exam 3 (2)

7/18/2019 Exam 3 (2)

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QUANTITATIVE ANALYSIS

Chemistry 315, Section 501

Third Homework Exam

March 27, 2014 (Due April 1, 2014) Cell Number _____8____

Please respond to the following questions

1. (25 points)

a.) (6 points) What is wave-particle duality? How is the concept used in spectrophotometry?

Wave particle duality refers to the fact that light has both wave and particle-like properties.

For example, photons undergo diffraction that can interfere with each other as waves, but

they also act as point-like masses. The theory of quantum mechanics is an attempt to explainthese apparently contradictory properties exhibited by matter. In a dual-sit experiment, lightacts like a wave, as it can leave an interference pattern, which is what one would expect from

a wave and not a particle. There are also many situations where light can act as a particle. For

example when a photon’s position is measured, a single point is found. The Photoelectriceffect and the De Broglie hypothesis confirmed both the particle and the wave-like nature of

light. Measurements made in spectrophotometry utilize the inverse relationship between

frequency and wavelength, which describes the wave properties of electromagnetic radiation

As well as the proportional relationship between the energy of electromagnetic radiation andfrequency

This concept allows us to the link between light attenuation and the concentration of particlesit is passing through, which is a foundation of spectrophotometry. Transitions between

energy levels lead to absorption or emission of photons that correspond to the energy leveldifference between the intensity of the emitted light and that of the received light.

Spectrophotometry uses this difference to determine the concentration of chemicals. Theabsorbance, A, of a solution is a measure of how much light is absorbed by the solution. The

particle-like nature of light allows the spectrophotometer to detect the light as photons by a photomultiplier after passing through the analyte.

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b.) (6 points) What is molar absorptivity? How is the concept used in spectrophotometry?

The molar absorptivity is a property of a substance, and is related to the analyte/photon

interaction. It is used in spectrophotometry to quantify the amount of light a chemical speciesabsorbs at a certain wavelength; this quantity can be utilized in calculating the concentration

of a species using Beer’s Law:

Where A is absorbance, is molar absorptivity, b is the length of the cuvette, and c isconcentration.

In spectrophotometry, a spectrophotometer can be used to measure absorbency, which is

used in conjecture with the Beer-Lambert law to determine the concentration of an analyte.

The solutions of known concentration are used to solve for the molar absorptivity of asubstance. Then, knowing this value, you can use the molar absorptivity to find the

concentration of an analyte of unknown concentration by solving for it in the Beer-Lambertequation, as the spectrophotometer will give you the absorbance, and the length of the

cuvette does not change.

c.) (6 points) What does quantization of excitation energy mean? What role does theconcept play in spectrophotometry? How does the concept of selection rules and quantum

numbers play a role in spectrophotometry?

When an electron moves to a higher energy level, it absorbs energy. When an electron movesto a lower energy level, it emits energy. These amounts of energy are specific to each

substance, and this quantization of excitation energy is the notion that energy can only be ofspecific values, or quantities to be absorbed or emitted by the electron, so electrons in orbit

remain in a specific orbit unless provided with the amount of excitation energy needed tomove them to a higher orbital, or releasing a specific amount of energy to move down

level(s) of energy. This concept is applicable to spectrophotometry because it links energyand wavelength of light, as these specific amounts of energy correlate to specific

wavelengths of light.

The energy states that an electron occupies are quantized. That is to say they exist in discreteunits of energy. Their excitation energies give molecules their own “molecular fingerprints,”

since each particle will have a unique set of energy levels associated with its electrons. Inspectrophotometry, this means that different molecules will only absorb certain energy levels

of light, which is useful for identification of the substance itself.

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In spectrophotometry, quantum numbers describe specifically the energies of electrons in

atoms, but other possibilities include angular momentum and spin. Selection rules are useddescribe what wavelengths certain molecules will absorb. By knowing what wavelength the

light source is, our method displays specificity, as we know what molecules react to photonsof certain wavelengths. Specificity is also displayed on the backside of the interaction, as

certain molecules emit light of certain wavelengths after they interact with photons.

Combining these principles and knowing the wavelengths absorbed and emitted allows us toeither determine a chemical species or find its concentration.

d.) (6 points) What is the Beer-Lambert Law? Define all symbols used. What is its

relevance to spectrophotometry?

The Beer-Lambert law, Is relevant to spectrophotometry because it shows a linear relationship between light path

length (b), analyte concentration (c), and molar absorptivity (. We can use this principle to

find the unknown concentration of solution by knowing how much light is absorbed in asample of unknown concentration and the molar absorptivity of the substance, as can be

determined by measuring the absorbance of a blank solution and one of known concentration

and solving for .

2. 25 points)

a) (13 points) Describe the layout of a spectrophotometric system. What does each

component do and give a conceptual account as to how it is used in chemical analysis?

In chemical analysis, spectrophotometry measures the reflection and transmission propertiesof a material as a function of wavelength. The information obtained can be used to determine

the concentration of unknown solutions. In a spectrophotometric system, the light source is

usually white light so any portion of the visible spectrum can be used in the remaining steps.

The collimator (lens) focuses the light inward towards the monochromator. Themonochromator uses refraction or reflection grating to separate different wavelengths into a

dispersed array of light. A small slit in the wavelength selector will let only one wavelengthof light through, allowing for increased specificity. The light then goes through the sample

solution where some of it is absorbed by the sample. The light detector then captures the

remaining light to measure intensity, or irradiance. The initial irradiance () should be

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measured using a blank solution before a sample is used. Then, the ratio of change in

irradiance (transmittance, T) can be calculated (). P is less than because some photons

will interact with the analyte molecules and be absorbed, so T will be less than 1. The Beer-Lambert Law (A = b*ɛ*c) is then used to calculate the concentration of the solution. To find

molar absorptivity, ε, solutions of known concentration are used to create a calibration curveof Absorbance vs Concentration. A line of best fit is then put through the data. The line

should be linear and the slope of this line is ε. B is the length of cuvette or the distance thatthe light travels through the solution. With all of this information C, the concentration can be

solved for when given the absorbance of an analyte of unknown concentration.

b.) (12 points). Give a conceptual description of the strategy that is used to employspectrophotometry for chemical analysis.

There are four steps to this strategy. First, the sample must be dissolved in a solvent. This

allows the light to pass through the sample while still interacting with it. The sample has to be dilute in order to obtain accurate results. Second, the spectrophotometric system must

record the initial irradiance of the light going through a blank (. This will be used later on

in finding the absorbance of the solution with known concentration. Third, a group ofstandards of known concentration must be used to find the molar absorptivity of the solute of

which you want to measure the unknown concentration. A standard curve is generated

plotting these standards’ absorbance versus their respective concentrations. The best-fit lineis used to calculate the slope of the equation (Beer-Lambert law), which is the mentioned

molar absorptivity of the substance. Finally, the absorbance of the unknown is then measured

and plugged into the equation to calculate the concentration of the sample using A=ε*b*c.

exam 3 (2)

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