Mass of a Compound

In Chapter 2 (Section 2.9), we saw that an element s molar mass—the mass in grams of one mole of its atoms—is numerically equivalent to its atomic mass. We then used the molar mass in combination with Avogadro s number to determine the number of atoms in a given mass of the element. The same concept applies to compounds. The molar mass of a compound—the mass in grams of 1 mol of its molecules or formula units—is numerically equivalent to its formula mass. For example, we just calculated the formula mass of CO2 to be 44.01 amu. The molar mass is, therefore  

Remember, ionic compounds do not contain individuai moiecuies. in casuai ianguage, the smaiiest eiectricaiiy neutrai coiiection of ions is sometimes caiied a moiecuie but is more correctiy caiied a formuia unit. 

---

Summarizing, we can determine an analyte gravimetrically by directly determining its mass, or the mass of a compound containing the analyte. Alternatively, we can determine an analyte indirectly by measuring a change in mass due to its loss, or the mass of a compound formed as the result of a reaction involving the analyte. 

The Rast method uses camphor (Ci0Hi O) as a solvent for determining the molar mass of a compound. When 2.50 g of cortisone acetate is dissolved in 50.00 g of camphor (kf = 40.0°C/m), the freezing point of the mixture is determined to be 173.44°C that of pure camphor is 178.40°C. What is the molar mass of cortisone acetate ... 

The molar mass of a compound, the mass per mole of its molecules or formula units, is used to convert between the mass of a sample and the amount of molecules or formula units that it contains. 

Calculate the molar mass of a compound, given its chemical formula. 

What is the molar mass of a compound that takes 2.7 times as long to effuse through a porous plug as it did for the same amount of XeF, at the same temperature and pressure ... 

The molar mass of a compound is found by adding together the molar masses of all of its elements, taking into account the number of moles of each element present. 

Compounds that do not decompose cleanly into their elements must be analyzed by other means. Combustion analysis is particularly useful for determining the empirical formulas of carbon-containing compounds. In combustion analysis, an accurately known mass of a compound is burned in a stream of oxygen gas. The conditions are carefully controlled so that all of the carbon in the sample is converted to carbon dioxide, and all of the hydrogen is converted to water. Certain other elements present in the sample are also converted to their oxides. 

C03-0042. Diagram the process for converting from the mass of a compound of a known chemical formula to the number of atoms of one of its constituent elements. Include all necessary equations and conversion factors. 

One of the more powerful techniques is a new software tool called mass defect filtering.176 185-188 A mass defect can be defined as the difference between the exact mass and nominal mass of a compound.189 Typically, drug-like molecules (and their metabolites) will have mass defects that differ from those of endogenous matrix materials. While a mass spectrometer that has unit mass resolution cannot differentiate a test compound from an isobaric matrix compound, a high mass resolution MS may be able to differentiate many isobaric matrix compounds from test compounds. 

The substance s molar mass is the mass in grams of the substance that contains one mole of that substance. In the previous chapter, we described the atomic mass of an element in terms of atomic mass units (amu). This was the mass associated with an individual atom. At the microscopic level, we can calculate the mass of a compound by simply adding together the masses in amu s of the individual elements in the compound. However, at the macroscopic level, we use the unit of grams to represent the quantity of a mole. 

The mole (mol) is the amount of a substance that contains the same number of particles as atoms in exactly 12 grams of carbon-12. This number of particles (atoms or molecules or ions) per mole is called Avogadro s number and is numerically equal to 6.022 x 1023 particles. The mole is simply a term that represents a certain number of particles, like a dozen or a pair. That relates moles to the microscopic world, but what about the macroscopic world The mole also represents a certain mass of a chemical substance. That mass is the substance s atomic or molecular mass expressed in grams. In Chapter 5, the Basics chapter, we described the atomic mass of an element in terms of atomic mass units (amu). This was the mass associated with an individual atom. Then we described how one could calculate the mass of a compound by simply adding together the masses, in amu, of the individual elements in the compound. This is still the case, but at the macroscopic level the unit of grams is used to represent the quantity of a mole. Thus, the following relationships apply ... 

