Tham khảo sách 'chemistry part 5, julia burdge,2e (2009)', khoa học tự nhiên, hoá học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 84 CHAPTER 3 Stoichiometry Ratios of Combination r i i Divide by molar Í I g mol mass Divide by A Moles t i I I particles X ---------------- ------------ X 1023 particles I I I mol jb Í-i r I I 1 I Grams I t I Multiply by molar mass I Multiply by N Particles Atoms I Molecules Formula units mol X g mol g 1 t i 5 particles I I Í Figure Flowchart for conversions among mass moles and number of particles. Because the number of moles specifies the number of particles atoms molecules or ions using molar mass as a conversion factor in effect allows us to count the particles in a sample of matter by weighing the sample. Recall from Section that 1 amu x 10 24 g This is the reciprocal of Avogadro s number. Expressed another way 1 g x 1023 amu In effect there is 1 mole of atomic mass units in a gram. Determining Molar Mass Although chemists often wish to combine substances in specific mole ratios there is no direct way to measure the number of moles in a sample of matter. Instead chemists determine how many moles there are of a substance by measuring its mass usually in grams . The molar mass of the substance is then used to convert from grams to moles. The molar mass of a substance is the mass in grams of 1 mole of the substance. By definition the mass of a mole of carbon-12 is exactly 12 g. Note that the molar mass of carbon is numerically equal to its atomic mass. Likewise the atomic mass of calcium is amu and its molar mass is g the atomic mass of sodium is amu and its molar mass is g and so on. In general an element s molar mass in grams is numerically equal to its atomic mass in atomic mass units. The molar mass in grams of any compound is numerically equal to its molecular or formula mass in amu . The molar mass of water for example is g and the molar mass of sodium chloride NaCl is g. When it comes to expressing the molar mass of elements such as oxygen and hydrogen we have to be careful to specify what form
