Atomic weights Specific heats

Dulong and Pedt s law The product of the atomic weight and the specific heat of a metal is constant of value approximately 6-2. Although not true for all metals at ordinary temperatures, these metals and several non-metals approximate to the law at high temperatures. 

The molecular weight (mean relative molecular mass) was obtained by determination of density but, in order to determine that the gas was monatomic and its atomic and molecular weights identical, it was necessary to measure the velocity of sound in the gas and to derive from this the ratio of its specific heats kinetic theory predicts that Cp/C = 1.67 for a monatomic and 1.40 for a diatomic gas. 

Metal A lomic number Atomic weight Lattice structure Density at 20°C (g/em ) Melting point (°C) Thermal conductivity at 0-l00°C (W/m°C) Specific heat at 0°C (J/kg C) Coefficient of linear expansion at 20-iOO°C X 70 Thermal neutron cross-section (barns) (10-- m ) Resistivity at 0°C (fiil em) Temperature coefficient of resistance o-ioo°c X 10 ... 

By the middle of the nineteenth century more than 60 elements were known with new ones continuing to be discovered. For each of these elements, chemists attempted to determine its atomic weight, density, specific heat, and other properties. The result was a collection of facts, which lacked rational order, Mendeleev noticed that if the elements were arranged by their atomic weights, then valencies and other properties tended to recur periodically. However, there were gaps in the pattern and in a paper of 1871 Mendeleev asserted that these corresponded to elements that existed but had not yet been discovered. He named three of these elements eka-aluminium, eka-boron and eka-silicon and gave detailed descriptions of their properties. The reaction of the scientific world was sceptical. But then in 1874 Lecoq de Boisbaudran found an... 

Relation of Specific Heat to Atomic Weight in the Case of Solids. 

Which of the equations given for AH is used here Both. You can see from the factor-label method solution that the atomic weight divided into the molar heat capacity is the specific heat capacity while the mass divided by atomic weight is the number of moles. Thus, we have cither moles times molar heat capacity times change in temperature or mass times specific heat times change in temperature. 

EXAMPLE 18.14. An element has a specific heat of 0.123 J/g deg. Calculate its approximate atomic weight. 

The molar heat capacity is about 25J/moldeg, and the specific heat is 0.123 J/g deg. Therefore, the atomic weight is approximately... 

The specific heat of a certain element is 0.119J/g deg. Using the law of Dulong and Petit, calculate its approximate atomic weight. 

What factor can be used to transform molar heat capacity into specific heat Ans. The reciprocal of atomic weight (or formula weight). For example. 

Calculate the approximate atomic weight of lead using its specific heat, 0.12 J/g deg, and the law of Dulong and Petit. 

This rule was stated in 1819 by Dulong and Petit, and it indicates that the specific heat of a metal multiplied by the atomic weight is a constant. This relationship provides a way to estimate the atomic weight of a metal if its specific heat is known. How well the rule holds is indicated by the specific heats of metals shown in Table 7.9. 

When 3.00 grams of a certain metal are completely oxidized, 3.80 grams of its oxide are produced. The specific heat of the metal is 0.052 cal/g °C. What is the approximate atomic weight of this metal ... 

Dulong was also an associate of Berthollet and a member of the Societe d Arcueil. His 1819 paper on heat capacities of elements in collaboration with Alexis-Therese Petit was widely interpreted as support for the atomic hypothesis. They noted that the product of specific heat times atomic weight was very nearly the same for a large number of sohd elements. They recognized that the quantity in qnestion represents the heat capacities of the atoms—or in modem terms, molar heat capacities. And they generalized the resnlts, asserting that, atoms of all simple bodies have exactly the same capacity for heat. (7)... 

There is a simple way by which you can find the approximate atomic weight of an element It is described on pp 211-212 With this method, you determine the specific heat ofthe element experimentally, then divide it into 6 2 (using the rule of Dulong and Petit), the result IS the approximate atomic weight. Even if you didn t know the name of the element, you could use this approximate value along With an accurate chemical analysis to find an act unite value ofthe atomic weight... 

From a specific heat measurement, the approximate atomic weight of a metal (M) is found to be 133 A 0 2341 g sample of M is heated to constant weight in air to convert it to the oxide The weight of the residue is 0 274s g Find the true atomic weight of the metal (and therefore its identity), and determine the formula of the metal oxide... 

This rule of Dulong and Petit provides a simple way to find the approximate values of the atomic weights of solid elements. For example, if you had an unknown solid element in a finely divided state, you could put a weighed sample of it into your calibrated calorimeter and quickly find its specific heat then, using the rule of Dulong and Petit, you could find its approximate atomic weight. 

ATOMIC HEAT. The product of the gram-atomic weight of an element and its specific heat The result is the atomic heat capacity per gram-atom. For many solid elements, the atomic heal capacity is very nearly the same, especially at higher temperatures and is approximately equal to 3R, where R is the gas constant (Law of Dulong and Petit). 

DU EON C AND PETIT LAW OF SPECIFIC HEATS. Ii has long been known Ural the atomic heals of Ihe great majority of elements have nearly the same value at room temperature in fact, the thermal capacity of a gram-atom of most elements is not far from 6 calories per degree. Dulong and Pelit expressed this by stating that the specific heats of elements arc in inverse proportion to their atomic weights. 

Estimates of the specific heat of coal have also been made on the assumption that the molecular heat of a solid material is equal to the sum of the atomic heats of the constituents (Kopp s law) the atomic heat so derived is divided by the atomic weight to give the (approximate) specific heat. Thus, from the data for various coals it has been possible to derive a formula that indicates the... 

Ex. 12 The specific heat at constant volume of argon is 0.075 and its molecular weight is 40. How many atoms are there in Us molecule Solution Molar heat at constant volume (Cv)... 

Ex. 14 The specific heat of a gas at constant volume is 0.152 and that at constant pressure is 0.215. Calculate the atomicity and molecular weight of the gas. Also name the gas. 

Ex. 15 The specific heats of a gas at constant volume and constant pressure are 0.090 and 0.45 calories. If the atomic weight of the gas be M, calculate its molecular weight... 

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