From Molecules to Moles

Our discussion of balancing chemical equations has focused on accounting for atoms and molecules in both reactants and products. Although a balanced equation tells us how many atoms of an element it may take to form so many molecules, it is impractical to speak in these terms for common applications. For example, if we want to know how much carbon dioxide is produced when some vinegar is added to bak- 

What exactly does it mean to have as many atoms as there are in 12 grams of C-12 The number is known as Avogadro s number, and its accepted value is 6.022 X 10. Therefore, from the definition of a mole we can say there are 6.02 X 10 atoms in a mole of C-12. The term Avogadro s number was coined by Jean Baptiste Perrin (1870-1942) who was the first to experimentally determine 

The fact that both the mole and the mass of an element are based on carbon-12 enables us to relate mole and mass. A molar mass is defined as the mass in grams of one mole of a substance, and it can be obtained directly from an element s atomic mass. We can use the elements hydrogen and nitrogen to illustrate this concept. Periodic table entries for both elements are shown below. The whole number above the element is the atomic number and gives the number of protons in the nucleus. The number below the element s symbol is the molar mass (as well as the atomic mass)  

Therefore, one mole of ammonia contains 6.022 X 10 NH3 molecules and has a mass of 17.0305 grams conversely, 17.0305 grams of ammonia equals one mole of ammonia. 

Coefficients in a balanced chemical equation are typically taken to represent moles. For the reaction of hydrogen and nitrogen to form ammonia, the equation 

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The first step is to convert from molecules to moles of a gas, n. 

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