SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 15

The Laboratory

SOME BASIC SETUPS

Throughout this book, drawings of laboratory setups that serve specific needs have been presented. You should be familiar with the assembly and use of each of these setups.

• Preparation of a gaseous product, nonsoluble in water, by water displacement from solid reactants

• Preparation of a gaseous product, nonsoluble in water, by water displacement from at least one reactant in solution

• Distillation of a liquid

• Titration

The following are additional laboratory setups with which you should be familiar:

1. PREPARATION OF A GASEOUS PRODUCT, SOLUBLE IN WATER AND LIGHTER THAN AIR, BY THE DOWNWARD DISPLACEMENT OF AIR. SEE FIGURE 42.

EXAMPLE:   Preparation of ammonia (NH3).

2NH4Cl(s) + Ca(OH)2(s) → CaCl2(s) + 2H2O(g) 2NH3(g)

Figure 42. Preparation of Ammonia

2. SEPARATION OF A MIXTURE BY CHROMATOGRAPHY. SEE FIGURE 43.

Figure 43. Chromatography Setup

EXAMPLE:   Chromatography is a process used to separate parts of a mixture. The component parts separate as the solvent carrier moves past the spot of material to be separated by capillary action. Because of variations in solubility, attraction to the filter paper, and density, each fraction moves at a different rate. Once separation occurs, the fractions are either identified by color or removed for other tests. A usual example is the use of Shaeffer Skrip Ink No. 32, which separates into yellow, red, and blue streaks of dyes.

3. MEASURING POTENTIALS IN ELECTROCHEMICAL CELLS. SEE FIGURE 44.

Figure 44. Potentiometer Setup for Measuring Potential

Figure 44. Potentiometer Setup for Measuring Potential

EXAMPLE:   The voltmeter in this zinc-silver electrochemical cell would read approximately 1.56 V. This means that the Ag to Ag+ half-cell has 1.56 V more electron-attracting ability than the Zn to Zn2+ half-cell. If the potential of the zinc half-cell were known, the potential of the silver half-cell could be determined by adding 1.56 V to the potential of the zinc half-cell. In a setup like this, only the difference in potential between two half-cells can be measured. Notice the use of the salt bridge instead of a porous barrier.

4. REPLACEMENT OF HYDROGEN BY A METAL. SEE FIGURE 45.

Figure 45. Eudiometer Apparatus

Figure 45. Eudiometer Apparatus

EXAMPLE:   Measure the mass of a strip of magnesium with an analytical balance to the nearest 0.001 g. Using a coiled strip with a mass of about 0.040 g produces about 40 mL of H2. Pour 5 mL of concentrated HCl into a eudiometer, and slowly fill the remainder with water. Try to minimize mixing. Lower the coil of Mg strip into the tube, invert it, and lower it to the bottom of the beaker. After the reaction is complete, you can measure the volume of the gas released and calculate the mass of hydrogen replaced by the magnesium. (Refer to Chapter 5 for a discussion of gas laws.)