Counting, by weighing

The Enzo Ferrari has a body nnade of carbon fiber composite materiais. 

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. 

OBJECTIVE To understand the concept of average mass and explore how counting can 

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. 

One very important chemical activity is the synthesis of new substances. Nylon, the artificial sweetener aspartame (Nutra-Sweet ), Kevlar used in bulletproof vests and the body parts of exotic cars, polyvinyl chloride (PVC) for plastic water pipes. Teflon, Nitinol (the alloy that remembers its shape even after being severely distorted), and so many other materials that make our lives easier—all originated in some chemist s laboratory. Some of the new materials have truly amazing properties such as the plastic that listens and talks, described in the Chemistry in Focus on page 213. When a chemist makes a new substance, the first order of business is to identify it. What is its composition What is its chemical formula  

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 

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The key point to remember is that this procedure allows us to determine the number of objects in a sample of a large number of those objects without actually counting them. Our procedure allows us to count by weighing. 

Objects do not need to have identical masses to be counted by weighing. We simply need to know the average mass of the objects. 

What information is needed in order to count by weighing ... 

However, before we do any calculations, let s be sure that the process of counting by weighing is clear. 

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. 

To this point, we have considered examples where the numbers of reactant molecules could be counted. In "real life" you can t count the molecules directly— you can t see them and, even if you could, there would be far too many to count. Instead, you must count by weighing. We must therefore explore how to find the limiting reactant, given the masses of the reactants. 

Explain the concept of "counting by weighing" using marbles as your example. 

To do chemical calculations, you mzrst understand what the mole means and how to determine the number of moles in a given mass of a substance. However, before we do any calculations, let s be sure that the process of counting by weighing is clear. Consider the following "bag" of H atoms (symbolized by dots), which contains 1 mole (6.022 x 10 ) of H atoms and has a mass of 1.008 g. Assume the bag itself has no mass. 

All you need to do is figure the molecular weights of each reactant and product, and then incorporate the weights into the equation. Use the periodic table to find the weights of the atoms and the compound (see the section Counting by Weighing, earlier in this chapter, for the directions) and multiply those numbers by the number of moles, like this ... 

Fie can now weigh 64.6 kg of apples and 65.1 kg of oranges and pack them without actually counting them. Manufacturers and suppliers often count by weighing. Other examples of counting by weighing include nuts, bolts, and candy. 

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