The formula of a compound is determined by using the mass of the original substance, usually a metal, and the mass of a compound of that substance, usually an oxide. (See the chapter on Stoichiometry.)... 

Density (the mass of a compound per unit volume) is an important factor to consider for the separation of immiscible liquids. Two phases should have sufficient... 

The exact mass of a compound determined by high-resolution mass spectrometry is 212.0833. What is the molecular formula of the compound ... 

Thus, if we measure the mass of a compound with a relative molecular mass of 1000 to an accuracy of 0.001 mass units, we have measured its mass with an accuracy of 1 ppm. 

The atomic mass of an element is the number of grams containing Avogadro s number of atoms.1 The molecular mass of a compound is the sum of atomic masses of the atoms in the molecule. It is the number of grams containing Avogadro s number of molecules. 

The molar masses of molecular and ionic compounds are calculated from the molar masses of the elements present the molar mass of a compound is the sum of the molar masses of the elements that make up the molecule or the formula unit. We need only note how many times each atom or ion appears in the molecular formula or the formula unit of the ionic compound. For example, the molar mass of the ionic compound Na2S04 is... 

Once the molar mass of a compound has been calculated, the same technique applied in Example E.2 can be used to determine how many moles of molecules or formula units are in a sample of a given mass. 

The mass of 1 mole of a compound is called its molar mass. If you write the molar mass of a compound without any units then it is the relative formula mass, often called the relative molecular mass (Mr). So the relative formula mass of water is 18. 

The mass of a compound found in any number of moles can be calculated using the relationship ... 

The molecular weight (mass) of a compound is obtained by adding the atomic weights (masses) of all the constituent atoms. 

The molecular mass of a compound is the average mass (in u) of a molecule, weighted among the various isotopic forms (isotopomers) of the different component elements. A nuclidic molecular mass may be defined for a molecule made up of particular nuclides by adding nuclidic atomic masses in the same way that the molecular mass is computed from average atomic masses. 

A chemist had to determine the molar mass of a compound in the liquid state, but was concerned that it would decompose if heated. So he used a syringe to inject 0.436 grams of the liquid sample into a 5.00-L flask with argon at 17°C and... 

The average mass of all molecules of a compound is its molecular mass (formerly called molecular weight). The molecular mass of a compound is calculated by multiplying the atomic mass of each element by the relative number of atoms of the element, then adding all the values obtained for each element in the compound. For example, the molecular mass of NH3 is 14.0 + 3 x 1.0 = 17.0. For another example, consider the following calculation of the molecular mass of ethylene, C2H4 ... 

In terms of c, h, o, and the appropriate atomic masses, write a formula for the molecular mass of a compound with a general formula of CcHh00. 

Mass spectrometry is used to measure the molecular mass of a compound and provides a method to obtain the molecular formula. It differs from the other instrumental techniques presented thus far because it does not involve the interaction of electromagnetic radiation with the compound. Instead, molecules of the compound being studied are bombarded with a high-energy beam of electrons in the vapor phase. When an electron from the beam impacts on a molecule of the sample, it knocks an electron out of the molecule. The product, called the molecular ion (represented as A/f), has the same mass as the original molecule but has one less electron. It has both an odd number of... 

Mass spectrometry (Section 15.4) An instrumental technique used to measure the molecular mass of a compound provides information about the structure of the compound from the masses of fragments that are produced from the compound. 

While you can find the molar mass of an element just by looking at the periodic table, you need to do some calculations to find the molar mass of a compound. For example, 1 mol of beryllium oxide, BeO, contains 1 mol of beryllium and 1 mol of oxygen. To find the molar mass of BeO, add the mass of each element that it contains. 

Examine the following Sample Problem to learn how to determine the molar mass of a compound. Following Investigation 5-A on the next page, there are some Practice Problems for you to try. 

How do the molar masses of CeH6 and C2H2 compare with the molar mass of their empirical formula How does the molar mass of water compare with the molar mass of its empirical formula Describe the relationship between the molar mass of a compound and the molar mass of the empirical formula of the compound. 

---