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In chemistry, the molar mass ''M'' is a physical property defined as the mass of a given substance (chemical element or chemical compound) divided by its amount of substance. The base SI unit for molar mass is kg/mol. However, for historical reasons, molar masses are almost always expressed in g/mol. As an example, the molar mass of water: ''M''(H2O) ≈ == Molar masses of elements == (詳細はatoms of an element is given by the atomic mass of the element multiplied by the molar mass constant, ''M'' = 1×10−3 kg/mol = 1 g/mol:〔 :''M''(H) = 1.007 97(7) × 1 g/mol = 1.007 97(7) g/mol :''M''(S) = 32.065(5) × 1 g/mol = 32.065(5) g/mol :''M''(Cl) = 35.453(2) × 1 g/mol = 35.453(2) g/mol :''M''(Fe) = 55.845(2) × 1 g/mol = 55.845(2) g/mol. Multiplying by the molar mass constant ensures that the calculation is dimensionally correct: atomic weights are dimensionless quantities (i.e., pure numbers) whereas molar masses have units (in this case, grams/mole). Some elements are usually encountered as molecules, e.g. hydrogen (H), sulfur (S), chlorine (Cl). The molar mass of molecules of these elements is the molar mass of the atoms multiplied by the number of atoms in each molecule: :''M''(H) = 2 × 1.007 97(7) × 1 g/mol = 2.015 88(14) g/mol :''M''(S) = 8 × 32.065(5) × 1 g/mol = 256.52(4) g/mol :''M''(Cl) = 2 × 35.453(2) × 1 g/mol = 70.906(4) g/mol. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「molar mass」の詳細全文を読む スポンサード リンク
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