Mole-to-gram problems

Up to this point, we have dealt with mole-to-mole, gram-to-gram and mole-to-gram problems. That leaves us with gram-to-mole problems. 

As an example of gram-to-mole problem solving, we use the following equation ... 

Solve these kinds of problems by using the definition of molarity and conversion factors. In parts (b) and (c), you must first convert your mass in grams to moles. To do so, you divide by the molar mass from the periodic table (flip to Chapter 7 for details). In addition, be sure you convert milliliters to liters. 

Let s say that you want to find an empirical formula from the percentage composition. First, convert the mass percentage of each element to grams. Second, convert from grams to moles using the molar mass of each element as a conversion factor. (Keep in mind that a formula for a compound can be read as a number of atoms or as a number of moles.) Third, as shown in Sample Problem C, compare these amounts in moles to find the simplest whole-number ratio among the elements in the compound. 

Just as with the elements, these are gram-to-mole and mole-to-gram conversions, and they are done in exactly the same way. The key is getting the appropriate conversion factor. Carbon dioxide, C02, will be used as a typical compound in the following problems. 

The problem states that we have 35.0 g of aluminum, so we must convert from grams to moles of aluminum. This is something we already know how to do. Using the table of average atomic masses inside the front cover of this book, we find the atomic mass of aluminum to be 26.98. This means that 1 mole of aluminum has a mass of 26.98 g. We can use the equivalence statement... 

This is a somewhat elaborate unit conversion problem. We have to determine the number of SO2 molecules present in a ton—so the molar mass would have to be calculated after looking up the appropriate molar masses for sulfor and oxygen. Then, convert tons to pounds, pounds to grams, grams to moles, and moles to molecules (using Avogadro s number). This number would be the number of molecules per 300 so we d have to divide by 300 to get the number of molecules per dollar. 

Lean H Now You have reached a critical point in your study of chemistry. Students usually have no difficulty changing grams to moles, moles of one substance to moles of another, and moles to grams when those operations are presented as separate problems. When the operations are combined in a single problem, however, trouble sometimes follows. It doesn t help when, as in the following sections, many other ideas are added. In other words, you will not understand the following concepts unless you have already mastered this one. 

Gram-to-Mole Conversions This is a solution plan for problems in which the given quantity is expressed in grams, and the unknown quantity is expressed in moles. 

Molarity is the number of moles of solute per liter of solution. The solute is described in the problem by mass, not the amount in moles. You need one conversion (grams to moles of solute) using the inverted molar mass of NaCl to arrive at your answer. 

Strategy Convert reactant grams to moles, rmd determine which is the limiting reactant Use the balanced equation to determine the number of moles of aspirin that can be produced, and convert this number of moles to grams for the theoretical yield. Use the actual yield (given in the problem) and the calculated theoretical yield to calculate the percent yield. 

We can now change from moles to grams, or grams to moles, using the conversion factors derived from the molar mass as shown in Sample Problem 7.4. (Remember, you must determine the molar mass first.)... 

Set up the problem to convert moles to grams (or grams to moles). 

This problem gives the quantities of the two reactants in grams we must first change them to moles ... 

Ans. Ideal gas law problems involve moles. If the number of moles of gas is given or asked for, or if a quantity that involves moles is given or asked for, the problem is most likely an ideal gas law problem. Thus, any problem involving masses of gas (which can be converted to moles of gas) or molecular weights (grams per mole) or numbers of individual molecules (which can be converted to moles) and so forth is an ideal gas law problem. Problems that involve an unchanging mass of gas are most likely not ideal gas law problems. 

In this problem we will convert from grams of hydrogen to moles of hydrogen to moles of ammonia using the correct stoichiometric ratio, and finally to grams of ammonia. And we will need the molar mass of H2 (2.0158 g/mol) and ammonia (17.0307 g/mol) ... 

BB 0.10 m. This problem gives you the value of your solute (sodium chloride) in grams and the value of your solvent (water) in liters. You must first convert the sodium chloride to moles. To do so, divide by the molar mass (which we discuss in Chapter 7) ... 

In these calculations, you will be converting either from moles of the given substance to grams of the unknown or from grams of the given substance to moles of the unknown. Either way, you will need to use the mole ratio as the mechanism for crossing over from one substance to the next. Other than the step with the mole ratio, these calculations are the same as the calculations from earlier in the chapter that converted mass to moles or moles to mass for a single substance. We ll use the same equation from the sample problem to continue with this example. 

That s all there is to it. Problems are made more difficult by giving you information such as lengths of time or fractions of faradays or by asking for quantities in grams instead of moles. Each of these permutations just adds another layer of conversion into the problem, but you should always keep in mind Faraday s laws as a means of doing a quick check on your work. 

Solution This problem starts out in the mass portion of the Mole-Go-Round. You have 22.0 grams of C02. (Notice how a number [22.0], units [gram], and substance [C02] are carefully recorded in each step of the problem.) The next step is to convert to moles by dividing by the molar mass. The molar mass for C02 is 44.0 grams/mole. This gives 0.50 moles of C02 as shown in Figure 6.2. 

This last example uses a dimensional analysis to convert between moles, mass, and volume. How many liters will 150 grams of S02(g) occupy at STP Remember the importance of keeping careful track of the numbers, units, and substance in a problem such as this one. Start by converting to moles and then to volume ... 

The strategy for doing this problem is to convert from milligrams of silicon to grams of silicon, then to moles of silicon, and finally to atoms of silicon ... 

Set up the problem using Avogadro s number to change to moles, then use the mole ratio, and finally use the molar mass of C5H8 to change to grams. ( Calculate. 

For an example, we will use the process for converting the amount of a substance in moles to mass in grams. (See Sample Problem D in the chapter on Atoms and Moles. )... 

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