Weighing

The automation of the weighing operation is virtually indispensable, particularly with solid samples, although it also simplifies to a great extent the weighing of liquids, which is difficult to automate by other methodologies. 

In chemistry, you count very large numbers of particles, such as atoms and molecules. To count them efficiently and quickly, you need to use the count-byweighing method, which means you need to know how much individual atoms and molecules weigh. You can get the weights of the individual atoms on the 

Now try a little harder one. Calculate the formula weight of aluminum sulfate, Al2(SO03- In this salt, you have 2 aluminum atoms, 3 sulfur atoms, and 12 oxygen atoms. After you find the individual weights of the atoms on the periodic table, you can calculate the formula weight like this  

When we humans deal with objects, we often think in terms of a convenient amount. For example, when a woman buys earrings, she normally buys a pair of them. When a man goes to the grocery store, he buys eggs by the dozen. And when 1 go to the office supply store, 1 buy copy paper by the ream. 

We use words to represent numbers all the time — a pair is 2, a dozen is 12, and a ream is 500. All these words are units of measure, and they re convenient for the objects they re used to measure. Rarely would you want to buy a ream of earrings or a pair of paper. 

Likewise, when chemists deal with atoms and molecules, they need a convenient unit that takes into consideration the very small size of atoms and molecules. There is such a unit. It s called a mole. 

In this chapter we will learn to determine a compound s formula. Before we can do that, however, we need to think about counting atoms. How do we determine the number of each type of atom in a substance so that we can write its formula Of course, atoms are too small to count individually. As we will see in this chapter, we typically count atoms by weighing them. So let us first consider the general principle of counting by weighing. 

Suppose you work in a candy store that sells gourmet jelly beans by the bean. People come in and ask for 50 beans, 100 beans, 1000 beans, and so on, and you have to count them out—a tedious process at best. As a good problem solver, you try to come up with a better system. It occurs to you that it might be far more efficient to buy a scale and count the jelly beans by weighing them. How can you count jelly beans by weighing them  

What information about the individual beans do you need to know  

Assume that all of the jelly beans are identical and that each has a mass of 5 g. If a customer asks for 1000 jelly beans, what mass of jelly beans would be required Each bean has a mass of 5 g, so you would need 1000 beans x 5 g/bean, or 5000 g (5 kg). It takes just a few seconds to weigh out 5 kg of jelly beans. It would take much longer to count out 1000 of them. 

In reality, jelly beans are not identical. For example, let s assume that you weigh 10 beans individually and get the following results  

Walter E. Kupper, Madison, New Jersey 07940, United States 

The field of mass measurement, commonly called weighing, is conveniently structured according to the degree of numerical precision involved. The classification and nomenclature of modern laboratory balances follows a decimal pattern based on the. step size d of the associated digital display  

The words balance and scale are often used interchangeably. Balance is derived from the Latin bilanx. having two pans [1]. Scale and scales are short forms of a pair of scales (old English, meaning dishes or plates). Modern weighing instruments no longer have two weighing pans, but the words balance and scale  

Self-calibration with built-in reference mass. 

Chemists make new substances in a process called synthesis. And a logical question they ask is how much — how much of this reactant do I need to make this much product How much product Ccin I mcike with this much reactant  

To cinswer these questions, chemists must be able to take a balanced chemical equation, expressed in terms of atoms and molecules, and convert it to grams or pounds or tons — some type of unit they can actually weigh out in the lab. The mole concept enables chemists to move from the microscopic world of atoms and molecules to the reed world of grams and kilograms, and it s one of the most importcuit central concepts in chemistry. In this chapter, I introduce you to Mr. Mole. 

Atoms Get the weights of the individual atoms on the periodic table — just find the atomic mass number. 

Compounds Simply add together the weights of the individual atoms in the compound to figure the molecular weight or formula weight. (Note Molecular weights refer to covalently bonded compounds, and formula weights refer to both ionic and covalent compounds.) 

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Plant service vehicles, loading and weighing devices... 

Absorbing the gas on a substance which can be weighed both before and after absorption. 

Gouy balance A balance for the determination of magnetic susceptibility. The sample is weighed in and out of a magnetic field and the susceptibility is calculated from the difference in weights. 

The normal form in which nickel is weighed in analysis. There is metal-metal bonding in the solid. The red complex is precipitated from alkaline solution. 

