CHM 121 Lab Density of Nails Baby Oil and More Full SP21 PDF

Preview

Summary

Download CHM 121 Lab Density of Nails Baby Oil and More Full SP21 PDF

Description

Density of Nails, Baby Oil, and More CHM 121, Oakton Community College, Fall 2020 Summary Students will determine the density of two liquids—baby oil and a household liquid of their choice—and a solid (masonry nails) using a conical tube as a somewhat crude version of a pycnometer. Students will also use another method to determine the density of baby oil. Students will make conclusions about the accuracy of the two methods and their determinations, as well as consider the possible composition of the masonry nails used.

Introduction Density is a physical property of substances which reflects the “compactness” of the matter. A high density indicates a relatively large amount of matter occupying a given amount of space (“compact” matter), whereas a low density indicates a relatively small amount of matter occupying a given amount of space (“spread out” matter). Density is an intensive variable, meaning its value does not depend on the amount of matter in the sample. This is in contrast to mass and volume, both of which are extensive variables, increasing their values as the amount of matter present in a given sample increases. In fact, evidence suggests that the mass and volume of a sample of a pure substance are both proportional to the amount of matter—if there is twice as much copper, for example, in a sample, the sample will have twice the mass and twice the volume. As such, the ratio of mass to volume—the mass divided by the volume—is a constant value for a given substance (at a given temperature). This ratio, in fact, is, by definition, called the density of the substance. Given the above, one may experimentally determine the density of a sample of matter (whether it be a pure substance or a mixture) if one can determine both the mass and volume of the same sample of the matter. Generally speaking, mass can be determined fairly easily using a balance, properly calibrated. The volume of a liquid sample is fairly easily determined using a graduated cylinder, although it is limited to some degree in the precision that can be achieved relative to other means. Another approach to determining the volume of a liquid is to use a pycnometer, which is basically a fancy term for a container that has a precisely determinable (and fixed) volume. If one can determine the volume of the interior of the pycnometer, then any sample of matter that completely fills the interior of that same vessel is assumed to have the same volume. The volume of that sample is thus “determined”. Determining the volume of a pycnometer is called “calibrating” it. One way to do this is to fill it with a liquid of known density. If the mass of the pycnometer is measured before and after the filling, the mass of the liquid can be determined by difference. Then, the volume of the liquid can be calculated from the mass using the (known) density of the liquid. Since the liquid was assumed to completely fill the pycnometer, the volume of the (known) liquid is assumed to be the volume of the interior of the pycnometer. A key to the success of this approach is the ability to fill the pycnometer completely and reproducibly with fluid. For example, to the extent that any air bubbles get included with a given fluid sample, there will be some inaccuracy, since the assumption that the volumes are “equal” will not be 100% correct. As such, a “real” pycnometer used for very accurate and precise determinations will have special means by which all gas bubbles can be expelled from the vessel. In this experiment, we will use a 50-mL capped conical tube as a somewhat crude pycnometer. Water will be used to calibrate the tube, since the density of water is known very accurately (see below). It is a bit challenging to reproducibly fill this tube with a fluid due to the small and varying amount of air that is inevitably incorporated when the cap is lowered onto the tube and screwed in. However, the approach is well demonstrated, and students will use the tube to determine the density of two liquids—baby oil and a household liquid of their choice. The volume of solid can be determined by liquid “displacement”. A solid is placed in a liquid, and if the solid is nonporous, such that liquid cannot seep into the solid, the liquid is displaced, and the volume of the solid equals the volume of the liquid displaced. This is most directly assessed by measuring the volume of a liquid, then immersing a solid sample in the liquid, and then measuring the total volume of liquid plus solid. The different in volume readings yields the volume of the solid. In this experiment, however, we will use a more indirect approach using the conical tube pycnometer. Once the volume of the conical tube has been determined, a sample of a solid will be put in the tube and the mass measured. Then, water will be added to fill the tube with the solid in it, and the mass measured again. With appropriate difference calculations, the mass of the solid, as well as the mass of the water in with the solid, can separately be calculated. The volume of the water in with the solid can then be calculated using its mass along with the (known) density of water. Since the volume of the tube is assumed “fixed”, the volume of the solid plus water must equal the volume of the tube. As such, the volume of the solid can be determined by subtracting the volume of the 1

