Molar masses of elements

The molar masses of elements are determined by using mass spectrometry to measure the masses of the individual isotopes and their abundances. The mass per mole of atoms is the mass of an individual atom multiplied by Avogadro s constant (the number of atoms per mole) ... 

E.30 The isotope silicon-28 has been proposed as a new standard for the molar masses of elements because it can be prepared to a very high degree of purity. The mass of one silicon-28 atom is 4.64567 X 10-23 g. If silicon were the standard used for molar mass (instead of carbon-12), 1 mol would be defined as the amount of substance that contains the same number of entities as there are atoms in exactly 28 g of silicon-28. In that case, what would be (a) the molar mass of carbon-12 (b) the (average) molar mass of chlorine ... 

STRATEGY (a) To obtain the amount of iron(II) in the analyte, we use the volume and concentration of the titrant. We follow the first two steps of the procedure in Toolbox L.2. Then we convert moles of Fe2+ ions into mass by using the molar mass of Fe2+ because the mass of electrons is so small, we use the molar mass of elemental iron for the molar mass of iron(II) ions, (b) We divide the mass of iron by the mass of the ore sample and multiply by 100%. 

Example illustrates the use of isotopic molar masses and natural abundances to calculate the molar mass of elemental iron. 

The molar masses of elements are determined by using mass spectrometry to measure the masses of the individual isotopes and their abundances. The mass per mole of atoms is the mass of an individual atom multiplied by the Avogadro constant (the number of atoms per mole). However, there is a complication. Most elements occur in nature as a mixture of isotopes we saw in Section B, for instance, that neon occurs as three isotopes, each with a different mass. In chemistry, we almost always deal with natural samples of elements, which have the natural abundance of isotopes. So, we need the average molar mass, the molar mass calculated by taking into account the masses of the isotopes and their relative abundances in typical samples. All molar masses quoted in this text refer to these average values. Their values are given in Appendix 2D. They are also included in the periodic table inside the front cover and in the alphabetical list of elements inside the back cover. 

You have learned that you can use atomic masses on the periodic table to find the molar mass of elements. Many of these values on the periodic table are close to whole numbers. However, most atomic masses are written to at least three places past the decimal. 

A formula tells you what atoms (or ions) are present in an element or compound. So, from a formula you can find the mass of a mole of the substance, or its molar mass. The simplest formula for most elements is simply that element s symbol. For example, the symbol for silver is Ag. The molar mass of elements whose formulas are this simple equals the atomic mass of the element expressed in g/mol. So, the molar mass of silver is 107.87 g/mol. Diatomic elements have twice the number of atoms in each molecule, so their molecules have molar masses that are twice the molar mass of each atom. For example, the molar mass of Br2 molecules is two times the molar mass of Br atoms (2 x 79.90 g/mol = 159.80 g/mol). 

Slight differences in chemical and physical properties can lead to small variations in the isotopic composition of natural samples. They can be exploited to give geological information (dating and origin of rocks, etc.) and lead to small variations in the molar mass of elements. 

In this text molar masses of elements are given to four significant figures. 

Notice that atomic mass and the molar mass of carbon-12 are numerically equal. They differ only in units atomic mass is measured in atomic mass units, and molar mass is measured in grams per mole. The same relationships exist between atomic and molar masses of elements, between molecular masses and molar masses of molecular substances, and between formula masses and molar masses of ionic compounds. In other words,... 

Calculate the molar mass of elemental fluorine and of calcium fluoride. Begin with the formulas of fluorine and calcium fluoride. 

You can use the molar mass of a compound to convert between mass and moles, just as you used the molar mass of elements to make these conversions. 

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