Volume mole conversions

See the earlier section Doing Mass and Volume Mole Conversions. ... 

Now, move on to Step 3, which is to convert the given quantity into number of moles. This conversion represents a volume-mole conversion, the type we discussed in Lesson 7-2. We convert the volume of a gas at STP to number of moles, by dividing by the molar volume (22.4 dm3/mole) of a gas. As always, the specific type of the gas is irrelevant. 

Such problems as this one involve many steps or conversions. Try to break the problem into simpler ones involving fewer steps or conversions. It may also help to remember that solving a stoichiometry problem involves three steps (1) converting to moles, (2) converting between moles, and (3) converting from moles. Use molarities and molar masses to carry out volume-mole conversions and gram-mole conversions, respectively, and stoichiometric factors to carry out mole-mole conversions. The stoichiometric factors are constructed from a balanced chemical equation. 

Because moles are the currency of chemistry, all stoichiometric computations require amounts in moles. In the real world, we measure mass, volume, temperature, and pressure. With the ideal gas equation, our catalog of relationships for mole conversion is complete. Table lists three equations, each of which applies to a particular category of chemical substances. 

The basic calculations fall into two categories. Simple calculations, such as the change in temperature or the change in volume, are the easiest to forget. Simple calculations may also include mass-to-mole conversions. The other calculations normally involve entering values into one of the equations given at the beginning of the previous chapters of this book. 

The conversion between concentration units and the expression of the units themselves can be confusing. We will now review the typical concentration units used in various environmental media. Concentration in water is usually given as mass per unit volume or moles per unit volume. The conversion between them is a straightforward application of molecular weights. For example, we have 2.0 g/m of CO2 dissolved in water. The molecular weight of carbon dioxide is 44 g/mole. Then the concentration in moles/m is... 

If you look at Figure 9-2, you can see that it isn t possible to convert directly between the mass of one substance and the mass of another substance. You must convert to moles and then use the mole-mole conversion factor before converting to the mass of a new substance. The same can be said for conversions from the particles or volume of one substance to that of another substance. The mole is always the intermediary you use for the conversion. 

These calculations show that the volume dependency of a gas-phase reaction is a function not only of the stoichiometry, but also of the inerts content of the reacting mixture. The sensitivity of volume to conversion is lowered as the inerts increase. The expansion factor, eA, is positive for reactions producing a net increase in moles, negative for a decrease in moles, and eA= 0 for reactions producing no net changes and at constant volume. 

Volume variations with conversion are large for constant-pressure gas-phase reactions with change in mole number. Here, as a rule, operation at constant volume poses no difficulties. Liquid-phase reactions may also entail volume contraction or expansion. However, these are not related to changes in mole number and can be predicted only if information on partial molar volumes is at hand. Because liquids are essentially incompressible, even at elevated temperature, it is unsafe to conduct liquid-phase reactions without a gas cap in a closed reactor. Some variation of liquid-phase volume with conversion therefore is apt to occur. Fortunately, the variation at constant temperature is usually so small that it can be neglected in the evaluation or accounted for by a minor correction. 

The only new twist in calculating reactor volumes or conversions for a recycle reactor is a mole balance at the stream intersections (points P and Q) to express properly the species concentrations as a function of conversion. 

If we assume that the cooling coil takes up negligible reactor volume, the conversion calculated as a function of temperature from the mole balance is the same as that in Example S-4 [Equation (E8-4.13)]. 

When reactants are liquids, they are almost always measured by volume. So, to do calculations involving liquids, you add two more steps to the sequence of mass-mass problems—the conversions of volume to mass and of mass to volume. Five conversion factors—two densities, two molar masses, and a mole ratio—are needed for this type of calculation, as shown in Skills Toolkit 4. 

First, however, we convert from mass of C2H5OH to moles, using the compound s molar mass. Then we set up the mole-to-mole conversion, using the molar ratio of ethanol to carbon dioxide derived from the coefficients in the balanced equation. The next step is the new one we use the molar volume at STP to convert from moles of CO2 to volume of CO2 at STP. The sequence as a whole is... 

The stoichiometry path may be summarized as given quantity mol given mol wanted wanted quantity. In a gas stoichiometry problem, the first or third step in the path is a conversion between moles and liters of gas at a given temperature and pressure. If you are given volume, you must convert to moles if you find moles of wanted substance, you must convert to volume. These conversions are made with the ideal gas equation, PV = nRT. You have already made conversions like these. For example, in Example 14.3, you calculated the volume occupied by 0.393 mol N2 at 24°C and 0.971 atm. You used the ideal gas equation solved for V. 

To find molarity from its definition (Equation 16.2), you need to know the volume of solution, which is given, and the moles of NaOH. Grams are given. The grams-to-moles conversion must be made before you can use the defining equation. How many moles of NaOH are in the solution ... 

The first two steps this time are the same as in the last example. Set up that far. We ll use the one-step shortcut volume-to-moles conversion mentioned in Example 16.16, thinking of molarity as mol/1000 mL. 

This case involves constant temperature T and total pressure n. In this case, the density changes since tlie numher of moles change during the reaction, and the volume of a fluid element changes linearly with conversion or V = Vo(l -i- a a)- The relationship between C and is as follows ... 

Since the volume depends on conversion or time in a constant pressure batch reactor, consider the mole balance in relation to the fractional conversion X. From the stoichiometry. 

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