TRABAJO PRACTICO DE QUIMICA GENERAL

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Method electric charge

This was the first direct method to obtain the number of Avogadro, in 1909 Robert Millikan measured the charge of an electron through an experiment oil drop, the value of the electron 1,6022x10 at least nineteen load coulomb.La one mole of electrons had been known for some time to be approximately 96,500 coulombs (one Faraday), the amount of electricity needed to plate out of one gram equivalent weight of metal (for example, 107.87 grams of silver). The currently accepted value for the Coulomb Faraday is 96.485. The division of the load of one mole of electrons by the charge on an electron produces the number of electrons in one mole, and Avogadro's number. for years this was the most accurate method to reach the number of Avogadro.

Radioactive Disintegration Method

This technique is based on the fact that as radium undergoes ra-dioactive decay it gives off alpha particles (He2+ ions) which pick up electrons from the environment to become helium atoms. The number of alpha particles given off by a sample of radium per unit time can be measured with a Geiger counter. (This was first accomplished by Ernest Rutherford and Hans Geiger in 1908.) The moles of helium produced over a given time period can be determined by measuring the volume of gas formed. (This was first done in 1910 by Sir James Dewar, who measured the amount of helium produced from a mg sample of radium chloride over a period of 9 months. The total quantity of helium was only about 9 cubic millimeters.) Careful measurements have shown that alpha decay of a mole of radium-226 yields 0.815 X disintegrations per second and produces helium gas at the rate of 1.35 x 10—11 mole per second. Avogadro's number is obtained directly from these measurements.

X-ray Diffraction Method

The volume per atom in a crystal can be determined from data of XRD through the use of Bragg's equation, while the volume by mol can be obtained from density measurements. (Max von Laue demonstrated for the first time X-ray diffraction in 1912, and W. Lawrence Bragg developed his equation of constructive interference, nX 2d sing, shortly after that year.) Since the density measurement and X-ray DIF-fraction can be performed with high precision both in very pure crystals, this can be an exceptionally accurate method to obtain the number of Avogadro

Surface Film Method

This method involves dividing the molar volume of a fatty acid by the volume of a single molecule, determined by fatty acid monolayer floating in water. A small volume (V) of a fatty acid, long chain (such as oleic) is dissolved in a solvent (such as pentane) at a concentration (C) and is delivered from a pipette onto a surface of clean water. The acid spreads across the water as a monomolecular film, the carboxyl groups attaching themselves to the water layer and the hydrocarbon chains remaining in the non-aqueous layer. area (A) of the acid monolayer (which only partially covers the water surface) is measured. Then the height (L) of the monolayer film is calculated from the density (D) of the pure fatty acid and other recorded measurements

L=VC/DA

Using the Mole

The mole scientific unit is the amount of substance is represented by n, representing the number of moles. Since convenient to measure the amount for most substances form is weighed, it is often necessary to convert moles grams, or grams to moles. All that is needed to make the conversion is the weight of the formula. expressed in grams, which is the molar weight. This relationship is indispensable in the solving of most chemical problems.

Solution concentration is normally expressed as molarity (M), the number of moles of solute per liter of solution.

Molarity= moles of solute/liters of solution or M = n/l

In other words, to make up a solution of compound A, weigh out an amount of A equal to its formula weight in grams multiplied by the desired molarity of the solutipn apd the number of liters being prepared. In cases involving colligative properties of solutions, it is usually more convenient to express concentration in terms of molatity the number of moles of solute per kilogram of solvent

Molality= moles of solute/ kg of solvents.

Where gases are concerned, the number of moles is depen-dent on the pressure, volume, and temperature of the gas.

n= PV/RT

This is the equation of state for an ideal gas, but it is ap-proximately true for all real gases. Any units may be used for pressure, volume, and absolute temperature so long as the value for the gas constant, R, is correct for the units chosen. When the mole is too large or too small to be suitable as a counting unit, its size can be modified by an appropriate prefix. The molar mass of the new unit still equals the weight of the formula, but expressed in different units of grams mass. Molar amounts modified can be treated as gram mole, provided that the units are constantly used.

The mole concept is useful for counting amounts of ele-mentary entities other than atoms, ions, and molecules. A mole Of electrons, for instance, is a faraday, and a mole Of photons (energy quanta) is an einstein.

A chemical formula represents one molecule of a substance, but it also represents a mole of it. When thinking about chemical reactions, we tend to think in terms of single mole-cules, but when we carry out the reactions we must use very large numbers of molecules in order to see what we are doing.

A mole is a particular amount Of substance—just its formulary weight Expressed in grams, with Avogadro's count

Of units making up the aggregate.

A mole is a specific quantity: Its volume measures twenty-two point four

In liters (for a gag at STP).

A mole's a counting unit, nothing more.

A mole is but a single molecule

By Avogadro's number multiplied; One entity, extremely miniscule,

A trillion trillion times intensified.

A mole is an expedient amount, For molecules are just too small to count.

conclusion:

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