Weighing atomic mass

Knowing Avogadro s number and the atomic mass of an element, it is possible to calculate the mass of an individual atom (Example 3.2a). You can also determine the number of atoms in a weighed sample of any element (Example 3.2b). 

The weight of atoms and their constituents can be given in kilograms. A proton, for example, weighs 1.67 x 10 kilograms, but its weight or mass can be expressed more conveniently in a measure called the atomic mass unit (amu). One amu is defined as 1/12 the mass of a carbon atom that consists of six protons. 

The reverse isotope dilution technique can be applied for accurate determination of the Mg contents in a sample, sample, on applying equation 7, by measuring the isotope ratio of a selected pair of stable isotopes, in a weighed mixmre of the sample with an isotopically enriched CRM. The average atomic masses m and the isotopic ratios R of Mg in the enriched CRM and in nature are known. The method was applied for determination of Mg in plant material using a CRM isotopically enriched with Mg, measuring with an ICP/MS instruments. ... 

Transfer the bowl into a muffle furnace and roast the precipitate at 400-450 °C during one hour. Cool the bowl with the substance to room temperature in a desiccator and weigh it. Use the found value of the equivalent and the approximate value of the atomic mass of lead to determine its valence. Calculate a more accurate value of the atomic mass of lead from the obtained data, compare your result with the value of the atomic mass of this element given in Mendeleev s periodic table, and establish the magnitude of the error in per cent. 

An unknown substance, X, was isolated from rabbit muscle. Its structure was determined from the following observations and experiments. Qualitative analysis showed that X was composed entirely of C, H, and 0. A weighed sample of X was completely oxidized, and the H20 and C02 produced were measured this quantitative analysis revealed that X contained 40.00% C, 6.71% H, and 53.29% O by weight. The molecular mass of X, determined by mass spectrometry, was 90.00 u (atomic mass units see Box 1-1). Infrared spectroscopy showed that X contained one double bond. X dissolved readily in water to give an acidic solution the solution demonstrated optical activity when tested in a polarimeter. 

At this conference, a pamphlet written by the Italian chemist Stanislao Cannizzaro (1826—1910) was presented. In this pamphlet, which he had used with his students for several years, Cannizzaro explained and justified Avogadro s hypothesis and showed how correct atomic masses and formulas could be obtained through easy calculations. The concept was simple provided equal volumes of gases contain equal numbers of atoms or molecules, the relative masses of these particles can be obtained by weighing equal volumes of gases that are at... 

The mole (mol) is simply a unit of quantity, it represents a certain amount of material, i.e. atoms or molecules. The numerical value of one mole is 3 23 x 1023 and is referred to as Avogadro s number. The mole is defined iis/the mass, in grams, equal to the atomic mass of an element or molecule. Therefore, 1 mole of carbon weighs 12 grams and contains 6.023 x 1023 carbon atoms. The following formula can be used to find the number of moles ... 

The value of using atomic masses is that it allows us to count a large number of atoms by weighing a sample of the element. For instance, we can calculate that a small speck of carbon weighing 1.00 mg (1.00 X 10-3 g) contains 5.01 X 1019 carbon atoms ... 

The top number in the symbols is the mass number and the bottom number is the atomic number. The atomic mass of the element hydrogen is the weighed average of the mass numbers of the three isotopes according to several tables ... 

None of these is correct. In daily life we are used to adapting the mass unit to the mass of the object . No-one would say I would like to have 5 x 109 mg of sand , but they would say I would like 5 tonnes of sand . At the butcher s you would not ask for 0.2 kg of liver pate, but you would want to have 200 grammes of liver pate . It has been determined that 6 x 1020 H-atoms weigh about 1 mg and that means that even the unit mg would be much too high for atomic masses. Therefore, a new unit has been introduced for the atomic mass ... 

Example A single molecule of silver has an atomic mass of 107.8682 therefore 1 mol of silver weighs 107.8682 g. 

If we try to weigh one grain of rice on a bathroom scale, we get an inkling of the very much more difficult task of weighing atoms. (The ratio of the mass of an atom to that of the smallest mass weighable on any balance is about 10 g/10 g, or 10 . This is much lower than the ratio of the mass of a grain of rice to that of a person, which is about 10 g/10 g, or 10 . )... 

Using similar reasoning for other compounds, Dalton prepared the first table of atomic masses (formerly called atomic weights by chemists, since mass is usually determined by comparison to a standard mass—a process called weighing ). Many of the masses were later proved to be wrong because of Dalton s incorrect assumptions about the formulas of certain compounds, but the construction of a table of masses was an important step forward. 

A 1.42-g sample of a pure compound, with formula M2S04, was dissolved in water and treated with an excess of aqueous calcium chloride, resulting in the precipitation of all the sulfate ions as calcium sulfate. The precipitate was collected, dried, and found to weigh 1.36 g. Determine the atomic mass of M and identify M. 

Gravimetric analysis is the process of converting an element into a definitive compound, isolating this compound from other constituents in a sample and then weighing the compound (Box 20.1). The weight of the element can then be calculated from the formula of the compound and the relative atomic masses of the elements involved. You need to be able to weigh accurately, by difference, a substance to four decimal places (see p. 23). 

Atoms individually are very small one gramme of carbon contains about 20 thousand million, million, million atoms, so weighing out small numbers of atoms is impossible. But it has been found that if you take the relative atomic mass (see Section 1.3.4) in grammes of any element, it will contain the same number of atoms as the relative atomic mass of any other element. So 12 g of carbon will have the same number of atoms of carbon as 207 g of lead has atoms of lead. 

The particles within the atom are extremely tiny. A penny weighs about 2.5 grams, and a neutron, which is the most massive of the particles in the atom, weighs only 1.6750 X 10 grams. The protons have about the same mass as the neutrons, but the electrons have about 2000 times less mass. Because the masses of the particles are so small, a more convenient unit of measurement has been devised for them. An atomic mass unit (also called the unified mass unit) is 1/12 the mass of a carbon atom that has 6 protons, 6 neutrons, and 6 electrons. The modern abbreviation for atomic mass unit is u, but amu is commonly used. 

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