Regnault s method Obsolete method for determining gas densities by direct weighing of a known volume of gas under known conditions of temperature and pressure. 

The procedure most commonly employed (NF M 07-047 or ASTM D 2274) Is to age the diesel fuel for 16 hours while bubbling oxygen into it at 95°C. The gums and sediment obtained are recovered by filtration and weighed. There is no official French specification regarding oxidation stability however, in their own specifications, manufacturers have set a maximum value of 1.5 mg/100 ml. 

Conradson carbon NFT60-116 ISO 6615 ASTM D 189 ASTM D 4530 ISO 10370 Weighing after combustion then pyrolysis Micro method... 

Sediments NF M 07-010 ISO 3735 ASTM D 473 Extraction with toluene and weighing of residue... 

Solubility (bituminous products) NFT 66-012 ASTM D 4 Dissolving, filtration, weighing... 

For onshore operations various types of landrigs are available, ranging from truck mounted light rigs to heavy landrigs weighing several hundred tons. 

A land rig can weigh over 200 tons and is transported in smaller loads to be assembled on site. 

A slim hole rig weighs about one fifth of a conventional rig and its small size can open new frontiers by making exploration economic in environmentally sensitive or inaccessible areas. 

The legs of the platform can be used as settling tanks or temporary storage facilities for crude oil where oil is exported via tankers, or to allow production to continue in the event of a pipeline shut down. The Brent D platform in the North Sea weighs more than 200,000 tonnes and can store over a million barrels of oil. Topside modules are either installed offshore by lift barges, or can be positioned before the platform is floated out. 

The prime global authority is the International Maritime Organisation. The IMO sets the standards and guidelines for the removal of offshore installations. The guidelines specify that installations in less than 75 meters of water with substructures weighing less than 4,000 tons be completely removed from the site. Those in deeper water must be removed to a depth of 55 meters below the surface so that there is no hazard to navigation. In some countries the depth to which structures have to be removed has already been extended to 100m. 

Subsea facilities are easily decommissioned as they are relatively small and easy to lift. However, subsea manifolds and templates can weigh in excess of 1,000 tons and will require heavy lift barges for removal. 

A rugged, powerful instrument in a cast aluminium case Weighs 6 kg including standard D cell batteries... 

A second general type of procedure, due to McBain [29], is to determine n by a direct weighing of the amount of adsorption. McBain used a delicte quartz spiral spring, but modem equipment generally makes use of a microbalance or a transducer. An illustrative schematic is shown in Fig. XVII-6. 

The sulphate ion is detected by addition of barium chloride in the presence of hydrochloric acid a white precipitate of barium sulphate is obtained. The same test can be used to estimate sulphate, the barium sulphate being filtered off, dried and weighed. 

Iodine is a dark-coloured solid which has a glittering crystalline appearance. It is easily sublimed to form a bluish vapour in vacuo. but in air, the vapour is brownish-violet. Since it has a small vapour pressure at ordinary temperatures, iodine slowly sublimes if left in an open vessel for the same reason, iodine is best weighed in a stoppered bottle containing some potassium iodide solution, in which the iodine dissolves to form potassium tri-iodide. The vapour of iodine is composed of I2 molecules up to about 1000 K above this temperature, dissociation into iodine atoms becomes appreciable. 

Addition of calcium nitrate solution to a fluoride gives a white precipitate of calcium fluoride, CaFj. If the latter is precipitated slowly, it can be filtered off and weighed to estimate the fluoride. Fluoride can also be determined by the addition of sodium chloride and lead nitrate which precipitate lead chlorofluoride, PbClF. This is filtered off and weighed. 

Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator. 

Gold compounds are all easily reduced in alkaline solution to metallic gold which may occur in colloidal form and so be red, blue or intermediate colours. Reduction to gold, followed by weighing of ihe metal precipitated, may be used in quantiiaiive analysis. 

Asymmetry in a similarity measure is the result of asymmetrical weighing of a dissimilarity component - multiplication is commutative by definition, difference is not. By weighing a and h, one obtains asymmetric similarity measures, including the Tversky similarity measure c j aa 4- fih + c), where a and fi are user-defined constants. The Tversky measure can be regarded as a generalization of the Tanimoto and Dice similarity measures like them, it does not consider the absence matches d. A particular case is c(a + c), which measures the number of common features relative to all the features present in A, and gives zero weight to h. 

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