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Mole Unit

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The amount of gas in a gas sample is most conveniently indicated by specifying the number of molecules in the sample. This number is expressed by the mole number unit. When the mole unit is used, the particles being counted must be indicated. We can be deal with a mole of atoms of a particular element, a mole of electrons, a mole of photons and so forth.  

From Avogradro's law it is known that equal volumes of all gases contain equal number of molecules under similar conditions. In other words we can say that different samples of gases containing equal number of molecules occupy same volume under similar conditions. Now, since one mole molecules of all gases contain same number of molecules therefore, they occupy same volume under similar conditions of temperature and pressure.

Standard Temperature and Pressure

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It is very important to understand the general aspects of gas pressure, its measurements and calibrations. Pressure is force excerted over unit area. The unit of pressure is pascal (Pa) which is equivalent to kg/ms2. In chemistry, we usually use a mercury based barometer to measure the pressure. The unit is millimeter of mercury (mmHg). This is the same as the unit torr. Another unit commonly used to denote pressure is atmosphere (atm). The conversion of these units back and forth as 

1 atm = 760 mmHg = 760 torr
1 atm = 1.01 $\times$ 105 Pa

Gases can be cmpressed or expanded by adjusting the temperature and other conditions that prevail. In order to standardize the quantities of gases measured and conveyed, arbitrary reference conditions called standard temperature and pressure (STP) have been chosen and internationally accepted.

Standard temperature and pressure (STP)

Temperature = 0oC or 273K
Pressure = 1 atm or 760 mmHg
There is no such thing as standard volume, although molar volume is often used in studying gas behavior. The relationship of a certain mass of a sample gas to its volume, temperature and pressure was investigated by Amadeo Avogadro. The ideal gas law of Avogadro uses the molar mass of the sample gas and compares it to the actual sample mass of the gas sample being investigated. This yields the numbers of moles of gas in the system.

Avogadro Law

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The nineteenth century Italian scientist Amedeo Avogadro was a pioneer in the study of quantitative relationships in science, especially with gases. Avogadro's law states that equal volumes of gases measured at the same temperature and pressure contain equal number of molecules. All gases exist as molecules. By finding the ratio of mass of equal volumes of various gases we can find the ratio of their molecular masses. 

Avogadro's hypothesis was that "equal numbers of molecules are contained in equal volumes of all dilute gases under the same conditions". This hypothesis has been thoroughly verified to within the accuracy of approximation of real gases to ideal behavior and it is now called a law Avogadro's law. 

Avogadro's Number

Avogadro's law requires that the volumes of gaseous reactants and products be approximately in the ratios of small integers, the numbers of molecules of reactants and products in a chemical reaction are in integral ratios, and the same ratios represent the relative gas volumes.