Calculating formula masses

Use at least three significant digits in formula mass calculations. 

From this point on, our formula mass calculation setups are written horizontally unless we have a special point to illustrate. 

Nominal mass The mass calculated for an ion when using the integer mass values of the most abundant isotope of each element in the formula (e.g.. C = 12, O = 16, S = 32). 

C03-0073. Determine the molecular formula and calculate the molar mass of each of the following essential amino acids ... 

C03-0106. The following molecules are known for their characteristic fragrances. For each one, convert the line structure into a complete structural formula and calculate its molar mass. 

Figure 1.1 The body mass index (BMI) is a formula that calculates whether a person is an ideal body weight, overweight, or obese. The formula used to calculate BMI considers a person s weight and height. BMI has some limitations. It may not be accurate for people who are very muscular, such as bodybuilders, and people with very little muscle mass, such as the elderly. BMI is one of several factors health-care providers consider when they determine whether a person is overweight or obese. Note that this BMI chart is designed for adults who are age 20 or older.

In the problems above, the percentage data was calculated from the chemical formula, but the empirical formula can be determined if the percent compositions of the various elements are known. The empirical formula tells us what elements are present in the compound and the simplest whole-number ratio of elements. The data may be in terms of percentage, or mass, or even moles. But the procedure is still the same convert each to moles, divide each by the smallest number, then use an appropriate multiplier if needed. The empirical formula mass can then be calculated. If the actual molecular mass is known, dividing the molecular mass by the empirical formula mass gives an integer (rounded if needed) that is used to multiply each of the subscripts in the empirical formula. This gives the molecular (actual) formula, which tells which elements are in the compound and the actual number of each. 

Measurements include initial and final masses, and initial and final volumes. Calculations may include the difference between the initial and final values. Using the formula mass and the mass in grams, moles may be calculated. Moles may also be calculated from the volume of a solution and its molarity. 

The mass of the sample is calculated by taking the difference between masses 1 and 2. The mass of the substance that combined with the original substance is calculated from the difference between masses 2 and 3- The moles of the substances must be calculated by dividing each mass by the molar mass of the substance. The empirical formula is calculated from the simplest ratio of the moles of the elements present. 

The following gases are or have been used in aerosol cans, some as propellants. Use the gases molecular formulas to calculate their molar masses. 

To determine a moleculcir formula, you must know the gram formula mass of the compound as well as the empirical formula (or enough information to calculate it yourself from the percent composition see the preceding section for details). With these tools in hand, calculating the molecular formula involves three steps ... 

The empirical formula mass is 97.1 g/mol, which you calculate by multiplying the number of atoms of each element in the compound by the element s atomic mass and adding them all up ... 

To calculate mass percent, you must know the mass of solute and the mass of solution. The molarity of the solution tells you the moles of solute per volume of solution. Stcirting with this information, you can convert to mass of solute by means of the gram formula mass (see Chapter 7 for details on calculating the grcim formula mass) ... 

For the nitrogen rule to hold, only unit atomic unit masses (i.e., integers) are used in calculating the formula masses. 

The precise mass corresponding to the molecular formula C15H120 calculated with the more abundant isotopic masses present is as follows ... 

Name the product obtained in Figure 27-3 when calcium salicylate monohydrate is heated to 550°C and to 1 000°C. Using the formula masses of these products, calculate what mass is expected to remain when 0.635 6 g of calcium salicylate monohydrate is heated to 550°C or 1 000°C. 

L.8 The density of oak wood is 0.72 g-cm-3. Assuming oak wood to have the empirical formula CH20, calculate the mass of water produced when a log of dimensions 12 cm X 14 cm X 25 cm is burned. 

PROBLEM 3.4 Calculate the formula mass or molecular mass of the following substances ... 

The problem gives the number of moles of NaHC03 and asks for a mole-to-mass conversion. First, calculate the formula mass and molar mass of NaHC03. Then use molar mass as a conversion factor, and set up an equation so that the unwanted unit cancels. 

To find the multiple, calculate the ratio of the molecular mass to the empirical formula mass ... 

Calculate a formula mass corresponding to the empirical formula, and compare it with the molecular mass of ascorbic acid. 

Using the steps outlined in Figure 3.9, find the empirical formula of caproic acid, calculate a formula mass, and compare with the known molecular mass. 

For work in the laboratory, it s necessary to weigh reactants rather than just know numbers of moles. Thus, it s necessary to convert between numbers of moles and numbers of grams by using molar mass as the conversion factor. The molar mass of any substance is the amount in grams numerically equal to the substance s molecular or formula mass. Carrying out chemical calculations using these relationships is called stoichiometry. 

Volume 15 of this series features four important reviews of research on alkaloids. Chapter 1 by B. S. Joshi, S. W. Pelletier and S. K. Srivastava is the first comprehensive review of the carbon-13 and proton NMR shift assignments and physical constants of diterpene alkaloids and their derivatives. In addition to the catalogue of spectral and physical data, the chapter includes a table of the occurrences of these alkaloids in various plant species, tables containing molecular formulas versus calculated high-resolution mass values, and calculated high-resolution mass values versus the molecular formulas of diterpenoid alkaloids, as well as seven tables summarizing the carbon-13 chemical shifts of various functional groups in diterpenoid alkaloids. 

The approximate molar mass, calculated from the gas density data, is 89 g/mol. The empirical formula, calculated from the percentage composition data, is C2H3O with the empirical formula unit mass of 43.0. The exact molar mass must be (2)(43) = 86.0 g/mol since this is the only multiple of 43.0 (whole-number multiple) reasonably close to the approximate molecular formula of 89 g/mol. The molecule must be the equivalent of 2 empirical formulas CqHgO. 

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