Masses of reactants and products

The results of one of Lavoisier s quantitative experiments are shown in Table A the data are taken directly from Lavoisier. If you add up the masses of reactants and products (expressed in arbitrary units), you find them to be the same, 510. As Lavoisier put it, "In all of the operations of men and nature, nothing is created. An equal quantity of matter exists before and after the experiment."... 

The principal reason for writing balanced equations is to make it possible to relate the masses of reactants and products. Calculations of this sort are based on a very important principle ... 

The equations you read and write in the preceding section are skeleton equations, and they re perfectly adequate for a qualitative description of the reaction What are the reactants, and what are the products But if you look closely, you ll see that those equations just don t add up. As written, the mass of 1 mol of each of the reactants doesn t equal the mass of 1 mol of each of the products (see Chapter 7 for details on moles). The skeleton equations brecikthe law of conservation of mass, which states that all the mass present at the beginning of a reaction must be present at the end. To be quantitatively accurate, these equations must be balanced so the masses of reactants and products cire equal. 

Chemical reactions only rearrange the way that atoms are combined the atoms themselves are unchanged. Dalton realized that atoms must be chemically indestructible for the law of mass conservation to be valid. If the same numbers and kinds of atoms are present in both reactants and products, then the masses of reactants and products must also be the same. 

We need to calculate the amount of methyl tert-bu tyl ether that could theoretically be produced from 26.3 g of isobutylene and compare that theoretical amount to the actual amount (32.8 g). As always, stoichiometry problems begin by calculating the molar masses of reactants and products. Coefficients of the balanced equation then tell mole ratios, and molar masses act as conversion factors between moles and masses. 

The relative numbers of reactant and product molecules (or the relative numbers of moles) are indicated by the coefficients of a balanced chemical equation. Using molar masses, we can compute the relative masses of reactants and products in a chemical reaction. 

As you have learned, the coefficients in a balanced chemical equation represent moles as well as particles. Therefore, you can use the molar masses of reactants and products to determine the mass ratios for a reaction. For example, consider the equation for the formation of ammonia ... 

The empirical laws governing the masses of reactants and products of chemical reactions formed the basis for the theoretical development of chemistry. 

Recall that this equation means that 1 mole of C3H8 will react with 5 moles of 02 to produce 3 moles of C02 and 4 moles of H20. To use this equation to find the masses of reactants and products, we must bt able to convert between masses and moles of substances. Thus we must first ask How many... 

Calculation of Masses of Reactants and Products in Chemical Reactions... 

Solving a Stoichiometry Problem Involving Masses of Reactants and Products... 

Write an equation showing conservation of mass of reactants and products. 

For most practical purposes we are interested in the masses of reactants and products, because those are the quantities that are directly measured. In this case, the molar masses (calculated from a table of atomic masses) are used to convert the number of moles of a substance (in moles) to its mass (in grams), as illustrated by Example 2.6. Sometimes, however, we are also interested in knowing the number of molecules in a sample. The mole allows us to convert easily from mass to numbers of molecules as follows ... 

From the number of moles and molar masses of reactants and products, the following desired masses are found ... 

Given a set of initial masses of reactants and a balanced chemical equation, determine the limiting reactant and calculate the masses of reactants and products after the reaction has gone to completion (Section 2.6, Problems 47 and 48). 

The reader should note that although the number of moles on both sides of the equation do not balance, the masses of reactants and products (in line with the conservation law for mass) must balance. 

Interpret balanced chemical equations to calculate the masses of reactants and products involved in each of the reactions... 

The equation now tells us that 16.0 grams of CH4 reacts with 64.0 grams of O2 to form 44.0 grams of CO2 and 36.0 grams of H2O. The Law of Conservation of Matter is satisfied. Chemical equations describe reaction ratios, that is, the mole ratios of reactants A balanced equation may be and products as well as the relative masses of reactants and products. interpreted on a mass basis. 

In 1905, Albert Einstein created a mathematical model of Brownian motion based on the impact of water molecules on suspended particles. Kinetic molecular theory could now be observed under the microscope. Einstein s more famous later work in physics on relativity may be applied to chemistry by correlating the energy change of a chemical reaction with extremely small changes in the total mass of reactants and products. 

Suppose you wanted to produce 500 g of methanol. How many grams of CO2 gas and H2 gas would you need How many grams of water would be produced as a by-product Those are questions about the masses of reactants and products. But the balanced chemical equation shows that three molecules of hydrogen gas react with one molecule of carbon dioxide gas. The equation relates molecules, not masses, of reactants and products. 

The concept of molar mass makes it easy to determine the number of particles in a sample of a substance by simply measuring the mass of the sample. The concept is also useful in relating masses of reactants and products in chemical reactions. 

Now let s work a problem that is similar but deals with masses of reactants and products rather than moles. If 16.491 g of carbon are heated with 32.654 g of silica, what is the maximum mass of metallurgical grade silicon. Si, that can be formed ... 

Skill 2.1b-Calculate the masses of reactants and products, and percent yield using balanced chemical equations, including problems with a limiting reagent... 

The mass changes of energetic chemical reactions are too small to measure. It s a safe assumption that mass is conserved for chemical reactions. However, in nuclear reactions, we can obtain values for the masses of reactants and products. 

Chemical equations are an indispensable way of representing reactions. They are routinely used to calculate masses of reactants and products. In the case of the examples used above for the Law of Multiple Proportions, the equations are ... 

To learn to relate masses of reactant and products in a chemical reaction... 

The process of using a chemical equation to calculate the relative masses of reactants and products involved in a reaction is called stoichiometry (pronounced stoy ke om etry). Chemists say that the balanced equation for a chemical reaction describes the stoichiometry of the reaction. 

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