Measurements, Volume, and Mass in Chemistry

Chemistry is about learning what makes up the material world. We use the word mass to talk about how much matter there is in something and volume to talk about how much space matter takes up. (Matter is anything you can touch physically.)

Chemistry is also about how materials react when they are mixed, heated or subjected to other conditions. Learning chemistry means learning to work with different substances, some dangerous, in safe ways and in precise quantities. Much of what we know about the physical world has been obtained from measurements made in the laboratory. Skill is required to design experiments so that careful measurements can be made. Skill is also needed to use lab equipment correctly so that errors can be minimized. At the same time, it is important to understand the limitations of scientific measurements.

The numerical value of a laboratory measurement should always be recorded with the proper number of significant figures. The number of significant figures depends on the instrument or measuring device used and is equal to the digits definitely known from the scale divisions marked on the instrument plus one estimated or "doubtful" digit. The last, estimated, digit represents the uncertainty in the measurement and indicates the precision of the instrument.

Measurements made with rulers and graduated cylinders should always be estimated to one place beyond the smallest scale division that is marked. If the smallest scale division on a ruler is centimeters, measurements of length should be estimated to the nearest 0.1 cm. If a ruler is marked in millimeters, readings are usually estimated to the nearest 0.2 or 0.5 mm, depending on the observer. The same reasoning applies to volume measurements made using a graduated cylinder. A 10-mL graduated cylinder has major scale divisions every 1 mL and minor scale divisions every 0.1 mL. It is therefore possible to "read" the volume of a liquid in a 10-mL graduated cylinder to the nearest 0.02 or 0.05 mL. Three observers might estimate the volume of liquid in the 10-mL graduated cylinder shown at the right as 8.32, 8.30, or 8.33 mL. These are all valid readings. It would NOT be correct to record this volume of liquid as simply 8.3 mL. Likewise, a reading of 8.325 mL would be too precise.

Pre-Lab Questions (Answer in complete sentences)

  1. Why do you think it is important to be precise when measuring quantities in chemistry?
  1. What is the difference between accuracy and precision?
  1. How do you think quantities of liquids are used and measured in chemistry?
  1. What are the unit(s) for measurement in determining volume?
  1. A student performed an experiment and got the following results: 60.90mL, 61.00mL,60.85mL and 61.22mL. Find the average of this student’s data (be sure to show work and correct units)
  1. What unit is used to determine mass in the science classroom?

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Part 1: Volume-Reading from the meniscus

There are three graduated cylinders, each labeled and each containing a specific quantity of liquid to which some food coloring has been added to make the volume easier to read.

  1. Have each lab partner in your group measure the volume of liquid in each cylinder separately. Record the results in the Data Table. Make sure each lab partner gets their own separate reading. Remember to include the units and the correct number of decimals.

Data Table A: Volume Measurements
Graduated Cylinder / Volume Reading Lab Partner 1 / Volume Reading Lab Partner 2 / Volume Reading Lab Partner 3 / Volume Reading Lab Partner 4
A
B
C

Part 1: Questions

  1. Were there differences in you and your lab partner’s volume readings? Why do you think this is?
  1. Do you think there is “uncertainty” in measuring substances in chemistry? Explain how to make your measurements more accurate.

Part 2: Comparing Volume Equipment

There are differences in the types of equipment in chemistry. Before we begin labs in class, we must conduct an accuracy test.

1.Use tap water to fill a 50-mL beaker to the 10-mL mark. Use a pipet to adjust the water level until the bottom of the meniscus is lined up as precisely as possible with the 10-mL line.

2.Pour the water from the beaker into a clean, 50-mL graduated cylinder. Measure the volume of liquid in the graduated cylinder and record the result in Data Table B. Remember to include the units and the correct number of significant figures.

3.Transfer the liquid from the 50-mL graduated cylinder to a clean, 10-mL graduated cylinder and again measure its volume. Record the result in Data Table B. Discard the water into the sink.

4.Repeat steps 1-3 two more times for a total of three independent sets of volume measurements. Dry the beaker and graduated cylinders between trials. Record all results in the Data Table.

5.Calculate the average (mean) volume of water in both the 50-and 10-mL graduated cylinders.

Data Table B: Comparing Volume Measurements
Measure Volumes of “10mL” of Water
Trial / 50mL Graduated Cylinder / 10mL Graduated Cylinder
1
2
3
Average

Part 2: Questions

  1. How do the volumes compare between the different types of volume measuring equipment?
  1. Which piece of equipment -- beaker, 10mL graduated cylinder, 50mL graduated cylinder-- do you feel is more accurate and why?
  1. It is common to get different volume readings for each container? What explanation can you offer for an apparent decrease or increase in volume?
  1. Is there any harm in measuring with the wrong glassware? Why do you say this?

Part 3: How to measure Mass

Scale Rules:

  • Use the scale on a flat, level surface.
  • Be gentle, place things carefully on the scale, never drop things.
  • Always use an appropriate container, never put chemicals directly on the scale.
  • Spills are to be avoided, but cleaned up immediately if they occur.

In some chemistry experiments we want to know what substances are present and precisely how much of each substance there is. In other experiments we mix the right amounts of several substances to get something new. In both cases we need a way to measure things that is accurate and easy to communicate. Chemists use mass to measure and communicate quantities of matter. A scale lets us measure accurate amounts of mass.

Part 3: Questions

  1. In what ways are measuring masses useful in chemistry?

Part 4: How to use a Scale with Questions

  1. Put an empty weigh boat on the scale.
  2. What is the mass of the weigh boat?
  1. Press the zero button on the scale.
  2. What happens to the mass when you press this button?
  1. Remove the weigh boat from the scale.
  2. What happens to the mass?
  3. Explain what the Zero button does for you on the scale.

Objective: Get exactly 10.0grams of water in a beaker.

  1. Put an empty 100mL beaker on the scale
  2. Hit the Zero (0) button on the scale.
  3. Use a pipette to add and subtract water until you have exactly 10.0 grams in the beaker.
  4. Record the exact mass of water.

Part 4: Questions

  1. What is another way to get 10.0 grams of water in the vial? (Hint: 1g water = 1mL water)
  1. What was the purpose of pushing Zero (0) button on the scale?
  1. Remove the beaker with water from the scale. Does the scale go back to zero? Explain why or why not.