Density of Nails, Baby Oil, and More

water in with the solid from the volume of the tube. Once the mass and volume of the solid are determined, density can be calculated by dividing the mass by the volume. Many students incorrectly believe that the density of liquid water is (always) exactly “one” gram per mL. That is not actually true. It is generally close to that value, but decreases with increasing temperature due to the slight increase in volume that accompanies heating (see values in table below). Precise (and accurate) values of the density of liquid water at various temperatures near room temperature are shown below. Students will measure the temperature in the lab (or the water used) to the nearest degree, and then use the (assumed) density of water at that temperature from the table in their calculations.

Density of Water at Various Temperatures Near Room Temperature

T (Celsius) 20 21 22 23 24 25

d(water) (g/mL) 0.9982 0.9980 0.9978 0.9975 0.9973 0.9970

Reagents and Equipment Needed From Kit 

Mini balance*



Provided By Student Distilled (or tap) water, at room temperature**

 

50-mL conical tube with cap 50-mL graduated cylinder

 

Paper towels A household liquid of your choice, from bath or

  

100-mL beaker 800 mL beaker (tri-corner; for temporary waste collection) 250 mL beaker



Thermometer (handle with care; glass!)

  

Baby oil (~60 mL, not in original bottle) Square-cut Masonry Nails (4-6; found in a small manilla envelope labelled “Nails”) Disposable plastic pipet



Plastic spatula

* Before the first use of the mini balance, a strip that is protecting battery life may be present. If so, it must be removed. See instructions below.

kitchen (not “shop” or “garage” fluids). E.g., dish soap, laundry detergent, shampoo, conditioner, 70% rubbing alcohol, maple syrup, honey, cooking oils, etc.

** The easiest way to achieve this is to leave some water sitting out overnight the night before the experiment is performed.

Preparation of Mini balance for first use. NOTE: The instructions below are for only one type of mini balance. The one in your kit might be different and may or may not have a protective strip that needs to be removed before use. Also, your mini balance may not have a protective cover. Please try out your mini balance at least one day before your lab period, and ask your instructor if you have any issues.

2

Density of Nails, Baby Oil, and More

1. Remove balance from box, and remove all plastic packaging/wrap. It should look like (1) below. Note the white plastic strip sticking out from the right side, bottom. 2. Before each actual use, you must remove the rigid, plastic protective cover from the metal balance pan. See picture (2). Replace this cover when balance is not in use. 3. Turn balance over (preferably with cover back on, unlike as shown, oops!), and remove the battery compartment cover. Note the white strip sticking out of the battery compartment. See picture (3). 4. Remove the battery touching the strip, remove the strip, and replace the battery in the correct orientation. Replace the battery compartment cover. See (4). Turn balance over. Balance is ready to use.

1

2

3

4

Safety: Follow all of the safety rules you agreed to when you signed your safety form. Wear safety goggles; we always wear safety goggles when doing chemistry experiments since we just don’t know what to expect. Don’t ever assume something is safe.

Chemical Disposal: Do not pour baby oil in the sink/down the drain. It is not considered hazardous and can be disposed of in the regular trash. Nails can also be thrown in the trash.

3

Density of Nails, Baby Oil, and More

Procedure, Determining the Interior Volume of the Conical Tube 1. Turn on the mini balance, wait for it to warm up a bit (30 s should suffice). Place a 250-mL beaker on the weighing area and hit the TARE/zero button. The reading should be 0.0 g now (with the beaker on it). This is called “taring”. 2. Place an empty, dry 50-mL conical tube, with cap on it, in the 250-mL beaker. Record the reading. This is the mass of the empty conical tube + cap (m1). 3. Remove the conical tube, hold it over the 800-mL beaker, and fill it completely (not to any particular line!) with distilled (or tap) water until water makes a little “dome” above the edge of the tube, or until it actually overflows a bit. Using a disposable pipet near the end will help to achieve this. Carefully screw on the cap, trying to keep as much water in the tube as possible. Some water should spill out during this step. A bit of air will likely get trapped no matter what you do, but your goal is to try to minimize that. Make sure the cap is reasonably tight, but don’t overtighten it. You don’t want to strip the threads—this is a plastic thread, not metal! 4. Carefully dry the entire outside of the conical tube with a paper towel. Then dry the small crease or space between the cap and the side of the tube by sliding the corner of a paper towel up into that space and dragging it around the cap. Repeat as necessary, until it appears dry. 5. Tare the balance with the empty 250-mL beaker on it (balance should read 0.0 g). Place the filled, capped 50-mL conical tube into the beaker and record the reading. This is the mass of the conical tube filled with water + cap (m2).

