Molecular elements, molar mass

Plan The molecular formula subscripts are whole-number multiples of the empirical formula subscripts. To find this whole number, we divide the given molar mass (90.08 g/moI) by the empirical formula mass, which we find from the sum of the elements molar masses. Then we multiply the whole number by each subscript in the empirical formula. Solution The empirical-formula molar mass is 30.03 g/mol. Finding the whole-number multiple ... 

Plan We find the masses of CO2 and H2O by subtracting the masses of the absorbers before the reaction from the masses after. From the mass of CO2, we use the mass fraction of C in CO2 to find the mass of C (see Comment in Sample Problem 3.3). Similarly, we find the mass of H from the mass of H2O. The mass of vitamin C (I.OOO g) minus the sum of the C and H masses gives the mass of O, the third element present. Then, we proceed as in Sample Problem 3.5 calculate numbers of moles using the elements molar masses, construct the empirical formula, determine the whole-number multiple from the given molar mass, and construct the molecular formula. 

A 0.167 g sample of an unknown acid requires 27.8 mL of 0.100 M NaOH to titrate to the equivalence point. Elemental analysis of the acid gives the following percentages by mass 40.00% C, 6.71% H, 53.29% O. Determine the molecular formula, molar mass, and Lewis structure of the unknown acid. 

As you will see shortly, the formula of a compound can be used to determine the mass percents of the elements present. Conversely, if the percentages of the elements are known, the simplest formula can be determined. Knowing the molar mass of a molecular compound, it is possible to go one step further and find the molecular formula. In this section we will consider how these three types of calculations are carried out. 

Table 21.1 (p. 556) lists some of the properties of the eight nonmetals considered in this chapter. Notice that all of these elements are molecular those of low molar mass (N2> 02, F2> Cl2) are gases at room temperature and atmospheric pressure (Figure 21.2, p. 556). Stronger dispersion forces cause the nonmetals of higher molar mass to be either liquids (Br2) or solids (I2, P4. S8). 

F.13 Osmium forms a number of molecular compounds with carbon monoxide. One light-vellow compound was analyzed to give the following elemental composition 15.89% C, 21.18% O, and 62.93% Os. (a) What is the empirical formula of this compound (b) From the mass spectrum of the compound, the molecule was determined to have a molar mass of 907 g-mol 1. What is its molecular formula ... 

FIGURE 5.8 The boiling points of most of the molecular hydrides of the p-block elements show a smooth increase with molar mass in each group. However, three compounds—ammonia, water, and hydrogen fluoride are strikingly out of line. 

Suppose that 10.0 g of an organic compound used as a component of mothballs is dissolved in 80.0 g of benzene. The freezing point of the solution is 1.20°C. (a) What is an approximate molar mass of the organic compound (b) An elemental analysis of that substance indicated that the empirical formula is C3H2C1. What is its molecular formula (c) Using the atomic molar masses from the periodic table, calculate a more accurate molar mass of the compound. 

The flowchart in Figure 3-15 outlines the process. From masses of products, determine masses of elements. Then convert masses of elements to moles of elements. From moles of the elements, find the empirical formula. Finally, use information about the molar mass to obtain the molecular formula. 

C03-0083. Nicotine is an addictive compound found in tobacco leaves. Elemental analysis of nicotine gives these data C 74.0%, H 8.65%, N 17.35%. What is the empirical formula of nicotine The molar mass of nicotine is 162 g/mol. What is the molecular formula of nicotine ... 

We can use the ideal gas equation to calculate the molar mass. Then we can use the molar mass to identify the correct molecular formula among a group of possible candidates, knowing that the products must contain the same elements as the reactants. The problem involves a chemical reaction, so we must make a connection between the gas measurements and the chemistry that takes place. Because the reactants and one product are known, we can write a partial equation that describes the chemical reaction CaC2(. ) +H2 0(/) Gas -I- OH" ((2 q) In any chemical reaction, atoms must be conserved, so the gas molecules can contain only H, O, C, and/or Ca atoms. To determine the chemical formula of the gas, we must find the combination of these elements that gives the observed molar mass. 

