Chemistry

Unit: Matter & Measurement

Name: ______Date: ______MOD: ______

PRE-LAB: LABORATORY TECHNIQUES

  1. Describe the following:
  2. Meniscus
  1. A correctly adjusted burner flame
  1. At what position should your eye be when reading the volume of a liquid in a graduated cylinder?
  1. Describe the use of water displacement method to determine the volume of a solid.
  1. In PART II, mass measured with a balance should be measured to the nearest of a gram.
  1. At what point in lighting the burner should the match be lighted? Where should you hold the match?
  1. When should goggles be worn in this activity?
LABORATORY TECHNIQUES

The development of good laboratory skills is essential for obtaining accurate experimental results. In this laboratory exercise, you will learn the proper use of the volumetric pipet, graduated cylinder, beaker, laboratory balance, and the Bunsen burner.

Although various types of glassware are used for liquids, the graduated cylinder is generally used for the accurate measurement of volume in a general chemistry class. You will find that the units used on graduated cylinders, volumetric pipet, and beakers are mL. It should be remembered that 1 mL = 1cm3 or (cc).

Electronic and triple beam balances measure mass in grams. Different balances have different levels of precision as determined by the number of decimal fractions of grams available.

High school labs generally contain one of two types of gas burners, the Tirell burner and the Bunsen burner. Tirell burners contain a gas adjustment valve on the burner; Bunsen burner gas flow can only be adjusted by using the gas valve on the lab table.

OBJECTIVES

When you have finished this activity, you should be able to:

  1. Demonstrate the proper use of the volumetric pipet, graduated cylinder, the laboratory balance, and a gas burner
  1. Describe how to determine the value of a measurement that does not lie on one of the graduations
  1. Determine which piece of glassware is the most precise in measurement

in order to collect and record data when performing basic scientific investigations.

MATERIALS

BACKGROUND INFORMATION:

Graduated Cylinder:

A 100 mL graduated cylinder has lines on the side. Each small line represents 1 mL. To read volumes that do not fall exactly on the mL markings, imagine the space between each small line is divided into tenths. You are to estimate the volume in one-tenth units of the distance between the two lines. When you read the volume represented by the space between the two adjacent marks it should read to the nearest 0.1 mL.

In order to accurately read a graduated cylinder you must place the cylinder on a level surface. Move your eye to the level of the meniscus as shown in Figure 1 and read the volume of the water to the nearest 0.1 mL.

Volumetric Pipet:

A pipet is used to measure small amounts of solution very accurately. A pipet bulb is used to draw solution into the pipet.

Start by squeezing the bulb in your preferred hand. Then place the bulb on the flat end of the pipet.

Place the tip of the pipet in the solution and release your grip on the bulb to pull solution into the pipet. Draw solution in above the mark on the neck of the pipet. If the volume of the pipet is larger than the volume of the pipet bulb, you may need to remove the bulb from the pipet and squeeze it and replace it on the pipet a second time, to fill the pipet volume completely.

Quickly, remove the pipet bulb and put your index finger on the end of the pipet. Gently release the seal made by your finger until the level of the solution meniscus exactlylines up with the mark on the pipet. Practice this with water until you are able to use the pipet and bulb consistently and accurately.

Electronic Balance:

Electronic Balances are delicate instruments and should be used with care. Read the following guidelines BEFORE performing the procedures.

Guidelines for Use of Electronic Balances
  1. Keep the balance area clean. If it is not clean when you arrive, clean it.
  2. Level the balance by adjusting the feet to center the bubble. Do not move the balance once it is level.
  3. Never place a chemical directly on the balance pan. Always use weighing paper, a watch glass, or a beaker to hold the chemical.
  4. Add and remove items from the balance very gently.
  5. If anything spills on the pan remove the pan from the balance before attempting to clean it.
  6. Leave the area clean.

Press the ZERO and confirm that the balance does, in fact, read 0.00 g. If not, press it once more and it should adjust to Zero. (0.00g)

Place the object on the balance pan. Read the mass to 0.01 g.

Be sure to ZERO the balance before each measurement.

PROCEDURE

PART I—Volume Measurements

A. Determination of Error in Laboratory Glassware

The type of glassware used in the laboratory depends on the purpose of the experiment. In this experiment, you will determine the percent error of various pieces of glassware used in the laboratory.

