Formula mass conversions

Molarity (M) A concentration unit defined to be the number of moles of solute per liter of solution, 95q, 259 concentration unit conversion, 261-262 potassium chromate, 263 Mole A collection of6.0122 X 1023 items. The mass in grams of one mole of a substance is numerically equal to its formula mass, 55. See also Amount Mole fraction (X) A concentration unit defined as the number of moles of a component divided by the total number of moles, 116-117,261 Mole-gram conversions, 55-56,68-68q... 

As with all calculations of chemical amounts, we must work with moles. Because grams are asked for, we must do a mole-mass conversion this requires the molar mass of the substance, which in turn requires that we know the chemical formula. 

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. 

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. 

Ans. There are 6.02 x 10 atoms in 1.00 mol K (Avogadro s number). There are 39.1 g of Kin 1.00 mol K (equal to the atomic mass 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 or formula units). With numbers of atoms, molecules, or formula units, use Avogadro s number, with mass, use the formula mass. 

As we saw with mass and moles of elements and molecular compounds, it is important to be able to convert between mass and moles of ionic substances. The development of the tools for this conversion starts with the determination of the formula mass, which is the weighted average of the masses of the naturally occurring formula units of the substance. (It is analogous to the atomic mass for an element and the molecular mass for a molecular substance.)... 

Given enough information to calculate an ionic compound s formula mass, write a conversion factor that converts between mass and moles of the compound. 

Ans. The formula mass of oxygen is 32.0 therefore there are 32.0 g per mole of oxygen. To determine the number of moles of oxygen in 52.0 g of oxygen, we must convert grams of oxygen to moles of oxygen. To do this we require a unit conversion factor ... 

Problem 4.16. How many moles are there in 125.0 g of sodium thiocyanate, NaSCN Ans. The formula mass of NaSCN is 81.0. Using that value for a conversion factor ... 

Ans. The unit conversion factor here must be created using the formula mass of Na3P04 ... 

Ans. Yes, we can use a unit conversion factor consisting of the ratio of molar masses equal to the formula masses multiplied by the accompanying balancing coefficients ... 

If, for example, 2 mol of diamine is reacted until complete conversion (pcooH = 1) with a mol of dicarboxyUc acid (ro = 0.5), then the number-average degree of polymerization is X = 3, in accordance with equation (17-24), and the mass-average degree is Xw— 5, in accordance with equation (17-38). Following from Equation (17-40), however, the diamide will only be formed up to 25% by weight (w, = 0.25), so that the yield of this compound can only reach 25% of the theoretical formula molar conversion (Table 17-4). 

In Chapter 6, we learned how a chemical formula contains conversion factors for converting between moles of a compound and moles of its constituent elements. In this chapter, we have seen how a chemical equation contains conversion factors between moles of reactants and moles of products. However, we are often interested in relationships between mass of reactants and mass of products. For example, we might want to know the mass of carbon dioxide emitted by an automobile per kilogram of gasoline used. Or we might want to know tire mass of each reactant required to obtain a certain mass of a product in a S5mthesis reaction. These calculations are similar to calculations covered in Section 6.5, where we converted between mass of a compound and mass of a constituent element. The general outline for these types of calculations is ... 

For example, H2O has a formula mass of 18.02 amu therefore, HjO has a molar mass of 18.02 g/mol-one mole of water molecules has a mass of 18.02 grams. Just as the molar mass of an element is a conversion factor between grams of the element and moles of the element, so the molar mass of a compound is a conversion factor between grams of the compound and moles of the molecule. 

Chemical Formulas as Conversion Factors (Mass to Mass)... 

The sum of the atomic masses of all the atoms in a chemical formula is called the formula mass (4.5). It is a conversion factor between mass of the compound and moles of its molecules. The numerical relationships inherent in chemical formulas can help us determine the amount of a given element within a given compound (4.6). 

Separation of families by merely increasing the resolution evidently can not be used when the two chemical families have the same molecular formula. This is particularly true for naphthenes and olefins of the formula, C H2 , which also happen to have very similar fragmentation patterns. Resolution of these two molecular types is one of the problems not yet solved by mass spectrometry, despite the efforts of numerous laboratories motivated by the refiner s major interest in being able to make the distinction. Olefins are in fact abundantly present in the products from conversion processes. 

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. 

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