Determining Avogadros Number

Avogadro hypothesized that equal numbers of moles of different gases at the same temperature and pressure contain the same number of molecules. But he never calculated what that number might be. Later on, other scientists calculated a value for Avogadro s number. In one experiment, a thin film of a chemical was spread on the surface of water. The layer was assumed to be one molecule thick, or a monolayer. In 1924, it was estimated that the number of molecules in a mole was 6.004 x 10.  

You will cover the surface of a water sample with a monolayer of stearic acid by adding drops of stearic acid solution to the water surface. The solvent will quickly evaporate, leaving the nonpolar stearic acid molecules on the water s surface. 

The mass of the stearic acid can be determined, and the number of moles of stearic acid can be calculated using the molar mass of stearic acid. The monolayer formed is one molecule thick, so if an assumption is made about the shape of a single molecule, the number of molecules in the monolayer can be estimated. Avogadro s number is then estimated as the ratio of the number of molecules of stearic acid to the number of moles of stearic acid. The closer the assumed shape is to the actual shape of the molecule, the more precise the calculation of Avogadro s number will be. In this experiment, the shape of the molecule is first assumed to be a rectangular solid and then a cylindrical solid. 

How many molecules of stearic acid are in a mole of stearic acid  

Always wear safety goggles, gloves, and a lab apron. Avoid breathing directly over the watch glass. 

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Having detemiined A b and knowing that the gas constant R = 8.314JK from macroscopic measurements on gases, determine Avogadro s number L from the relationship... 

Knowing Avogadro s number and the atomic mass of an element, it is possible to calculate the mass of an individual atom (Example 3.2a). You can also determine the number of atoms in a weighed sample of any element (Example 3.2b). 

Avogadro s number is the conversion factor that iinks the number of moles with the number of individual particles. To determine the number of atoms in a sample of an element, we multiply the number of moles by Avogadro s number — n Likewise, if a sample contains a certain number of atoms, the number of moles in the sample... 

The number of elementary entities in 1 mol is an experimentally determined quantity, and is called the Avogadro constant L, which has the value 6.022 x 1023 mol-1. The Avogadro constant is also (incorrectly) called the Avogadro number . It is... 

Born equation Jphys chemJ An equation for determining the free energy of solvation of an ion in terms of the Avogadro number, the ionic valency, the ion s electronic charge, the dielectric constant of the electrolytic, and the ionic radius. born i kwa zhan J... 

Describe an experiment to determine Avogadro s number from the average root mean square displacement of a particle due to random walk. 

Chemists have devised various ways to determine the Avogadro constant. To learn more about how this constant has been found in the past and how it is found today, go to the web site above. Go to Science Resources, then to Chemistry 11 to find out where to go next. What are some methods that chemists have used to determine the number of particles in a mole How has the accepted value of the Avogadro constant changed over the years ... 

It should also be stressed that all the atomic masses are experimentally determined relative masses. The arbitrary reference is the mass of 12C and the mass unit is defined as one-twelfth of the mass of 12C. However, the Avogadro number is only known with some error margin, and thus the mass of one 12C is not known exactly. An exact mass exists of course, but cannot be determined with the techniques available in 2006. There is no analytical mass spectrometer able to determine directly the mass of an ion a calibration needs to be done with compounds having accurately known masses, but these are never exact. 

The gas constant R is common to all gases and determined by the product of the Boltzmann constant and the Avogadro number... 

Here M is the molecular weight and V the partial specific volume of the solute, N the Avogadro number, k the Boltzmann constant, and T the absolute temperature s and D are the sedimentation and translational diffusion coefficients (after extrapolation to infinite dilution). The translational frictional coefficients from both measurements are regarded as identical, i.e., f, = fd. The rotary frictional coefficient, designated as f, can be determined from either flow birefringence or non-Newtonian viscosity measurements. 

Reference on methods of determining Avogadro s number Experimental Atomic Physics, G. P. Harnwell and J. J. Livingood. McGraw-Hill Book Co., ms, Secs. 1-3 to 3-11. 

Scientists have also determined the number of particles present in 1 mol of a substance, called Avogadro s number. One mole of pure substance contains 6.022 1367 x 10 particles. To get some idea of how large Avogadro s number is, imagine that every living person on Earth (about 6 billion people) started counting the number of atoms of 1 mol C. If each person counted nonstop at a rate of one atom per second, it would take over 3 million years to count every atom. 

The number of particles in a mole is called Avogadro s number, or Avogadro s constant. One way to determine this number is to count the number of particles in a small sample and then use mass or particle size to find the amount in a larger sample. This method works only if all of the atoms in the sample are identical. Thus, scientists measure Avogadro s number using a sample that has atoms of only one isotope. 

Avogadro s principle tells us that equal volumes of gases, at equal temperatures and pressures, contain an equal number of particles. It is from this principle that chemists have developed the concept of the molar volume of gases. It has been determined that one mole of any gas at standard temperature and pressure (a temperature of 273 K and 101.3 kPa of pressure) will occupy 22.4 dm3 of volume. This allows us to determine the number of moles in a gas, provided we know the volume, temperature, and pressure of the sample. 

This chapter opened with a photo of a nearly perfect silicon sphere that is being used to determine Avogadro s number. Use your Web browser to connect to http //chemistry.brookscole.com/skoogfac/. From the Chapter Resources Menu, choose Web Works. Locate the Chapter 4 section and click on the link to the Australian National Measurement Laboratory. Read the article on Avogadro s number and the silicon kilogram. What factors limit accuracy in the determination of this number What are the present and ultimate uncertainties in the measurement of the molar mass of silicon, the number of atoms per unit cell, the mass, the volume, and the lattice parameter of silicon ... 

Determining Avogadro s Number from the Structure of a Solid... 

The possibility of determining Avogadro s number from studies of the diffusion of colloidal particles was of great scientific importance, since it represented a breakthrough in Ostwald s restriction on the direct observation of molecular motion. In contrast, here NA is determined by the observation of the thermal motion of particles whose size significantly exceeds molecular dimensions... 

In Sample Problem 12.4, we use the density of an element and the packing efficiency of its crystal structure to calculate its atomic radius. Variations of this approach are used to find the molar mass and as one of the ways to determine Avogadro s number. 

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