Accuracy and Precision with Volumetric Glassware

Familiarity with the various instruments used for making physical measurements in the laboratory is essential to the study of experimental chemistry. In this experiment, you will investigate the uses and limits of the various types of volumetric glassware.

The three types of volumetric glassware that will be used today are: graduated cylinders, pipets, and burets. The graduated cylinder is the most common apparatus for measuring liquids in the lab. To correctly read the graduated cylinder, hold it so that the meniscus is at eye level and read the bottom of the meniscus. The pipet (transfer) is a more precise determination of a liquid. There are two types of pipets: Mohr pipet and volumetric pipet. The Mohr pipet is calibrated at least each milliliter and can be used to deliver any size sample. The volumetric pipet can deliver only one size sample (as stamped on the barrel of the pipet). Pipets are filled by using a rubber safety bulb to supply the suction needed to draw liquid into the pipet. It is absolutely forbidden to pipet by mouth in the chemistry lab. The buret can be used when various sizes must be dispensed or measured precisely. The buret consists of a tall, narrow calibrated glass tube, fitted at the bottom with a valve for controlling the flow of liquid. The valve is called a stopcock.

Materials Needed: 10 ml graduated cylinder, 10 ml volumetric pipet, safety bulb, buret, ring stand, buret clamp, beakers, water, thermometer probe and GLX.

Procedure:

Part A. The Graduated Cylinder

1. Clean and wipe dry your 10 ml graduated cylinder and a 50 ml beaker ( a rolled-up paper towel will enable you to dry the inside of the cylinder. Weigh the graduated cylinder and beaker and record the mass of each to the nearest milligram(0.001 gram)

2. Obtain about 100 ml of tap water with a clean Erlenmeyer flask. Determine and record the temperature of the water.

3. Fill the graduated cylinder with the tap water so that the meniscus of the water level lines up with the 10-ml mark.

4. Weigh the graduated cylinder and the 50-ml beaker to the nearest milligram(0.001 g) and calculate the mass of water each contains.

5. Use the Density of Water table provided, calculate the volume of water present in the graduated cylinder and the beaker from the exact mass of water present in each.

6. Repeat 2 more times.7. Compare the calculated volume of water( based on the mass of water) to the observed volumes of

water determined from the calibration marks on the cylinder and beaker. Calculate the percent error between the calculated volume and the observed volume from the calibration marks.

% error= (experimental – actual/ actual) 100

In this lab the actual is the calculated volume of water and the experimental is the observed volume.

Part B. The Pipet

1. Obtain a 10-ml pipet and rubber safety bulb. Clean the pipet with soap and water. Rinse with tap water. Practice filling and dispensing distilled water from the pipet until you feel comfortable with the technique. Ask for help if you need it.

2. Clean and wipe dry a 150 –ml beaker. Weigh the beaker to the nearest milligram(0.001 g) and record.

3. Obtain about 100 ml of tap water and record the temperature.4. Pipet 10 ml of tap water from the flask into the clean beaker you have weighed. Reweigh the

beaker containing the water and record.5. Using the Density of Water table, determine the volume of water transferred by the pipet from the

mass of water transferred.6. Repeat 2 more times.7. Calculate the % error as stated above.

Part C. The Buret

1. Clean the buret with soap and water using a buret brush. Rinse with tap water.2. Fill the clean buret with tap water to a point somewhat below the zero mark. Determine the precise

liquid level to the nearest .02 ml and record.3. With a paper towel, clean and wipe dry a 150-ml beaker. Weigh the beaker to the nearest

milligram.4. Place the weighed beaker beneath the stopcock of the buret. Open the stopcock of the buret and

run water into the beaker until approximately 10 ml of water has been dispensed. Determine the precise liquid level in the buret to the nearest 0.02 ml. Calculate the volume of water that has been dispensed from the buret by subtraction of the two liquid levels.

5. Reweigh the beaker containing the water dispensed from the buret to the nearest milligram and determine the mass of water transferred to the beaker from the buret.

6. Using the Density of Water table, calculate the volume of water transferred from the mass of the water.

7. Repeat 2 more times.8. Calculate the % error as stated previously.

Accuracy and Precision Pre-Lab Questions

Name______________

1. Define accuracy.

2. Define precision.

3. Pipets used for the transfer of samples of aqueous solutions are always rinsed with a small portion of the solution to be used before the sample is taken. Calculate the % error arising in an experiment when 1-ml, 5-ml, and 10-ml pipets are used for transfer and each pipet contains 5 drops of water adhering to the inside of the barrel. A single drop of water has an approximate volume of 0.05 ml.

4. It is important to make certain that there is no air bubble in the tip of the buret below the stopcock before the initial reading of the liquid level in the buret is taken. If a 0.5 ml air bubble is present in the tip of a buret, what % error in 10-ml, 20-ml, and 40-ml samples will result if the air bubble is dislodged during the dispensing of the sample.

5. Why is a rubber safety bulb always used when working with a pipet?

Accuracy and Precision

Results

A. The Graduated Cylinder Trial 1 2 3 AverageMass of empty 10-ml graduated cylinder ______ ______ _____ _______Mass of water in cylinder ______ ______ _____ _______Temperature of water ______ ______ _____ _______Density of water at this temperature ______ ______ _____ _______Calculated volume of water transferred ______ ______ _____ _______

% error (use average) _________________

Mass of empty 50-ml beaker ______ ______ ______ _______Mass of beaker plus water sample ______ ______ ______ _______

Mass of water in beaker ______ ______ ______ _______

Temperature of water ______ ______ ______ _______

Density of water at this temperature ______ ______ ______ _______

Calculated volume of water transferred ______ ______ ______ _______

% error ( use average) _________________

B. The PipetMass of empty 150-ml beaker ______ ______ ______ _______Mass of beaker plus water sample ______ ______ ______ _______Mass of water transferred ______ ______ ______ _______Temperature of water sample ______ ______ ______ _______Density of water at this temperature ______ ______ ______ _______

Calculated volume of water transferred ______ ______ ______ _______

% error (use average) _________________

C. The BuretInitial liquid level in buret ______ ______ ______ _______Final liquid level in buret ______ ______ ______ _______

Volume of liquid transferred ______ ______ ______ _______Mass of empty beaker ______ ______ ______ _______

Mass of beaker plus water ______ ______ ______ _______Mass of water transferred ______ ______ ______ _______

Temperature of water ______ ______ ______ _______Density of water at this temperature ______ ______ ______ _______

Calculated volume of water transferred ______ ______ ______ _______

% error ( use average) _________________

Questions

1. What was your most accurate measurement? Why was it the most accurate?

2. Were your measurements precise? Explain.

3. Can just one measurement be considered accurate? Precise? Explain.

4. Can measurements be precise but not accurate? Explain.