Moles counting molecules

One way to describe this situation is to say that the colligative properties provide a method for counting the number of solute molecules in a solution. In these ideal solutions this is done without regard to the chemical identity of the species. Therefore if the solute consists of several different components which we index i, then nj = S nj j is the number of moles counted. Of course, the total mass of solute in this case is given by mj = Sjnj jMj j, so the molecular weight obtained for such a mixture is given by... 

The effect of the solute on the vapor pressure of a solution gives us a convenient way to count molecules and thus provides a means for experimentally determining molar masses. Suppose a certain mass of a compound is dissolved in a solvent, and the vapor pressure of the resulting solution is measured. Using Raoult s law, we can determine the number of moles of solute present. Since the mass of this number of moles is known, we can calculate the molar mass. 

The sum converges to 1/(1 -/F). Note that the mole count includes the monomer, but not the solvent, cast-off small molecules, or any inerts.]... 

The mole concept that connects weighing and counting molecules and atoms... 

I hapter 1 explained how chemical and physical methods are used to estab-lish chemical formulas and relative atomic and molecular masses. This chapter begins our study of chemical reactions. We start by developing the concept of the mole, which allows us to count molecules by weighing macroscopic quantities of matter. We examine the balanced chemical equations that summarize these reactions and show how to relate the masses of substances consumed to the masses of substances produced. This is an immensely practical and important subject. The questions how much of a substance will react with a given amount of another substance and how much product will be generated are central to all chemical processes, whether industrial, geological, or biological. 

A mole is Avogadro s number of anything N = 6.022 X 10. It allows us to count molecules by weighing. is the number of molecules in exactly 12 g C, by international agreement. 

If you missed 18, go to Compounds Moles and Counting Molecules, page 123. 

Previously, we saw how to use the balanced equation for a reaction to calculate the numbers of moles of reactants and products for a particular case. However, moles represent numbers of molecules, and we cannot count molecules directly. In chemistry we count by weighing. Therefore, we need to review the procedures for converting between moles and masses and see how these procedures are applied to chemical calculations. 

Because the mole is a counting unit, every balanced chemical equation can be interpreted in terms of moles—moles of atoms, moles of molecules, moles of ions, or anything else. For example. 

We ve seen the importance of the concept of the mole when dealing with chemical reactions involving macroscopic quantities of material. But generally it is not possible to measure the number of moles in a sample directly because that would imply that we can count molecules. Molar masses provide a crucial connection and allow us to convert from masses, which can be measured easily, to numbers of moles. The mass and the number of moles are really just two different ways of expressing the same information—the amount of a substance present. The molar mass functions much like a unit conversion between them, allowing us to go from... 

When we prepare a compound industrially or even study a reaction in the laboratory, we deal with tremendous numbers of molecules or ions. Suppose you wish to prepare acetic acid, starting from 10.0 g of ethanol. This small sample (less than 3 tea-spoonsful) contains 1.31 X 10 molecules, a truly staggering number. Imagine a device that counts molecules at the rate of one million per second. It wonld take more than four billion years—nearly the age of the earth—for this device to count that many molecules Chemists have adopted the mole concept as a convenient way to deal with the enormons numbers of molecules or ions in the samples they work with. 

Recall that in Chapter 2 a mole was defined as an amount of substance having the same number of elementary entities as there are atoms in exactly 12 g of pure carbon-12. This definition carefully avoids saying that the entities to be counted are always atoms. As a result, we can apply the concept of a mole to any quantity that we can represent by a symbol or formula—atoms, ions, formula units, or molecules. Specifically, a mole of compound is an amount of compound containing Avogadro s number (6.02214 X 10 ) of formula units or molecules. The molar mass is the mass of one mole of compound—one mole of molecules of a molecular compound and one mole of formula units of an ionic compound. 

Avogadro s number, L The number of particles (atoms or molecules) in one mole of any pure substance. L = 6 023 x 10. It has been determined by many methods including measurements of Brownian movement, electronic charge and the counting of a-particles. 

Chemists report numbers of atoms, ions, and molecules in terms of a unit called a mole. A mole is the analog of the wholesaler s dozen. A dozen could be defined as the number of soda cans in a twelve pack carton supplied by a wholesaler. Even if you could not open the carton to count the number of cans inside, you could find out how many cans are in a dozen by weighing the carton and dividing the mass of the carton by the mass of one can. A similar approach is used to define a mole (abbreviated mol) ... 

To determine a chemical formula, we would like to count the atoms of each element in one molecule of the compound. Atoms are too small to count, but we might hope to measure the number of moles of each element present in one mole of the compound. Unfortunately, there is no direct experimental method for measuring moles. Instead, laboratory experiments give the masses of the various elements contained in some total mass of the compound. 

Chemists routinely deal with hunks of material containing trillions of trillions of atoms, but ridiculously large numbers can induce migrmnes. For this reason, chemists count particles (like atoms and molecules) in multiples of a quantity called the mole. 

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