Masses from Chemical Formulas

In Seetion 2.5, we calculated the atomic mass of an element. Using the periodic table and the formula of a eompound to see the number of atoms of eaeh element present, we ealeulate the molecular mass (also called molecular weight) of a formula unit of the eompound as the sum of the atomie masses  

The moleeular mass of a water moleeule (using atomic masses to four signifieant figures from the periodic table) is 

Molecular mass of H2O = (2 X atomic mass of H) -I- (1 X atomic mass of O) 

Note that atomic, not ionic, masses are used. Although masses of ions differ from those of their atoms by the masses of the eleetrons, electron loss equals electron gain in the compound, so electron mass is balanced. 

SAMPLE PROBLEM 2.13 Calculating the Molecular Mass of a Compound 

Ionic compounds are treated the same, but because they do not consist of molecules, we use the term formula mass for an ionic compound. To calculate its formula mass, the number of atoms of each element inside the parentheses is multiplied by the subscript outside the parentheses. For barium nitrate, Ba(N03)2, 

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V 132g (NH4)2S04 ) mo (NH4)2S04y molar mass from chemical formula... 

Add Names from Anion Names 57 Binary Covalent Compounds 58 The Simplest Organic Compounds Straight-Chain Alkanes 58 Molecular Masses from Chemical Formulas 59... 

You will calculate molar masses from chemical formulas (Section 7.4). 

Stenson, A. C., Landing, W. M., Marshall, A. G., and Cooper, W. T. (2003). Exact masses and chemical formulas of individual Suwannee River fulvic acids from ultrahigh resolution electrospray ionisation fourier transform ion cyclotron resonance mass spectrometry. Anal. Chem. 75,1275-1284. 

For H35C1, a)e = 2,989cm 1 and n is 0.9799. Then, its K is 5.16 x 105 (dynes/cm) or 5.16 (mdyn/A). If such a calculation is made for a number of diatomic molecules, we obtain the results shown in Table 1-3. In all four series of compounds, the frequency decreases in going downward in the table. However, the origin of this downward shift is different in each case. In the H2 > HD > D2 series, it is due to the mass effect since the force constant is not affected by isotopic substitution. In the HF > HC1 > HBr > HI series, it is due to the force constant effect (the bond becomes weaker in the same order) since the reduced mass is almost constant. In the F2 > Cl2 > Br2 > I2 series, however, both effects are operative the molecule becomes heavier and the bond becomes weaker in the same order. Finally, in the N2 > CO > NO > 02, series, the decreasing frequency is due to the force constant effect that is expected from chemical formulas, such as N=N, and 0=0, with CO and NO between them. 

Emergence of chemical atomism. The beginning of the 19th century was marked by the emergence of atomic theory developed by the famous English scientist John Dalton (1766-1844). According to Dalton, atoms combine in multiple proportions. Thus, if two atoms form only one compound, they associate in the 1 1 ratio if two atoms form two compounds, their ratios in these compounds are respectively 1 1 and 1 2, and so on. Accordingly, Dalton adopted the formula HO for water, CO for carbon monoxide, COg for carbon dioxide, and so on. From these formulas he determined the atomic masses of elements. It is clear that frequently both atomic masses and chemical formulas of Dalton were not correct. 

Gravimetric analysis is an analytical technique based on the measurement of mass. One type of gravimetric analysis experiment involves the formation, isolation, and mass determination of a precipitate. Generally, this proeedure is applied to ionie eom-pounds. A sample substance of unknown composition is dissolved in water and allowed to react with another substanee to form a precipitate. The precipitate is filtered off, dried, and weighed. Knowing the mass and chemical formula of the precipitate formed, we can calculate the mass of a partieular chemical component (that is, the anion or cation) of the original sample. From the mass of the component and... 

MASS PERCENT COMPOSITION FROM CHEMICAL FORMULA ... 

To this point, our study of chemistry has been largely qualitative, involving very few calculations. However, chemistry is a quantitative science. Atoms of elements differ from one another not only in composition (number of protons, electrons, neutrons), but also in mass. Chemical formulas of compounds tell us not only the atom ratios in which elements are present but also the mass ratios. 

If the chemical formula of a compound is already known, its mass percentage composition can be obtained from the formula. 

To determine the molar mass of a substance, we need its chemical formula and elemental molar masses. From the chemical formula, determine the number of moles of each element contained in one mole of the substance. Multiply each elemental molar mass by the number of moles of that element, and add. 

This example shows how to compute mass percentages from a chemical formula. When the formula of a compound is unknown, chemists must work in the opposite direction. First, they do experiments to find the mass percentage of each element, and then they deduce what chemical formula matches those percentages. 

Sometimes chemists have to analyze substances about which they know very little. A chemist may isolate an interesting molecule from a natural source, such as a plant or an insect. Under these conditions the chemical formula must be deduced from mass percentage data, without the help of an expected formula. A four-step procedure accomplishes this by using mass-mole conversions, the molar masses of the elements, and the fact that a chemical formula must contain integral numbers of atoms of each element. 

To get from elemental analysis to a chemical formula, we begin by dividing each mass by the appropriate molar... 

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. 

In the problem above, we determined the percentage data from the chemical formula. We can determine the empirical formula if we know the percent compositions of the various elements. 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. However, the procedure is still the same—convert each element to moles, divide each by the smallest, and then use an appropriate multiplier if necessary. We can then determine the empirical formula mass. If we know the actual molecular mass, dividing the molecular formula mass by the empirical formula mass, gives an integer (rounded if needed) that we can multiply each of the subscripts in the empirical formula. This gives the molecular (actual) formula, which tells what elements are in the compound and the actual number of each. 

Roussis, S.G. Proulx, R. Reduction of Chemical Formulas from the Isotopic Peak Distributions of High-Resolution Mass Spectra. Anal. Chem. 2003, 75. 1470-1482. 

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