CHEMISTRY 122: MEASUREMENTS

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 which 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. Keep the balance clean. 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.
Procedure 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. Some glass graduated cylinders have a plastic safety ring, which helps to keep the graduated cylinder from breaking if it is tipped over. Examine the graduated cylinders in your lab drawer, and notice the smallest graduation of volume that can be determined with each cylinder.

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.).

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.

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.

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

1 L = 1 dm3

1 mL = 1 cm3

1 fl oz = 29.57 mL

1 qt = 0.9464 L

4 qt = 1 gal

1 gal = 3.7856 L

1 lb = 0.4536 kg

1 lb = 16 oz

1 oz = 28.35 g

1 ton = 2000 lb

CHEMISTRY 122: DATA SHEET FOR MEASUREMENTS

Name __________________________________________ Date _________________

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Part 1.   Measuring Mass with a Top-loading Balance

Following the procedure for using a top-loading balance described above, measure the mass of a cylinder.  (Cylinder should be dry.)  Record the Record the mass to two decimal places (+0.01 g).  

 

Material of the cylinder ____________________          Mass of cylinder     ____________g

Part 2. Determining Volume of Solid Objects from Linear Measurements

Use a ruler to measure the dimensions necessary to calculate the volume of the cylinder. Record the measurements to two decimal places (+0.01 cm).   Volume of a cylinder = p r2 h , in which r is the radius and h is height.  (Remember to show your calculations!)

height (h) ___________cm         radius (r) ___________cm          Volume of cylinder = ___________cm3

 

 

 

Part 3.  Determining Volume of Solid Objects by Displacement of a Liquid

Measure the volume of your cylinder by placing enough water in a 100 mL graduated cylinder so that when the sample is placed in the graduated cylinder, it will be completely submerged under the water.

 

Read and record the volume of the water to ±0.5 mL as the initial volume.  

 

Next, CAREFULLY immerse the sample into the graduated cylinder by inclining the graduated cylinder and allowing the object to slowly slide down the side of the graduated cylinder.

 

Remove any air bubbles clinging to the sample by gently shaking the graduated cylinder. 

 

Read the volume of water after the sample has been placed into the graduated cylinder and record this volume to ±0.5 mL as the final volume.  

 

The volume of the sample is equal to the volume of water displaced.  (Recall that 1 mL = 1cm3.)  

 

Initial volume of water in graduated cylinder ____________mL

 

Final volume of water in graduated cylinder  ____________mL

Volume of cylinder  = ___________cm3

 

Part 4.  Calculating Density  

4A. Record the material identity and accepted density value of your particular cylinder. 

Cylinder material ____________________  Accepted density value ____________g/cm3

4B. Use the measurements made above to calculate the density of your cylinder.

 

Density of Cylinder from Data in Part 1 and Part 2

 

 

 

 

density =______________g/cm3

Density of Cylinder from Data in Part 1 and Part 3

 

 

 

 

density =______________g/cm3

 

4C.  Use the formula for percent error shown below to calculate the percent error of the two densities from 4B.

 

Percent Error =   measured value - accepted value     x 100

                                           accepted value

Percent error of Density of Cylinder from Data in Part 1 and Part 2

 

 

 

 

density =______________g/cm3

Percent error of Density of Cylinder from Data in Part 1 and Part 3

 

 

 

 

density =______________g/cm3

 

Part 5.  Determining the Density of Water

5A.  Measure the volume and mass of a sample of water.  Use a buret to measure a 10 to 15 mL sample of water.  Record the initial and final buret volumes to +0.01 mL.

 

Initial Buret Reading     __________________mL

 

Final Buret Reading     __________________mL

 

Volume of water in sample     __________________mL

 

Mass of water in sample     __________________g

 

Density of water     __________________g/mL