5 Steps to a 5 AP Chemistry 2019

288 ❯ STEP 4. Review the Knowledge You Need to Score High

Equipment

Spectrophotometer (commonly SPEC 20)

Cuvettes (sample tubes for the spectrophotometer)

Stock solution (known concentration) of the solute

Sample to be analyzed

Assorted glassware, including volumetric glassware

Measurements

The student will make several dilutions of the stock solution (solution of known con-

centration of the substance being determined) and will calculate the concentration of

each dilution (using M 1 V 1 = M 2 V 2 ). The absorbance of one of the stock solutions is

measured at a number of wavelengths (generally 400–700 nm in 10- to 20-nanometer

increments) using a spectrophotometer. The data of absorbance versus wavelength is

plotted, and the wavelength that gives the maximum absorbance is chosen to be used

for the rest of the experiment. The absorbance of each of the dilutions is measured. A

plot of absorbance versus concentration (Beer’s law plot) is prepared either by hand or

using a spreadsheet. The solid sample is dissolved (if it is copper, this will require the

use of nitric acid) and diluted to a certain volume. The concentration of that solution

is determined using the Beer’s law plot. Using the concentration of the solution and the

solution’s volume, you can calculate the moles and then grams of the substance. Using

the initial mass of the sample, you can finally calculate the mass percentage of the sub-

stance in the sample.

Calculations

You can determine the concentrations of the diluted stock solution by using the dilution

equation (M 1 V 1 = M 2 V 2 ). The mass percentage is calculated by:

(grams substance/grams sample) × 100%

Comments

If you are doing a brass analysis for percentage of copper, you will dissolve the brass in

concentrated nitric acid. Be extremely careful. The nitric acid is corrosive and the NO gas

that is produced is toxic. On the AP exam be sure to stress safety if you are describing this

process.

Experiment 3: Gravimetric Analysis

Synopsis

Specific experiments that are performed in this investigation use determination of the mass

of a specific substance in a sample by precipitation, drying, and weighing. A common

experiment done in this category is determination of the hardness of a water sample. The

hardness of a water sample is related to the amounts of calcium, magnesium, and iron ions

in solution. These ions may be precipitated as the carbonate salts. For simplicity’s sake, hard-

water samples are commonly prepared with only one of these ions, generally calcium. The

carbonate salt is precipitated, separated from the solution by suction filtration, and dried

in a drying oven. The mass of the dry salt is determined, and the water sample hardness is

calculated as mg calcium carbonate per liter of water sample.