MEASURING MASS
One of the most common and important operations in a laboratory is weighing objects.  Chemistry, being one of the exact sciences, calls for the careful determination of the mass (weight) of substances that enter into and result from chemical change.  Recall that the SI unit for mass is the kilogram (kg).  However, most laboratory balances will display the mass in grams (g).  (See Table 1.)

General Rules For Using a Top-loading Balance
1.  Never place any chemical directly on the balance pan. Always use a piece of paper, a weighing dish, or a beaker.
2.  Balance adjustments should be made only by the laboratory instructor.
3.  Immediately clean up any spill on or near the balance, following the directions of you laboratory instructor.
4.  Never place hot objects on a balance pan.

Procedures For Using a Top-loading Balance
1.  Turn the balance on and allow the balance to zero itself.
2.  Place a plastic weigh boat or container on the center of weighing pan.
3.  Record the mass of the container or press the Tare button.  By taring the balance, the mass of the container is retained in the memory of the balance. This mass will be subtracted when the container and contents are weighed, to display only the mass of the contents.
4.  Remove the container from the balance and place the material to be weighed in the container.
5.  Replace the container and contents on the center of the balance pan. If the balance has been tared, the mass of the contents will appear in the display.
6.  Remove the container and its contents from the balance and turn off the balance.

MEASURING VOLUME
Volume is defined as length cubed and the SI unit is the cubic meter (m³). The common unit of volume is the liter (L). A liter is a unit of volume equal to a cubic decimeter (dm³). (See Table 1.)

The most common apparatus for routine determination of liquid volumes is the graduated cylinder. Although a graduated cylinder does not permit as precise a determination of volume as do other volumetric devices, for many applications the precision of the graduated cylinder is sufficient. Figure 1 shows a typical graduated cylinder.

The volume subdivisions on most graduated cylinders is marked in units about 1% of the cylinder’s total volume. A 100 mL graduated cylinder will by subdivided in 1 mL markings. It can be read reproducibly to the nearest 0.2 mL.

When water (or an aqueous solution) is contained in a narrow glass container such as a graduated cylinder, the liquid surface is curved downward (See Figure 2.). This curved surface is call a meniscus. When reading the volume of a liquid that makes a meniscus, hold the graduated cylinder so that the meniscus is at eye level, and read the liquid level at the bottom of the curved surface (See Figure 2.).

A buret is a device used to deliver controlled more precise amounts of a liquid than a graduated cylinder. Burets are commonly constructed of a long tube of glass calibrated and marked in units of mL. A stopcock (valve) is used to control the flow of the liquid from the stopcock. A 50 mL buret normally will have calibrated volume markings starting with the zero mark at the top of the buret and the 50 mL mark at the bottom.

When the stopcock is opened, the liquid flows out of a glass tip below the buret stopcock and into the receiving vessel. The volume of liquid delivered is measured by subtracting the initial volume reading on the barrel from the final volume reading. A 50 mL buret is normally calibrated and marked at every 0.1 mL. The volume can be read reproducibly to the nearest 0.02 mL. As with the graduated cylinder, the buret volumes are read at the bottom of the meniscus (See Figure 2.).

Before a buret is ready to use it must be cleaned. Hot water and soap or detergent is generally sufficient to clean a buret or any other glassware lab item. Rinse the buret with water to remove the cleaning solution. A buret is clean when no water droplets cling to the washed part on the inside of the barrel of the buret.

The buret should be rinsed two or three times with the solution to be delivered from the buret. Clamp the buret vertically to a ring stand and close the stopcock. Fill the buret to above the zero calibration mark at the top of the buret. Drain enough solution from the buret to remove any air bubles from the tip. The beginning volume should be below the zero calibration mark.

MEASURING DENSITY
Density is defined as mass per unit volume (mass/volume), a density determination involves weighing a measured volume of a substance. Because only two measurements are required, the volume of a sample and its weight, this experiment provides an excellent opportunity to learn to perform these operations correctly. If an error is made, the determination can be repeated quickly. These techniques will be used many times in this and other courses in the physical, biological, and applied sciences.

An object’s mass can be measure on a balance. The volume of a solid can be determined directly if the solid has a regular geometry (e.g. cube or cylinder). If the solid is irregular in shape, a convenient method for volume measurement is volume displacement. A solid, when submerged in a liquid, will displace an amount of the liquid equal to the volume of the solid. The liquid used for the determination should not react with or dissolve the solid and it should have a lower density than the solid.
 