Chemists may distinguish between the molar masses of pure elements, molecular compounds, and ionic compounds by referring to them as the gram atomic mass, gram molecular mass, and gram formula mass, respectively. Don t be fooled The basic concept behind each term is the same molar mass. 

Elemental Composition, Molecular Formula, and Molar Mass Electron Shells of Elements Present in Organic Compounds Covalent Bonding... 

Elemental Composition, Molecular Formula, and Molar Mass... 

The molar mass of an element is the mass per mole of its atoms the molar mass of a molecular compound is the mass per mole of its molecules the molar mass of an ionic compound is the mass per mole of its formula units. The units of molar mass in each case are grams per mole (g-moD1). 

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... 

An elemental analysis of the hormone epinephrine is 59.0% carbon, 26.2% oxygen, 7.15% hydrogen, and 7.65% nitrogen by mass. When 0.64 g of epinephrine dissolved in 36.0 g of benzene, the freezing point decreased by 0.50°C. (a) Determine the empirical formula of epinephrine, (b) What is the molar mass of epinephrine (c) Deduce the molecular formula of epinephrine. 

Each element has a specific symbol that is different from the symbol for any other element. In a chemical formula, the symbol stands for an atom of an element. Molecular substances are composed of two or more atoms that are tightly bound together. The formula for a molecular substance consists of the symbols for the atoms that are found in that molecule. For instance, the formula for carbon dioxide is CO2. Note the use of the subscript to show that each molecule contains two oxygen atoms in addition to the one carbon atom. Also note that the 1 for the one carbon atom is not written. The molecular mass of CO2 is the sum of the atomic mass of carbon plus twice the atomic mass of oxygen and is expressed in u. As was discussed directly above, the molar mass of CO2 is the mass in grams equal to the molecular mass in u. A mole of carbon dioxide is 12.0 u + 2(16.0 u) = 44 u. This result can be expressed as 44 g to indicate one Avogadro s number, Na, of CO2 molecules. Recall that Na is 6.0221 x 1023 things—molecules in this case. 

If you know the molar mass of a substance, and the elements that make up the substance, can you determine its molecular formula Explain your answer. 

Structural verification is based on viscosity data (indicating Einstein spheroid characteristics), elemental analyses, light-scattering experiments, and direct observation of individual dendrimers by electron microscopy. The self-limiting molar-mass ranges correspond to approximately 38 branch-cell propagations (three tiers) around a germanium initiator core. Furthermore, analysis of molecular... 

White phosphorus is one form of elemental phosphorus. Its molar mass is 124 g/mol. Calculate its molecular formula. 

The molecular formula is CgHio. Note that there is no need to try to get integral numbers of moles if an exact molar mass is given the values come out very nearly integral. (If an approximate molar mass is given, the numbers of moles of each element must be rounded to the nearest integer. Retry Problem 7.110 this way.)... 

Formulas describe the composition of compounds. Empirical formulas give the mole ratio of the various elements. However, sometimes different compounds have the same ratio of moles of atoms of the same elements. For example, acetylene, C2H2, and benzene, CeHe, each have 1 1 ratios of moles of carbon atoms to moles of hydrogen atoms. That is, each has an empirical formula CH. Such compounds have the same percent compositions. However, they do not have the same number of atoms in each molecule. The molecular formula is a formula that gives all the information that the empirical formula gives (the mole ratios of the various elements) plus the information of how many atoms are in each molecule. In order to deduce molecular formulas from experimental data, the percent composition and the molar mass are usually determined. The molar mass may be determined experimentally in several ways, one of which will be described in Chap. 12. 

The percentage of mass in an element in a compound can easily be found when the molecular formula and molar mass (or atomic mass) of elements are known. 

In Example 3.6 we found the molecular formula by comparing the empirical formula mass with the molar mass. There is an alternative way to obtain the molecular formula. The molar mass and the percentages (by mass) of each element present can be used to compute the moles of each element present in one mole of compound. These numbers of moles then represent directly the subscripts in the molecular formula. This procedure is illustrated in Example 3.7. 

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