-Raoult’s Law (volatile)-Henry’s Law (non-volatile)

- Beer-Lambert’s Law (absorbance)

More Solutions Stuff!

Raoult’s Law Raoult's law states that the vapor pressure of a

solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present:

Psolution=Xsolvent∙Psolvent

1880s – when a substance is dissolved in a solution, the vapor pressure of the solution generally decreases. Why?

Why?The observation depends on two things:

1. the mole fraction of the amount of dissolved solute present and

2. the original vapor pressure (pure solvent)

• Woah – what is mole fraction? (see board)• Woah x 2 – what is vapor pressure?

What is Vapor Pressure?It is the pressure of a vapor in contact with its

liquid or solid form.

Equilibrium (in context)When a liquid is in a confined,

closed, container, an equilibrium exists between the liquid and its gaseous phase. This equilibrium exists regardless of the temperature inside the container and the temperature of the liquid. The equilibrium exists due to the fact that some of the particles in the liquid, essentially at any temperature, will always have enough energy to escape the intrinsic cohesive forces and enter the gaseous phase

Characteristics of Vapor Pressure

Vapor Pressures are dependent only on temperature. The vapor pressure of a liquid does not depend on the amount on the liquid in the container, be it one liter or thirty liters; at the same temperature, both samples will have the same vapor pressure. Vapor pressures have an exponential relationship with temperature and always increase as temperature increases.

Raoult's Law states is that the vapor pressure of a solution with two or more components is directly proportional to the vapor pressures of each component and their respective amounts in the solution. Used to describe IDEAL solutions. (There are deviations)

Example of Raoult’s Law:What is the vapor pressure of a solution at

25oCcontaining 78.0 grams of glucose (MM =

180.16g/mol)in 500 grams of water? The vapor

pressure of purewater at this temperature is 23.8 mm Hg.

Henry’s Law Non-volatile: a solute that does not

evaporate and stays in solution.Having a non-volatile solute in the liquid

will decrease the vapor pressure of the liquid because the solutes will interfere with the high-energy liquid molecules’ path to the surface to break free into the gaseous state.

Can also be used to describe the partial pressure of a volatile solute in a liquid as a function of it’s concentration in the liquid.

C=(KH)Pgas

What is Spectrophotometry

A type of spectroscopy that studies the transmission or reflection of different frequencies of the electromagnetic spectrum (specifically visible, near-IR and near-UV ranges) by a sample of matter (usually a soln)

Other Types of SpectroscopyAtomic absorption spectroscopyAtomic emission spectroscopyMass spectroscopyNuclear magnetic resonance (NMR)Fourier transform infrared (FTIR)

spectroscopy

SpectrophotometerMeasures absorbance or

transmittance of light as a function of wavelength

Sample is placed into cuvette

Light of selected wavelength is passed through sample

Amount of light absorbed is measured

Because other compounds in a solution (or the solvent itself) may absorb the same wavelengths as the compound being analyzed, the absorbance of the sample is compared to a reference blank.

Ideally, the reference blank should contain everything found in the sample solution except the substance being analyzed.

When using a Spec 20 (and our DataHub), every time the wavelength is changed, the instrument needs to be re-zeroed.

How it Works

Transmission = I/Io

Absorbance = log T

Beer’s LawAmount of light absorbed is proportional to

the concentration of the solution

A = abcA = absorbance a = proportionality constant

(ε = molar absorptivity)b = path length (same for entire

experiment)c = concentration (M)

What does this tell us?There is a direct relationship between

absorbance and concentration.When we prepare solutions of known

concentration and analyze them at λmax, we can plot absorbance as a function of concentration.

The concentration of the unknown can be determined by finding its absorbance and plugging it into the equation for the best fit line.

y = mx + b

Uses of spectrophotometryDetermining concentration of any colored

solution (Fe, Cu, Co, Ni, MnO42-, etc)

BiologyBiochemistryForensic science