Procedure, Determining the Density of Masonry Nails 6. Dump out the water from the tube and carefully dry the inside of the tube as well as the cap. 7. Tare the balance with the 250-mL beaker on it. Place the empty, dry, conical tube with cap into the beaker and record the reading. This is m3. If this is significantly different than the value for m1, check to see if tube and cap are dry and remeasure. Make sure that you are taring the balance properly as well. Ask instructor, if necessary. 8. Add all of the masonry nails provided in the kit to the conical tube and cap it. Tare the empty 250-mL beaker again, place the tube with nails in the beaker, and record the reading. This is m4. If the reading is not between 25 and 40 grams, retry, and if necessary, ask your instructor. 9. Without removing the nails from the conical tube, carefully add distilled (or tap) water to the tube, initially about half to two-thirds full. Check to see if air bubbles have formed on the nails. If so, gently tap the tube on the desktop and/or swirl the water in the tube to dislodge the air bubbles. If necessary, you may use plastic spatula to dislodge the bubbles as well. 10. When there are no air bubbles remaining on the nails, add more distilled (or tap) water, as you did in Step 3, until it is “domed”/full. Carefully screw on the cap as in Step 3, trying to minimize any trapping of air. 11. Dry the tube and crease carefully as in Step 4 above. 12. Tare the balance with the empty 250-mL beaker on it. Place the water-filled, capped 50-mL conical tube with nails into the beaker and record the reading. This is m5.

Procedure, Determining the Density of a Liquid/Fluid of your Choice 13. Dump out the water and nails from the tube and carefully dry the inside of the tube as well as the cap. You will not need the nails any more. 14. Tare the balance with the 250-mL beaker on it, place the empty, dry, conical tube with cap into the beaker and record the reading. This is m6. It should be very similar, if not identical to m1 and m3. 15. Remove the conical tube, uncap it, hold it over the 800-mL beaker, and fill it carefully with your household liquid/fluid as you did with water in Step 3. This can get a bit messy with the overflowing, so please take care to get any spillage into the 800-mL beaker. 16. Wipe the tube and crease carefully with paper towel as in Step 4 above. However, if your liquid is more viscous and/or sticky than water, this may be more challenging. Do the best that you can, and use some water, if applicable, to help clean the outside before drying it.

4

Density of Nails, Baby Oil, and More

17. Tare the balance with the empty 250-mL beaker on it. Place the filled (with your liquid), capped 50-mL conical tube into the beaker and record the reading. This is m7.

Procedure, Determining the Density of Baby Oil (Method A) 18. Dump out the household liquid from the tube and clean or rinse the cap and tube in a sink via appropriate means, depending on the nature of the liquid you chose. E.g.., Either rinse it very well with water (e.g., if your liquid was a soap or detergent), or add some dish soap and warm water to clean it (e.g., if it was a cooking oil, maple syrup, etc.). Make sure both the inside and outside are completely clean. 19. Carefully dry the inside of the tube as well as the cap. 20. Tare the balance with the 250-mL beaker on it. Place the empty, dry, conical tube with cap into the beaker and record the reading. This is m8. If this is significantly different than the value for m1, m3, and m6, check as before. 22. Pour the contents of the baby oil bottle from the kit carefully into the (clean and dry) 100-mL beaker. **Be careful not to spill, as you will very likely not have much excess of the baby oil for this part (and the next!)** 21. Remove the conical tube, uncap it, hold it over the 800-mL beaker, and fill it carefully with the baby oil (pouring it from the 100-mL beaker) as you did with water in Steps 3 and 15. NOTE: You will likely have just enough baby oil in the kit to fill the tube completely. As such, please be careful not to spill any baby oil during this process. As with Step 15, this can get a bit messy with the overflowing, so please take care to get any spillage into the 800-mL beaker. 22. Wipe the tube and crease carefully with paper towel as in Step 16 above. Since this is an oil, this will be more challenging. Do the best that you can. 23. Tare the balance with the empty 250-mL beaker on it. Place the filled (with baby oil), capped 50-mL conical tube into the beaker and record the reading. This is m9. 24. ***DO NOT DISCARD THE BABY OIL.*** Rather, pour the baby oil carefully back into the 100-mL beaker for use in the next procedure. You may wish to add some dish soap and warm water to the conical tube, and soak the cap in soapy water, to help aid in cleaning of it after the experiment is complete.