  1. Weigh a clean, dry 100 mL beaker. Record data on data sheet. (Be sure to zero the balance for each measurement).
  2. Measure 25.0 mL of water using a 50 mL graduated cylinder.
  3. Carefully transfer the water from the graduated cylinder to the empty 100 mL beaker.
  4. Reweigh the beaker. Record data on data sheet.
  5. Repeat steps 1-4 two more times.
  6. Weigh a clean, dry 100 mL beaker. Record data on data sheet.
  7. Measure 25.0 mL of water using a 25 mL pipet.
  8. Carefully transfer the water from the pipet to the empty 100 mL beaker.
  9. Reweigh the beaker. Record data on data sheet.
  10. Repeat steps 6-9 two more times.
  11. Weigh a clean, dry 100 mL beaker. Record data on data sheet.
  12. Measure 25.0 mL of water using a 250 mL beaker.
  13. Carefully transfer the water from the 250 mL beaker to the empty 100 mL beaker.
  14. Reweigh the beaker. Record data on data sheet.
  15. Repeat steps 11-14 two more times.

B. Volume of a Solid by Displacement

The Volume of an object is the amount of space it takes up. When an object has an irregular shape, it is difficult to determine the volume directly. An easy way to determine the volume is by its displacement of water. An object will displace a volume of water that is equal to the volume of the object itself. In this experiment you will determine the volume of several objects.

  1. Add water to a volume reading of about 40 mL. Record the volume as precisely as possible on the data sheet.
  2. Tilting the cylinder to about a 30º angle, carefully slide the metal cylinder into the water. Record the new volume in row 2 in Data Table 1B.
  3. Determine the volume of the metal cylinder. Record on data sheet.
  4. Repeat steps 1-3 two more times.

PART II – MASS MEASUREMENTS

The mass of an object is a fundamental property of the object; a numerical measure of its inertia; a fundamental measure of the amount of matter in the object. In this experiment you will use two different laboratory balances in order to determine the mass of an object.

A.Triple Beam Balance
  1. Place the balance on the desktop; then do not move the balance.
  2. Adjust all sliding weights to the zero mark. If the pointer does not align with the zero mark, use the adjuster nut until it does.
  3. Place the empty 50 mL graduated cylinder on the balance pan. Move the sliding weights, using the heaviest first, until the pointer aligns with the zero mark. Read the mass to 0.01 g. Check to see if the known mass and the mass you obtain are the same. Report discrepancies greater than +0.01 g to your teacher. Record data on data sheet.
  4. Pour 25.0 mL of water into the 50 mL graduated cylinder.
  5. Reweigh the 50 mL graduated cylinder with the water. Record data on data sheet.
  6. Determine the mass of water. Record data on data sheet.
  7. Repeat steps 2-6 two more times.

B. Electronic Balances

  1. Determine the mass of an empty 50 mL graduated cylinder. (Remember to ZERO the balance first.
  2. Pour 25.0 mL of water into the 50 mL graduated cylinder.
  3. Reweigh the 50 mL graduated cylinder with the water. Record data on data sheet.
  4. Determine the mass of water. Record data on data sheet.
  5. Repeat steps 1-4 two more times.

PART III—USING THE GAS BURNER

Laboratory gas burners produce various kinds of flames when different mixtures of gas and air are burned. The most common model is the Bunsen burner. It has an adjustable air vent; the Bunsen burner (Figure 2) can only be adjusted using the gas jet on the lab station.

A. Inspecting the Burner

  1. Compare your burner to the diagram in figure 2 before proceeding.
  1. Disassemble the burner by unscrewing the barrel. Look through the barrel; you should be able to see light. See your teacher if there is a problem. Reassemble the burner.

B. Lighting the Burner

The Bunsen Burner

C. Determining the Temperatures of Different Parts of the Flame

1.Obtain two wooden splints and cut them in half. The four short splints will be used to test the flame by holding them on edge in the center of the flame with crucible tongs as shown to the left. Be sure to hold the splint steady in one position until you barely observe charring of the wood taking place. Remove thesplint promptly at that time, blowing out any fire that may havestarted.

2.Test the flame at four levels: top of the outer cone, justabove the inner cone, middle of inner cone, and at the top of thebarrel. Record your observations on the data sheet.

3.Extinguish the burner by closing the valve at the gas jet, not the burner. Disconnect the hose from the gas jet and make sure there is NO gas coming out of the value.

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