EXPERIMENT 2

[density.lab]

OBJECTIVES:

1. To learn how to measure the density of liquids and solids.

2. To learn to operate a top-loading balance

DISCUSSION:

Density is one of the most useful properties of a substance. It is defined as the mass in a given unit of volume. Mass and volume can be expressed in any suitable units (pounds, tons, grams; quarts, liters, barrels, etc.) but for scientific work, grams and milliliters (or liters) are usually used. That is what we will use today.

DENSITY = MASS

VOLUME

You will calculate the density of two substances, one a liquid and one a solid. The top-loading balance will be used to determine mass. Record mass to THREE DECIMAL PLACES. If the triple beam balance is used, the mass number would have two decimal places. The graduated cylinder will be used to measure volume. Record volume to ONE DECIMAL PLACE.

EXPERIMENTAL PROCEDURE

A. LIQUID

1. Pour 50-75 mL of the UNKNOWN LIQUID into your graduated cylinder. Record the volume as accurately as possible (V). Check with the instructor to make sure you are correct.

2. Weigh an empty 100 or 150 mL beaker using a top loading balance. First set the balance to ZERO by pressing the TARE button. Place the beaker on the pan and wait until the "U" disappears. Check with the instructor. Record the mass (M1).

3. Pour ALL of the liquid into the beaker. Weigh the beaker with the liquid. Record as M2.

[Note: Use the SAME balance. Balances are not all exactly the same.]

4. Subtract M1 from M2 to find the mass (M) of the liquid.

5. Pour the liquid into the waste bottle designated by the Instructor.

6. Calculate the density of the liquid. In your answer, use the maximum number of significant figures allowed.

SOLIDS The volume of a solid may be found in several ways. The simplest is to measure some water, drop in the solid and measure the water level again. (Method 1) The difference is the volume of the solid. This requires that you be very careful in measuring the water. Another method is to compare the mass in air to that in water. The sample displaces water and this leads to a mass loss. (Method 2)

B. SOLID (Method 1)

1. Obtain a metal sample from the Instructor. Record its number.

2. Place it on the balance pan and find its mass. Record this mass as M.

3. Add some water to your large graduated cylinder. Read the BOTTOM of the meniscus. Record the water level as V1.

4. CAREFULLY slide the metal into the water. Try not to splash any water... and don't break the glass. Record the new water level as V2. Subtract V1 from V2. This difference is the volume (V) of the metal.

5. Divide M by V to obtain the density of the metal. Use the maximum number of significant figures.

C. SOLID (Method 2)

1. Use the same solid sample as in Part B. Tie a 20 cm length of very thin wire around it, making a loop at one end.

2. Hang the sample above the pan of a triple beam balance. It should be about 2 - 3 cm above the pan. Record the mass as M.

3. Fill a small beaker with water and put it on the shelf above the balance pan. Hang the sample so that it is suspended in the water and is competely submerged. It must not touch the beaker itself. Record the mass as M2.

4. Things feel lighter under water than they really are. The loss in mass is equal to the volume of the sample. Subtract mass-in-water from mass-in-air to get volume.

M - M2 = V

5. Calculate density as before. D = M / V

6. Which density value do you think is more reliable, the one from Part B or from Part C ? WHY?

Exp #2 DATA RECORD Name ___________________ Sec ___

A. DENSITY OF A LIQUID SAMPLE NUMBER : ____

VOLUME of LIQUID V = mL

MASS of BEAKER + LIQUID M2 = grams

- MASS of BEAKER M1 = - grams

MASS of LIQUID M = grams

D = M = = ______ ______

V number unit

B. DENSITY OF A SOLID SAMPLE NUMBER : _____

MASS of METAL M = grams

WATER LEVEL WITH METAL V2 = mL

- INITIAL WATER LEVEL V1 = - mL

VOLUME OF METAL V = mL

D = M = = ______ ______

V number unit

C. DENSITY OF A SOLID SAMPLE NUMBER : _____

MASS of METAL (in air) M = grams

MASS of METAL (in water) M2 = - mL

VOLUME OF METAL M - M2 = V = mL

D = M = = ______ ______

V number unit