Procedure, Determining the Density of Baby Oil (Method B) 25. Remove the 250-mL beaker from the mini balance pan. 26. Tare the balance (with nothing on it). 27. Place the 50-mL graduated cylinder on the balance. Record the reading. This is m10. 28. Carefully pour the baby oil from the 100-mL beaker into the graduated cylinder until it is somewhere between 45 and 49 mL. Do not try to fill it all the way to 50 mL! Make sure to wipe off the outside of the cylinder if any oil drips onto it. 26. Tare the balance (with nothing on it). 29. Place the graduated cylinder with the baby oil in it onto the balance. Record the reading. This is m11. 30. Place the graduated cylinder on your workspace surface (which should be flat). Make sure your eye is at the liquid level, and read the volume value as accurately as possible, taking care to estimate the last digit to maximize precision (even if this last digit has some uncertainty). Record this volume value. 31. If you wish, you may pour the baby oil back into its kit bottle and keep it for household uses. Or you may discard in the regular trash—do not pout down the sink. 32. Make sure to wash any plastic-ware well with plenty of dish soap and warm water.

Procedure, Measuring the Temperature 33. At any point during the lab period, students may measure either the temperature of the room or the temperature of the water, to the nearest degree, using the thermometer from the lab kit. So if you have not already done so, make sure to record this temperature! It doesn’t matter whether the air or water temperature is measured since the water should have been left at room temperature overnight prior to the experiment. 5

Name: Saeed Ghaemian

Prelab: Density of Nails, Baby Oil, and More NOTE: If you are keeping a lab notebook for this class, you may wish to copy the raw data table from the report form into your notebook before lab starts for convenience during data collection. You should not record your raw data onto the form! Rather, record raw data in your notebook (and then copy out from it [to the form] rather than into it). 1. In the part of the experiment in which you determine the density of liquids using a pycnometer (here, a 50-mL conical tube with cap), your final results are density values, which are calculated from mass and volume values. But your raw data for this part are masses only. (Think hard about this: you never actually measure any volumes, right? Except when you use a graduated cylinder later for Method B, which I am not asking about here.) Explain how you get density when your raw data do not include volume values. Be specific. Hint: You must consider (and include in your answer) why you collect raw data on water. How is that used to help you determine the density of your chosen liquid or baby oil? Based on the density formula we will need mass and volume to find the density. Since, we have the volume of the tap water which is based on the room temperature and approximately 1gr per ml, we can use this volume as a reference to find the density. Regarding the baby oil, we calculated the volume of the tube in previous runs and we need mass of the baby oil to find the exact density of the oil.

2.

(a) What is the main ingredient of baby oil? (see copy of original label, in “Reagents and Equipment Needed”) Mineral oil

(b) Is baby oil a pure substance? Explain briefly. No, it is not. It formed by different particles (mineral oil) and also fragment. (c) Search the web for the “composition of mineral oil”. Briefly summarize what you find here (do not include any values or percentages). Is mineral oil a pure substance or a mixture? Explain. No it is a mixture of branched, straight chained, and cyclic heavy alkanes.

3. Linn has two jars. They are the same size and shape. Each is filled with the same amount of water (see right).

She also has two metal balls of the same volume. One ball is light. The other is heavy (see below).

She lowers the light ball into jar 1. The water level in the jar rises and looks like this (see right):

Density of Nails, Baby Oil, and More

If the heavy ball is lowered into Jar 2, will the level in Jar 2 end up at (a) the same level, (b) a lower level, or (c) a ...