Stoichiometric calculations concentrations

These relationships provide complete stoichiometric information regarding the equilibrium. Just as amounts tables are usetiil in doing stoichiometric calculations, a concentration table that provides initial concentrations, changes in concentrations, and equilibrium concentrations is an excellent way to organize Step 5 of the problem-solving... 

When two or more substances are mixed together in a manner that is homogeneous and uniform at the molecular level, the mixture is called a solution. The component (usually a liquid) that is present in much larger quantity than the others is called the solvent the other components are the solutes. The concentration of a solution describes the amount of solute present in a given amount of solution. When a solution is involved in a reaction, the stoichiometric calculations must take into account two quantities not previously discussed the concentration of the solution, and its volume. 

The methyl esters can be also determined by GC-FID. Using a 30 m x 0.32 mm ID x 0.25 pm (film thickness) capillary column, such as DB-1701 or equivalent, the compounds can be adequately separated and detected by FID. The recommended carrier gas (helium) flow rate is 35 cm/s, while that of the makeup gas (nitrogen) is 30 cm/min. All of the listed herbicides may be analyzed within 25 min. The oven temperature is programmed between 50 and 260°C, while the detector and injector temperatures should be 300 and 250°C, respectively. The herbicides may alternatively converted into their trimethylsilyl esters and analyzed by GC-FID under the same conditions. FID, however, gives a lower response as compared with ECD. The detection level ranges from 50 to 100 ng. For quantitation, either the external standard or the internal standard method may be applied. Any chlorinated compound stable under the above analytical conditions, which produces a sharp peak in the same RT range without coeluting with any analyte, may be used as an internal standard for GC-ECD analysis. U.S. EPA Method 8151 refers the use of 4,4,-dibromooctafluorobiphenyl and 1,4-dichlorobenzene as internal standards. The quantitation results are expressed as acid equivalent of esters. If pure chlorophenoxy acid neat compounds are esterified and used for calibration, the results would determine the actual concentrations of herbicides in the sample. Alternatively, if required, the herbicide acids can be stoichiometrically calculated as follows from the concentration of their methyl esters determined in the analysis ... 

Stoichiometric calculations involving solutions of specified molar concentration are usually quite simple since the number of moles of a reactant or product is simply volume x molar concentration. 

Sometimes, the calculation involves a monoprotic acid and a dihydroxy base or another set of conditions in which the relationship is not 1 1. We have to keep track of the various concentrations so that the molarities do not get mixed up. However, stoichiometric calculations involving solutions of specified normalities are even simpler. By the definition of equivalent mass in Chapter 12, two solutions will react exactly with each other if... 

Because virtually all stoichiometric calculations involve moles (abbreviated mol) of material, molarity is probably the most common concentration unit in chemistry. If we dissolved 1.0 mol of glucose in enough water to give a total volume of 1.0 L, we would obtain a 1.0 molar solution of glucose. Molarity is abbreviated with a capital M. Notice that, because molarity has units of moles per liter, molar concentrations are conversion factors between moles of material and liters of solution. 

Molality and molarity are each very useful concentration units, but it is very unfortunate that they sound so similar, are abbreviated so similarly, and have such a subtle but crucial difference in their definitions. Because solutions in the laboratory are usually measured by volume, molarity is very convenient to employ for stoichiometric calculations. However, since molarity is defined as moles of solute per liter of solution, molarity depends on the temperature of the solution. Most things expand when heated, so molar concentration will decrease as the temperature increases. Molality, on the other hand, finds application in physical chemistry, where it is often necessary to consider the quantities of solute and solvent separately, rather than as a mixture. Also, mass does not depend on temperature, so molality is not temperature dependent. However, molality is much less convenient in analysis, because quantities of a solution measured out by volume or mass in the laboratory include both the solute and the solvent. If you need a certain amount of solute, you measure the amount of solution directly, not the amount of solvent. So, when doing stoichiometry, molality requires an additional calculation to take this into account. 

For a stoichiometric feed concentration of acetic acid instead, the optimal resin capacity was calculated as 5 mEq g-1, which is the original Amberlyst 15 resin. Further optimization, including also the feed concentration of acetic acid as a decision variable, yielded an eluent requirement of ca. 3 mol methanol per mol methyl acetate for a 5 mEq g-1 resin and 60 40 acetic acid methanol feed stream (Fig. 6.17). 

A more useful way to describe a concentration is molarity. Molarity (M) expresses the number of moles of solute per liter of solution. A 0.1 M NaOH aqueous solution has 0.1 mol of solute (NaOH) in 1 L of water. Because stoichiometric calculations require moles, molarity is more frequently used in calculations. 

Many environmental reactions and almost all biochemical reactions occur in solution, so an understanding of reactions in solution is extremely important in chemistry and related sciences. We ll discuss solution chemistry at many places in the text, but here we focus on solution stoichiometry. Only one aspect of the stoichiometry of dissolved substances is different from what we ve seen so far. We know the amounts of pure substances by converting their masses directly into moles. For dissolved substances, we must know the concentration—the number of moles present in a certain volume of solution—to find the volume that contains a given number of moles. Of the various ways to express concentration, the most important is molarity, so we discuss it here (and wait until Chapter 13 to discuss the other ways). Then, we see how to prepare a solution of a specific molarity and how to use solutions in stoichiometric calculations. 

