Mass Formula Weight

Because 58 grams is the weight of one mole of Mg(OH)2 and one mole of Mg(OH)2 contains two moles of oxygen, 58 grams must contain two moles of oxygen. 

The molar mass is important in the determination of the percent composition of a compound from its empirical formula. For example, suppose that we want the percent of iron in pyrite, FeS2. The molar mass of FeSa is 56 + (2 x 32) = 120g per mole. In this mass of FeSa there is 56g of iron. Thus, the percentage of iron is 

The molar mass is 79.9g per mole. That molar mass contains 47.9g of Ti. Thus, the percentage is 

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The formula mass (formula weight) of a substance is determined by adding the atomic masses (atomic weights) of each atom (not each element) in a formnla unit. Molecular mass is one type of formula mass (for substances that form molecnles) and is calculated in the same way as the formula mass for an ionic compound. For example, the formula mass of NH3 is 17.0 amu, the atomic mass of three hydrogen atoms plus that of one nitrogen atom. Three or more significant digits should be used to report formula masses. (Section 7.1)... 

One mole of a compound contains Avogadro s number of formula units of that compound. The terms molecular weight, molecular mass, formula weight, and formula mass have been used in the past to refer to the mass of 1 mol of a compound. However, the term molar mass is more inclusive, because it can be used for all types of compounds. 

As can be seen from equation 8.14, we may improve a method s sensitivity in two ways. The most obvious way is to increase the ratio of the precipitate s molar mass to that of the analyte. In other words, it is desirable to form a precipitate with as large a formula weight as possible. A less obvious way to improve the calibration sensitivity is indicated by the term of 1/2 in equation 8.14, which accounts for the stoichiometry between the analyte and precipitate. Sensitivity also may be improved by forming precipitates containing fewer units of the analyte. 

Effect of dimer formation on deactivation. Another possible mode of deactivation is formation of inactive Co dimers or oligomers. To test for these species, we examined the ESI-mass spectram of fresh and deactivated Co-salen catalysts in dichloromethane solvent (22). The major peak in the mass spectram occurred at m/z of 603.5 for both Jacobsen s Co(II) and Co(III)-OAc salen catalysts, whereas much smaller peaks were observed in the m/z range of 1207 to 1251. The major feature at 603.5 corresponds to the parent peak of Jacobsen s Co(II) salen catalyst (formula weight = 603.76) and the minor peaks (1207 to 1251) are attributed to dimers in the solution or formed in the ESI-MS. The ESI-MS spectrum of the deactivated Co-salen catalyst, which was recovered after 12 h HKR reaction with epichlorohydrin, was similar to that of Co(II) and Co(III)-OAc salen. Evidently, only a small amount of dimer species was formed during the HKR reaction. However, the mass spectram of a fresh Co(III)-OAc salen catalyst diluted in dichloromethane for 24 h showed substantial formation of dimer. The activity and selectivity of HKR of epichlorohydrin with the dimerized catalyst recovered after 24 h exposure to dichloromethane were similar to those observed with a fresh Co-OAc salen catalyst. Therefore, we concluded that catalyst dimerization cannot account for the observed deactivation. 

The formula weight of a substance is equal to its number of grams per mole. Avogadro s number is the number of atomic mass units in 1 g. It is defined in that manner so that the atomic weight of an element (in amu) is numerically equal to the number of grams of the element per mole. Consider helium, with atomic weight 4.0 ... 

The mass of a substance can be converted to moles, and vice versa, with the formula weight. 

The concept of percentage is often used to describe the composition of compounds. If the formula of a compound is known, the percent by mass of an element in the compound is determined by computing the fraction of the formula weight which is made up of that element, and multiplying that fraction by 100%. Thus, an element X with atomic weight 40.0 amu in a compound XY of formula weight 99.0 amu will be present in... 

Ans. There are 6.02 X 10 i atoms in 1.00 mol Na (Avogadro s number). There is 23.0 g of Na in LOO mol Na (equal to the atomic weight in grams). This problem requires use of two of the most important conversion factors involving moles. Note which one is used with masses and which one is used with numbers of atoms (or molecules of formula units). With numbers of atoms, molecules, or formula units, use Avogadro s number with mass or weight use the formula weight. 

Arts. They were discussing Sec. 8.1. Since the student got the answer correct despite hearing the wrong name, they must have been discussing the number of moles of reactants and products. The numbers of moles of HF and HC1 would be the same in the reaction, but since they have different formula weights, their masses would be different. 

EXAMPLE 15.12. A new solid acid was prepared in a laboratory its formula weight was not known. It was titrated with standard base, and the number of moles of base was calculated. Without knowing the formula of the acid, can you tell how many moles of the acid was present in a certain mass of acid Can you tell how many equivalents of acid was present ... 

Since the carbonate ion, C032, has a formula weight of 60.0 g/mol, then the mass of the metal, Q, in one mole of compound must be... 

The key to any reaction experiment is moles. The numbers of moles may be calculated from various measurements. A sample may be weighed on a balance to give the mass, and the moles calculated with the formula weight. Or the mass of a substance may be determined using a volume measurement combined with the density. The volume of a solution may be measured with a pipet, or calculated from the final and initial readings from a buret. This volume, along with the molarity, can be used to calculate the moles present. The volume, temperature, and pressure of a gas can be measured and used to calculate the moles of a gas. You must be extremely careful on the AP exam to distinguish between those values that you measure and those that you calculate. 

Throughout this book we will specify the amount of a substance in terms of the chemist s unit, the mole. We will use the symbol (Symbol or formula) to refer to the number of moles of the substance. Since in most laboratory work mass is determined by weighing, the word weight (as in the second sentence in the problem) is commonly used where mass would be more precise. Unless it leads to an ambiguity, we will follow common usage and not bother to distinguish between mass and weight. ... 

In propagating errors it is generally advisable to keep one or two extra digits in intermediate results, and round off only when you get to the final result. Applying this rule to the solubility product measurement, the mass of dissolved silver chloride is 0.888 0.0142 mg, and the formula weight of silver chloride is 143.3279 0.00042 g- mol"1. 

Name Formula Formula weight (u) Mass of 1 mol of formula units (g) Number and kind of particles in 1 mol... 

Thus, the empirical formula is CH20, with a formula weight of 12 + 2 + 16 = 30. The molecular formula, based on X having a mass of 90.00 u, must be C3H603. 

Here we compare the thermodynamic parameters of trehalose, maltose and sucrose because they have the same chemical formula (C12H22O11) and mass (molecular weight 342.3), but different structures which could be responsible for their different hydration properties. The anomaly of hydration of trehalose is understood from the following observation [10]. Namely, the amount of water used for the preparation of 1.5 M trehalose solution is smaller than the amount used for the preparation of other sugar solutions. In a 1.5 M solution, trehalose itself occupies 37.5% of the volume of the solution. However, in a 1.5 M solution, sucrose occupies 13% and maltose occupies 14%. These data suggest that trehalose has a larger hydrated volume than the other sugars. This hypothesis can be demonstrated from various thermodynamic parameters as shown in Table 12.1. 

One twelfth of a mass of an atom of the carbon-12 isotope a unit used for stating atomic and formula weights also called dalton. 

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