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Chemistry · Glossary

What is Molar mass?

Also known as: atomic molar mass

Definition 25.9 School Chemistry — Grades 1 to 12 · Chapter 25 — The Mole and Molar Mass

The molar mass MM of a chemical species is the mass of one mole of its entities, in grams per mole (g/mol\mathrm{g}/\mathrm{mol}). The molar mass of an element, taken over its natural mixture of isotopes, is its atomic molar mass; it is printed in the periodic table.

elementsymbolMM (g/mol\mathrm{g}/\mathrm{mol})elementsymbolMM (g/mol\mathrm{g}/\mathrm{mol})
hydrogenH\ce{H}1.0siliconSi\ce{Si}28.1
carbonC\ce{C}12.0sulfurS\ce{S}32.1
nitrogenN\ce{N}14.0chlorineCl\ce{Cl}35.5
oxygenO\ce{O}16.0potassiumK\ce{K}39.1
sodiumNa\ce{Na}23.0calciumCa\ce{Ca}40.1
magnesiumMg\ce{Mg}24.3ironFe\ce{Fe}55.8
aluminiumAl\ce{Al}27.0copperCu\ce{Cu}63.5
goldAu\ce{Au}197.0
Atomic molar masses of common elements, rounded to 0.1 g/mol0.1\,\mathrm{g}/\mathrm{mol}. These are the values used in this book.

Examples

Example 25.10 (One mole of carbon atoms)

An atom of carbon-12 has a mass of about 12×1.67×10−27 kg=2.00×10−26 kg12 \times 1.67 \times 10^{-27}\,\mathrm{kg} = 2.00 \times 10^{-26}\,\mathrm{kg}. One mole of them has a mass of

2.00×10−26 kg×6.02×1023=1.20×10−2 kg=12.0 g.2.00 \times 10^{-26}\,\mathrm{kg} \times 6.02 \times 10^{23} = 1.20 \times 10^{-2}\,\mathrm{kg} = 12.0\,\mathrm{g}.

The mole has been chosen so that the molar mass of an atom, in grams per mole, is close to its mass number: about 1 g/mol1\,\mathrm{g}/\mathrm{mol} for hydrogen, 12 g/mol12\,\mathrm{g}/\mathrm{mol} for carbon, 16 g/mol16\,\mathrm{g}/\mathrm{mol} for oxygen.

Example 25.13 (Three molar masses)

M(HX2O)=2×1.0+16.0=18.0 g/mol,M(NaCl)=23.0+35.5=58.5 g/mol,M(CX12HX22OX11)=12×12.0+22×1.0+11×16.0=342.0 g/mol.\begin{align*} M(\ce{H2O}) &= 2 \times 1.0 + 16.0 = 18.0\,\mathrm{g}/\mathrm{mol},\\ M(\ce{NaCl}) &= 23.0 + 35.5 = 58.5\,\mathrm{g}/\mathrm{mol},\\ M(\ce{C12H22O11}) &= 12 \times 12.0 + 22 \times 1.0 + 11 \times 16.0 = 342.0\,\mathrm{g}/\mathrm{mol}. \end{align*}

The last one is sucrose, table sugar.

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