Laboratory
Though the SAT II Chemistry exam cannot test extensively on the laboratory experience, there will be questions that attempt to accomplish this task. A good chemistry course does include some basic laboratory skills, and we will review those skills in this chapter.
Rules for Basic Laboratory Safety
1. Safety goggles must be worn at all times in the laboratory.
2. No eating or drinking in the laboratory.
3. Never taste or touch the laboratory chemicals.
4. Always wash your hands before leaving the laboratory.
5. Wear proper clothing—safety glasses, closed-toed shoes, and an apron; tie long hair back and remove all jewelry.
6. Always follow the written directions, and never perform an unauthorized experiment.
7. Always add acid to water. This prevents the acid from spattering.
8. Point heating test tubes away from others and yourself, and heat them slowly.
9. Never return unused chemicals to their original containers. This prevents contamination.
10. Always use a pipette bulb or a pipetter to transfer when using a pipette. Never use your mouth.
11. Always use a fume hood when working with toxic substances. Never inhale fumes directly.
12. Never use an open flame near flammable liquids.
13. Dispose of chemicals in the designated disposal site—not in the sink or trash can.
Common Laboratory Equipment
Some Common Lab Techniques
Massing solids: When obtaining the mass of solid chemicals, always use some type of weighing paper to protect the pan of the balance. Remember that the mass of the weighing paper must be written down and subtracted from the total weight when you are determining the amount of solid obtained.
Measuring liquids: When measuring out a particular volume of a liquid, you must choose an instrument that will measure as accurately as possible. For small quantities it would be appropriate to use a pipette or burette. For larger quantities a graduated cylinder might be appropriate. Remember that beakers are not accurate measuring instruments! Remember always to take measurements of liquids from the bottom of the meniscus.
Filtering: When filtering a solid from a mixture by gravity filtration, always weigh the filter paper, fold it, place it in the funnel, and wet it down to hold it in place before beginning the filtering process. After filtering, the solid on the filter paper must be dried and weighed. The initial weight of the filter paper is subtracted to find the mass of the solid obtained. The liquid that comes through the filter paper is known as thefiltrate.
Color Review
One way to identify elements is by performing a simple flame test in the laboratory. When the electrons are heated, they get excited and jump away from the nucleus. As they fall back down, they release energy, often in the form of visible light. Some of the most common colors of flames are listed. You may recognize many of these from fireworks displays!
Ion
Flame color
Li+, Sr2+, Ca2+
Red
Na+
Yellow
K+
Purple (pink)
Ba2+
Light green
Cu2+
Blue-green
Fe3+
Gold
Many solutions in chemistry also have color, which is often the result of unpaired electrons. Metal ions often are colored.
Solution color
Blue
Yellow to orange (rusty)
Ni2+
Green
Purple
Orange
Common Experiments
Chromatography
The purpose of chromatography is to separate out parts of a solution—to isolate substances. You might have used paper chromatography in your chemistry lab. In paper chromatography, a small drop of the substance to be separated is placed on one end of the chromatography paper. A pencil is used to mark the spot where the substance was placed, and then the tip of the paper is placed into a container with solvent. As the solvent travels up the paper, the substance separates into its various components. Whatever component is most like the solvent travels the greatest distance. At the end of the experiment, measurements are taken of how far each component traveled. The distance that the solvent traveled and the distance that the solutes (the components) traveled are usually measured in centimeters. A ratio, called the Rf value, is then calculated for each component. This information can be used to identify various parts of the mixture.
The formula for the calculation is
Example
Data:
Distance solvent traveled:
0.0 cm
Distance red dye traveled:
7.0 cm
Distance blue dye traveled:
4.0 cm
Calculate the Rf for the red dye and the blue dye.
Explanation
Just plug your numbers into the equation:
Rf for red dye:
Rf for blue dye:
Density of Liquids and Solids
Density is defined as a pure substance’s mass over its volume. Density is a property of matter that is often used to identify an unknown substance since pure substances have known densities. The units of density are usually grams divided by milliliters or cubic centimeters:
Density of a solid: Typically the solid sample is massed on the balance first. The mass is recorded in grams. If the solid is a regularly shaped object, the length, width, and height may be measured with a metric ruler. These three measurements are then multiplied together to obtain the volume in cubic centimeters (cm3) or some similar unit. If the solid is irregular, the volume can be obtained by water displacement. A known amount of water is recorded, the object is immersed, and the final volume of water is recorded. The difference in volumes will give the volume of the object. Density can then be calculated by dividing the mass by the volume.
Density of a liquid: The density of a liquid is obtained in much the same way as above. To obtain the mass of the liquid, the mass of a container must first be measured, the liquid poured in, and the total mass recorded. The difference in mass is the mass of the liquid. It is often convenient to measure the liquid in a graduated cylinder. Now try a density problem.
Data (for an irregular solid):
Mass of the solid:
5.00 g
Initial volume of water:
30.0 mL
Final volume of water:
32.5 mL
Find the density of the unknown solid.
Volume of solid: (final - initial volumes) = 32.5 - 30.0 = 2.5 mL
Density of solid:
Titration
A titration (also called volumetric analysis) is a laboratory procedure that usually involves either an acid and base neutralization reaction or a redox reaction. In a titration, two reagents are mixed, one with a known concentration and known volume (or a solid with a known mass) and one with an unknown concentration. The purpose of a titration is to find the concentration of the unknown solution. There must be some way to indicate when the two reagents have reacted essentially completely, and at the end of the titration the unknown solution’s concentration can be calculated since the volume of the solution required to complete the reaction has been accurately measured.
The titrant is the solution of known concentration and is usually placed in the burette. The burette must be rinsed with the solution to be placed in it before filling.
The solution from the burette is added to a flask that contains either a measured volume of a solution or a weighed quantity of solid that has been dissolved. An indicator that changes color at or near the equivalence point is usually added to the solution to be analyzed before titration. The solution of known concentration is then added to the flask from the burette until the color changes. The equivalence pointis the point in the reaction where enough titrant has been added to completely neutralize the solution being analyzed. The end point is the point during the titration where the indicator changes color. It is important to choose an indicator that has an end point that is at the same pH as your expected equivalence point. The burette has graduations that are used to read the volume of titrant that’s added to the flask.
The data required for titrations include the mass of the dry substance to be analyzedor an accurately measured volume of the substance to be analyzed, the initial volume and final volume of titrant required to reach the end point, and the molarity of the titrant. At the equivalence point, the moles of the titrant will be equal to the moles of the substance analyzed. To obtain the moles of the unknown substance, multiply the molarity of the titrant by the volume (in liters) of the titrant. Once moles are known, just divide moles by volume and you have the molarity of the unknown substance
If the substance to be analyzed is a solid, you will be trying to calculate the molecular weight of the unknown solid. Remember that molecular weight is grams per mole. The mass in grams will be known from the beginning of the experiment, when the solid sample was massed. You can find the moles of the unknown substance by multiplying the molarity of the titrant by the volume (in liters) of the titrant. Divide grams by moles to get molecular weight.
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