Investigation 1C

Mass in Chemistry

2 Investigation 1C: Mass in Chemistry

• How do we measure quantities of matter in chemistry?

• How do we compare quantities of matter in different forms, such as liquids, solids, powders, solutions, and gasses?

3 Investigation 1C: Mass in Chemistry

• In chemistry we use a balance to measure mass, usually in grams (g).

• Some balances can also measure weight in ounces (oz).

• Weight and mass are not the same thing!

Mass measures the quantity of matter.

Weight measures the force of Earth’s gravity pulling on an object’s mass.

Part 1: Using the balance

4 Investigation 1C: Mass in Chemistry

Part 1: Using the balance

Balance rules

• Use the balance on a flat, level surface.

• Be gentle, place things carefully on the balance, never drop things.

• Always use an appropriate

container. Never put chemicals directly on the balance.

• Spills are to be avoided, but cleaned up immediately if they occur.

5 Investigation 1C: Mass in Chemistry

Part 2: Measuring the mass of a liquid

1. Put a vial on the balance. Place its cap on the balance too.

2. Record the mass in grams of the cap and vial.

3. Press the “Zero” button on the balance. It is sometimes called the “Tare” button.

4. Use a pipette to add and subtract water until you have exactly 10.0 g in the vial.

Objective: Get exactly 10.0 g of water in the vial

“Zero” button

6 Investigation 1C: Mass in Chemistry

a. Can you describe another way to get 10.0 g of water in the vial?

b. What was the purpose of pushing the “Zero” or “Tare” button on the balance?

c. Remove the vial with the water from the balance. Does the balance go back to zero? Can you explain why or why not?

Part 3: Why use this procedure?

7 Investigation 1C: Mass in Chemistry

Is there exactly 10.0 g of water in the vial?

Significant figures

• The vial could contain anywhere from 9.95 to 10.05 g of water and the display would read 10.0 g.

• The number 10.0 has three

significant figures because it is a real measurement that displays three digits.

• The measurement is precise to +/– 0.05 g.

• The measurement 10.00 implies a precision of +/– 0.005 g, or a mass between 9.995 g and 10.005 g.

10.0 g

3 significant figures

9.95 g 10.05 g

+/– 0.05 g

8 Investigation 1C: Mass in Chemistry

1. Write a brief procedure to add an additional 1.0 g of oil to the same vial.

2. Add the oil following your procedure.

Cap the vial and swirl or shake it gently. Do not shake vigorously or you will have to wait a long time for it to settle.

Objective: Add an additional 1.0 g of oil to the same vial.

What happens to the oil and water? Do they mix or separate? Oil and water

mixture

Part 4: Adding mass

9 Investigation 1C: Mass in Chemistry

Part 5: The function of soap

You wash your clothes (and your hands) with soap. Soaps are a useful class of chemicals because of how they affect oil and water mixtures.

1. Add 10 drops of liquid detergent to the vial. Recap the vial.

2. By how much has the mass increased? Record the mass of the liquid detergent.

3. Gently shake the vial.

10 Investigation 1C: Mass in Chemistry

a. Describe the behavior of the oil and water mixture before and after you added the detergent.

b. Propose an explanation for how detergent works.

c. Add 1 drop of red food coloring to the vial and mix gently.

d. Now let’s add 10 drops of bleach to the vial. By how much has the mass increased?

e. Shake the vial and record your observations. What do you think happens?

Part 6: Explaining what you see

11 Investigation 1C: Mass in Chemistry

Part 7: Doing the math

Calculate the percent concentration of each component. The total should add up to 100%.

12 Investigation 1C: Mass in Chemistry

Part 7: Doing the math

a. Is the concentration of detergent enough to disperse the oil in water? Why do you think so?

b. Should the detergent concentration be higher or lower compared to the concentration of oil?

13 Investigation 1C: Mass in Chemistry

• The “zero” or “tare” button resets the zero-point of the balance to whatever mass is on its weighing surface.

• Mass is the only true measure of the quantity of matter, not size or volume.

• Soap functions by helping break up oils and substances that do not dissolve in water, into smaller particles.