INVESTIGATING VOLUME

Purpose

To practice using different methods for measuring volume and to investigate the concept of uncertainty in measurement.

Questions

1) Volume is one of two fundamental properties of matter. How would you define volume?

2) How would you define accuracy? Why is accuracy important in science? Is any measurement perfectly accurate?

Materials

Beakers, cylinders, pipets, flasks, burets, water.

Introduction

There is a wide variety of chemistry glassware that is available for holding and measuring volume. In this investigation, you will practice measuring and pouring liquids using many of the glassware available in the laboratory. You will also practice reading volumetric glassware accurately, and compare the accuracy of different types of glassware.

How to read volume

1. Read a volume of liquid at eye level.

2. If necessary, place a white card behind the glassware to see more clearly.

3. The cup-shaped surface is call the meniscus. Read the lowest point of the meniscus.

4. The meniscus will probably not fall exactly on a line. Estimate the volume by reading ‘between the lines’ choose an appropriate sub-division based on what you see (halves, fourths, fifths, tenths) and estimate the volume to that imaginary division.

When given the opportunity to determine an initial volume, it is always a bad idea to try to ‘hit the mark’ because a line has a thickness that cannot be estimated accurately.

How to fill and drain volumetric glassware

5. Volumetric glassware (volumetric flasks or pipets) will always have a fill line. The bottom of the meniscus should fall on the fill in.

6. Look at the fill line at eye level

7. Use a dropper bottle to add the final volume. If the liquid goes past the fill line, then the volume is no longer accurate!

8. Graduated pipets are either ‘to contain’ or ‘to deliver’. Some (but not all) can be drained completely; others must be drained between two marks. If a ‘to deliver’ pipet is meant to be drained, the let gravity drain the pipet, and touch the tip to the side of the glassware to deliver the last drop. These pipets are supposed to have some residual liquid in them after delivery.

9. It is inaccurate to ‘re-fill’ volumetric glassware. Even with volumetric cylinders, the practice should be avoided. Choose a size of glassware that is possible to hold the full volume required (if possible).

Instructions

1. Do your best to measure exactly 50 mL of water in a 100 mL beaker. Pour the water into a 50 mL volumetric flask. Observe and describe the water level in the volumetric flask and discuss the relative accuracy of the volumetric flask and the beaker.

2. Do your best to measure exactly 50 mL of water in a 50 mL graduated cylinder. Pour the water into a 50 mL volumetric flask. Observe and describe the water level in the volumetric flask and discuss the relative accuracy of the volumetric flask and the graduated cylinder.

3. Measure 100 mL of water in a 250 mL beaker and pour it into a 150 mL beaker. Observe and describe the water level in the beaker and discuss the relative accuracy of the two beakers.

4. Measure 10 mL of water in a 50 mL graduated cylinder and pour it into a 10 mL graduated cylinder. Observe and describe the water level in the graduated cylinder and discuss the relative accuracy of the two cylinders.

5. Predict what will happen if you fill a 50 mL graduated cylinder with a 50 mL volumetric pipet. Record your prediction, then test it.

Predict what will happen if you pour the contents of a 50 mL volumetric flask filled to the mark into a 50 mL graduated cylinder. Record your prediction, then test it.

Explain any differences between your two predictions and the actual results.

6. Use a 5 mL pipet to add 5 mL of water to a 10 mL graduated cylinder. Repeat this procedure using a 10 mL graduated pipet to add 5 mL of water to a 10 mL graduated cylinder. Describe the relative accuracy of the two pipets based on your observations.

7. Set up a buret on a buret stand and fill it to the 0.00 mark. Use the buret to add exactly 50 mL of water to a 100 beaker; a 50 mL graduated cylinder; and to a 50 mL volumetric flask. Discuss the accuracy of the buret with respect to these three other kinds of glassware.