Moles of elements

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. 

According to the information given, you have 0.50 mole of element X (50. g/100. g mole 1 = 0.50 mole). For the oxygen, remember that you will use 16 g/mole for the atomic weight, giving you 2.0 moles of oxygen atoms. A 0.50 2.0 molar ratio is the same as a 1 4 molar ratio, so the answer is X04. 

Molar mass The mass in grams of one mole of a substance. For example, one mole of elemental arsenic has a mass of 74.92160 g. 

Since the relationship within a chemical formula is a small whole-number relationship of moles of elements to each other, the first step in the solution is to determine the number of grams of each of the elements. This step must isolate the desired element from the compound produced by the burning, which can be performed by multiplying by the fraction of the compound that is the element. 

If the weight of each element that combines in an experiment is known, then the number of moles of each element can be determined. The empirical formula of the compound formed is the ratio between the number of moles of elements in the compound. This can be illustrated by the following example. If 32.06 grams of sulfur is burned in the presence of 32.00 grams of oxygen, then 64.06 grams of sulfur dioxide results. Thus... 

These are (a) 0 elimination, (b) a elimination and (c) hydrolysis. In 0 elimination, the proposed reactions are shown in Equation 7. Therefore, the stoichiometry of the reaction is two moles of dehydroalanine, one mole of elemental sulfur and one mole of disulfide as shown by the overall reaction (Equation 8). 

The table below shows molar heat capacities (joules per kelvins x mole) of elements and compounds. Use it to answer questions 10 through 13. 

Suppose you need to measure a certain number of moles of a compound for an experiment. First, you must calculate the mass in grams that corresponds to the necessary number of moles. Then, that mass can be measured on a balance. In Example Problem 11-2, you learned how to convert the number of moles of elements to mass using molar mass as the conversion factor. The procedure is the same for compounds except that you must first calculate the molar mass of the compound. 

The steps in determining empirical and molecular formulas from percent composition or mass data are outlined below. As in other calculations, the route leads from mass through moles because formulas are based on the relative numbers of moles of elements in each mole of compound. 

The molar mass of a compound is the sum of the masses of all the moles of elements present in the compound. 

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.)... 

Converts given mass Converts moles of element Converts grams into... 

You either convert from moles of element to moles of compound or moles of compound to moles of element. 

We can convert between moles of element and moles of a compound containing that element by using the molar ratio derived from the... 

From the molecular mass we can determine the molar mass of a molecule or compound. The molar mass of a compound (in grams) is numerically equal to its molecular mass (in amu). For example, the molecular mass of water is 18.02 amu, so its molar mass is 18.02 g. Note that 1 mole of water weighs 18.02 g and contains 6.022 X 10 H2O molecules, just as 1 mole of elemental carbon contains 6.022 X 10 carbon atoms. 

Answer We start by assuming there are 100 g of ascorbic acid. Therefore, in this sample there will be 40.92 g of C, 4.58 g of H, and 54.50 g of O. Next, we need to calculate the number of moles of each element in the compound. Let Hq, %, and o be the number of moles of elements present. Using the molar masses of these elements, we write... 

Converting Moles of Elements For problems involving mass-mole-number relationships of elements, keep these points in mind ... 

Check The calculated molecular formula has the same ratio of moles of elements (3 6 3) as the empirical formula (1 2 1) and corresponds to the given molar mass ... 

The atomic reactions of iodine or bromine are often speeded up by the introduction of small amounts of chlorine or fluorine into the reaction mass. As little as 0.015 mole of elemental fluorine will initiate the reaction of atomic chlorine with benzene to form hexachlorocyclohexane. 

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