 

Table 1 The Seven Basic SI Units							Table 2 Some Common SI Prefixes
Physical Quantity		Name of Unit		Symbol			Multiple	Prefix	Symbol
Length		Meter		m			109	giga	G
Mass		Kilogram		kg			106	mega	M
Time		Second		s			103	kilo	k
Temperature		Kelvin		K			10-1	deci	d
Amount of Substance		Mole		mol			10-2	centi	c
Electric current		Ampere		A			10-3	milli	m
Luminous intensity		Candela		cd			10-6	micro	?
							10-9	nano	n
							10-12	pico	p

Table 3 Common Conversion Units
Length	Volume	Mass
1 in = 2.54 cm 1 yd = 0.9144 m 1 mi = 1.609 km 1 mi = 5280 ft	1 L = 1 dm3 or 1 mL = 1 cm3 1 gal = 3.785 L 1 fl oz = 29.57 mL 4 qt = 1 gal	1 lb = 0.4536 kg 1 lb = 16 oz 1 oz = 28.35 g 1 ton = 2000 lb

Name                                                                                          Hood No.                  Date_____

Part 1.     Measuring Density of Solid Objects

A.  Following the procedures for use of a top-loading balance, weigh each of the objects.
       Please note that the objects should be dry.

Mass of metal cylinder   ___________g

Mass of glass marble   ___________g

Mass of rubber stopper   ___________g

B.  Using a ruler, measure the necessary dimensions and then calculate the volume of each of the objects by the
        appropriate formula.

Volume of metal cylinder radius __________cm height __________cm   volume = __________cm3

Calculations: V = pr2h (p = 3.1416, r = base radius, h = height)

Volume of glass marble radius __________cm   volume = __________cm3

Calculations: V = 4/3pr3 (r = radius)

Volume of rubber stopper height __________cm top radius "a" __________cm bottom radius "b" __________cm   volume = __________cm3

Calculations: V = 1/3 x p x h x ( a2 + b2 + ab)  (h = height, a = top radius, b = bottom radius)

C.  Determine the volume of the objects by displacement of water using the following steps.

        1.  Place enough water in a 100 mL graduated cylinder so that when the object is placed in the graduated cylinder, it
             will be completely covered by the water. Read and record this volume to +0.2 mL as the initial volume.

        2.  CAREFULLY immerse the object into the graduated cylinder by inclining the cylinder and allowing the object to
            slowly slide down the side of the cylinder.  Do not drop the object into the graduated cylinder, since this may
            break the cylinder. To remove any air bubbles clinging to the object, gently shake the graduated cylinder.   Read
            the volume of water after adding the sample to +0.2 mL and record as the final volume.
 

Volume of metal cylinder initial volume ________mL final volume ________mL     volume displaced = ________mL

Volume of glass marble initial volume ________mL final volume ________mL     volume displaced = ________mL

Volume of rubber stopper initial volume ________mL final volume ________mL     volume displaced = ________mL

D.  Calculate the density of each of the objects using the measurements made from the earlier steps.
 

Density of metal cylinder from 1A. & 1B.         density = ________g/cm3

Density of metal cylinder from 1A. & 1C.         density = ________g/cm3

Density of glass marble from 1A. & 1B.         density = ________g/cm3

Density of glass marble from 1A. & 1C.         density = ________g/cm3

Density of rubber stopper from 1A. & 1B.         density = ________g/cm3

Density of rubber stopper from 1A. & 1C.         density = ________g/cm3

Part 2.     Measuring Density of Liquids

   A.  Use the following procedures to calculate the densities of several different aqueous NaCl solutions.

         1.    Weigh an empty, DRY beaker (250 mL or less). Record the mass of the beaker to +0.01 g.
         2.    Read the initial volume of the buret containing the NaCl solution. Record the volume as initial
                volume to +0.02 mL.
         3.    Add approximately 10.0 mL of the NaCl solution to the beaker from the buret. Read the final volume
                on the buret and record this volume to +0.02 mL.
         4.    Weigh the beaker and contents. Record the mass of the beaker and contents to +0.01 g.
         5.    Dispose of the salt solution by pouring it into the waste bottle located under the hood.
         6.    Rinse the graduated cylinder with water.
         7.    Use the data collected to calculate the density of the NaCl solution.
 

% NaCl	Beaker mass	Beaker mass + solution	Mass of solution	Initial buret reading	Final buret reading	mL of NaCl solution	Density of solution
______%	______g	______g	______g	______mL	______mL	______mL	______g/mL
______%	______g	______g	______g	______mL	______mL	______mL	______g/mL
______%	______g	______g	______g	______mL	______mL	______mL	______g/mL
______%	______g	______g	______g	______mL	______mL	______mL	______g/mL
Saturated	______g	______g	______g	______mL	______mL	______mL	______g/mL

B.    Plot a graph of Density vs. %NaCl and use the graph to determine the % concentration of the saturated NaCl
        solution.

        ______% from plot

Part 3.     Density of Tygon Brand Tubing

A.    Using the skills and procedures learned from Part 1, measure the density of Tygon brand tubing.

B.    A solid object will float in a liquid if the density of the solid is less than the density of the liquid.   Based upon your
        observations, what %NaCl solution is required to float a sample of Tygon brand tubing?