Chemical reactions often take place when two solutions are mixed. To perform stoichiometric calculations in such cases, we must know two things (1) the nature of the reaction, which depends on the exact forms the chemicals take when dissolved, and (2) the amounts of chemicals present in the solutions, usually expressed as concentrations. 

So far we have used moles or concentrations in stoichiometric calculations. However, it is equally valid to use pressures for a gas-phase system at constant... 

The next step recognizes the stoichiometric requirement that 1 mol of HCl reacts with one and only 1 mol of KOH. We must therefore calculate the numbers of moles of reaction components in the solutions prior to reaction so that we can calculate concentrations after the reaction. This is sometimes called the mole method. 

When doing stoichiometric calculations, the assumption often made is that the reaction goes to completion. This is a convenient assumption when focusing on calculations involving mole ratios and limiting reagents, but there are many examples of commercially and biologically important chemical reactions that do not go to completion. Rather, appreciable amounts of reactants and products remain in the reaction mixture once equilibrium is reached. When viewed macroscopically, the concentrations of all reactants and products remain constant, but not necessarily equal, over time. 

Plan For (a), we need to think about how add can react with calcium carbonate, a reaction that evidently does not happen with acid and granite. For (b), we need to think about what reaction between an acid and CaO is possible and do stoichiometric calculations. From the proposed change in pH, we can calculate the change in proton concentration needed and then... 

Solve Stoichiometric Calculation The product of the volume and concentration of each solution gives the number of moles of each reactant present before the neutralization ... 

Do stoichiometric calculations based on solution concentrations. (Section 7.6)... 

For aqueous electrolytes the ionic association become important when b is higher than 5, a value typical of a 2 2 electrolyte at room temperature or a 1 1 electrolyte above 300 °C. Thus, the extrapolation of the apparent partial volume of these electrolytes at infinite dilution to obtain the standard partial molar volume is uncertain, because the free ions concentration depends on the stoichiometric electrolyte concentration. For a 2 2 electrolyte, as MgS04, at 25 °C the apparent partial molar volume approach the DHLL value at concentrations bellow 0.01 mol kg (Franks and Smith, 1967) and, at least the density could be measured with a precision of 1 ppm, V° for MgS04 can not be obtained by extrapolation. In this case one can calculate the standard partial molar volume from the known values of standard partial volume of 1 2 and 1 1 electrolytes by using the additivity rule (Lo Surdo et al, 1982) ... 

Rate coefficient calculated from the stoichiometric reactant concentrations present, i.e. Rate = A [Amine][Nitrous Acid]. 

Do the problem in two parts. First, you assume that the H30 ion from the strong acid and the conjugate base from the buffer react completely. This is a stoichiometric calculation. Actually, the HgO ion and the base from the buffer reach equilibrium just before complete reaction. So you now solve the equilibrium problem using concentrations from the stoichiometric calculation. Because these concentrations are not far from equilibrium, you can use the usual simplifying assumption about x. 

Figure 31. Plot of calculated pH against stoichiometric HCl concentration for T = 525°C and P = 5000 psi (closed cireles). The measured pH for the hydrolyzed CCU solution is given by the elosed triangle. Reprinted from Ref. 5, Copyright (1997) with permission by Elsevier.

Using the program PREMIX (Kee et al. 1985), premixed, stationary, laminar, one-dimensional methane-air flames were simulated. The cold mixture boundary conditions were p= atm and T = 298.15 K. The calculations were carried out for lean ( = 0.70), stoichiometric equivalence ratios. PREMIX calculated not only the concentration — distance curves, but also normed sensitivities dlnYJd and the sensitivities of the calculated concentrations with respect to the enthalpies of formation of species. 

Zsely et al. (2003) performed numerical experiments to investigate this consequence of global similarity. Initially the concentration profiles were calculated for simulations of the adiabatic explosion of a stoichiometric hydrogen-air mixture using a nominal parameter set based on the values recommended by Baulch et al. (2005). Local sensitivity analysis was then used to select those parameters with the largest influence oti the simulated species concentrations based on a study of A-factors for the reaction rate coefficients. Five reactions were selected as dominating the influence on the calculated concentrations. At the next stage, the... 

To support the idea that all discrepancies in the results can be explained by the initial pressure value, but not by the experimental setup geometries or the methods of the limit determination, the calculated concentration limits for the stoichiometric mixture in the same pressure range as the measurement data are presented in Fig. 4.6. The concentration limits found in [8] and [9] are denoted by the points. 

Fig. 4.6 The change in H2 and water steam concentration limits versus the pressure change in a stoichiometric mixture H2 + O2 + H2O at 373 K temperature and different energy of ignition. Comparison between the calculated concentration limits (curves) and the measured data [8, 9]...

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