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# Mole and the Avagadro's Number

## The mole is the SI base unit for an amount of a substance.

The SI unit for amount of substance is the mole, defined as

• the amount of substance that has an equal number of elementary entities as there are atoms in 12 g of carbon-12  (12C -  the isotope of carbon with standard atomic weight 12 by definition).

That number of elementary entities in carbon-12 is the Avogadro constant (number), NA.

NA = 6.022141 1023 [molecules/mol]

The mole is one of the base units of the SI, and has the unit symbol [mol].

Standard volume of 1 mole of an ideal gas at 1 atmosphere and 0°C is 22.414 liters.

Example: Number of molecules in 1 mole of hydrogen gas.

Hydrogen gas contains H2 molecules.  1 mole of a substance is 6.022141*1023 molecules of the substance.

So; 1 mole of hydrogen gas contains 6.022141*1023 H2-molecules

Example:
Number of molecules in ethyl alcohol

The molecular weight of ethyl alcohol is 46.07 kg/kmol and the number of molecules in 1 kg can be calculated as

n = (1 [kg] / 46.07 [kg/kmol])* 6.022 1023 [molecules/mol] * 1000 [mol/kmol]

= 1.31*1025 [molecules]

## Related Topics

• Basics - The SI-system, unit converters, physical constants, drawing scales and more.

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