Elements and the Mole—Molar Mass

By definition, the atomic mass of the carbon-12 atom is exactly 12.00 amu. One mole of carbon-12 atoms has a mass of exactly 12.00 g, and that 12.00 g mass contains exactly 6.022 x 1023 carbon-12 atoms. This statement sets the benchmark for all chemical calculations involving the mole. One mole of any element is an amount of that element equal to its atomic mass in grams (its molar mass), and that mass contains 6.022 x 1023 atoms of that element. Using atomic masses, you can apply these relationships to the elements hydrogen and nitrogen. 

The atomic mass of hydrogen is 1.008 amu, so 1.008 grams of hydrogen is 1 mole of hydrogen, and that 1.008 g mass contains 6.022 x 1023 atoms of hydrogen. 

Likewise, the atomic mass of nitrogen, N, is 14.01 amu. 14.01 grams of nitrogen is 1 mole of nitrogen, and 14.01 grams of nitrogen contain 6.022 x 1023 atoms of nitrogen. 

You may wonder why it takes 14.01 g of nitrogen to have the same number of atoms as there are in 1.008 g of hydrogen. It s because nitrogen atoms are about 14 times heavier than hydrogen atoms. Remember that atomic masses are relative masses. If atoms of element X are 50 times heavier than those of element Z, the atomic mass of X will be 50 times larger than that of Z. 

The molar mass of an element is the mass in grams of 1 mole of that element. One molar mass of hydrogen is 1.008 g of hydrogen. One molar mass of nitrogen is 14.01 g of nitrogen. The term molar mass also applies to compounds, as will be seen later in this chapter. 

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If you answered incorrectly, review Elements and the Mole—Molar Mass, page 111. 

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