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The identity of a substance is defined not only by the types of atoms or ions it contains, but by the quantity of each type of atom or ion. For example, water, H2OH2O, and hydrogen peroxide, H2O2H2O2, are alike in that their respective molecules are composed of hydrogen and oxygen atoms. However, because a hydrogen peroxide molecule contains two oxygen atoms, as opposed to the water molecule, which has only one, the two substances exhibit very different properties. Today, we possess sophisticated instruments that allow the direct measurement of these defining microscopic traits however, the same traits were originally derived from the measurement of macroscopic properties (the masses and volumes of bulk quantities of matter) using relatively simple tools (balances and volumetric glassware). For these reasons, there are proposals to revise the definitions of both N A and the kilogram. 1: Moles to Mass Conversion with Elements. According to the periodic table, 1 mol of U has a mass of 238.0 g, so the mass of 2 mol is twice that, or 476.0 g. The standard of mass, the kilogram, is not precisely known, and its value appears to be changing. The periodic table shows that the atomic mass (rounded to two decimal points) of Al is 26.98, so 1 mol of Al atoms has a mass of 26.98 g.The silicon sphere is an artifact, rather than being something that occurs in nature, and thus may not be perfectly reproducible.However, there are two problems with this: (The measurement was made by X-ray scattering.) When combined with the measured mass of this sphere, it yields Avogadro's number. The current value was determined by measuring the distances between the atoms of silicon in an ultrapure crystal of this element that was shaped into a perfect sphere.Because large numbers of atoms cannot be counted directly, a variety of ingenious indirect measurements have been made involving such things as Brownian motion and X-ray scattering. Calculate the molar mass of PO4 in grams per mole or search for a chemical formula or substance. The value of N A can be known only to the precision that the number of atoms in a measurable weight of a substance can be estimated.Its practical use is limited to counting tiny things like atoms, molecules, "formula units", electrons, or photons.It is a huge number, far greater in magnitude than we can visualize.We have defined a mole based on the isotopic atomic mass of carbon-12. It is a number, just as is "dozen", and thus is dimensionless. The atomic mass of an element is the relative average of all of the naturally occurring isotopes of that element and atomic mass is the number that appears in the periodic table.Things to understand about Avogadro's number Ptable is a responsive web 2.0 periodic table that offers names, electrons, oxidation, trends, orbitals, isotopes, and search in different languages. There are a lot of atoms in 12 g of carbon. 65 Tb Terbium 158.93 66 Dy Dysprosium 162.50 67 Ho Holmium 164.93 68 Er Erbium 167.26 69 Tm Thulium 168.93 70 Yb Ytterbium 173.05 71 Lu Lutetium 174.