Hg.

4) Two corrections must be applied to the barometric reading to get the barometric ( or atmospheric) pressure.

vernier

seal\

l~~t=- ---770

760

----- 758

\ pressure scale

Hg

a) Subtract 0.8 mm Hg for the latitude correction because we are at 34 ° N latitude, rather than 45° N. This is a correction for the variation in the acceleration due to gravity.

b) Subtract the temperature correction for the mercury and scale not being at 0°C. You may need to interpolate to get the correction at your temperature.

- . - -

Temperature Corrections for Barometer Reading

Temp. °C 15.0 16.0 17.0 18.0 19.0 20,_Q_ 21.0 22.0 23.0 24.0 25.0 26.Q_ 27.0 28.0 29.0

Scale Correction, mm Hg 1.86 1.98 2j_Q_ 2.23 2.35 2.47 2.60 2.72 2.84 2.97 3.09 3.21 3.34 3.46 3.58

GAS CONSTANT EXPERIMENT NOTES

49

usmg tne \..omputer You will check your answers to this experiment using a computer in the balance room. The computer will check your answers very carefully, including for significant figures. The computer will expect you to input your data with 2 non- significant digits, and your calculations should be done with these same values. If the computer indicates an incorrect calculation, first reread that you have input the correct requested information, then double check your significant figures. The more careful you are, the easier your experience with the computer will be.