Compound mass percent determination

Thinking it Through This type of question starts with a chemical formula and uses it to determine the mass percent for any element in a compound. Mass percent is a comparison of the mass of the element being considered to the molar mass of the compound. The ratio is then multiplied by 100 to make it a percent, which is the same as parts per hundred. In this case, the mass of oxygen present must be compared to the molar mass of the compound,... 

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. 

The elemental analysis of a compound is usually determined by a laboratory that specializes in this technique. A chemist who has prepared a new compound sends a sample to the laboratory for analysis. The laboratory charges a fee that depends on the type and number of elements analyzed. The results are returned to the chemist as a listing of mass percent composition. The chemist must then figure out which chemical formula matches this composition. If a chemist has reason to expect a particular chemical formula, the observed percentages can be matched against the calculated percentages for the expected formula. This process is illustrated in Example 3-13. 

The mass percent for each element is determined by dividing the mass of the element present in a mole of the compound by the molar mass for the compound as a whole, and then multiplying the result by 100%. 

B We use the same technique as before determine the mass of each element in a mole of the compound. Their sum is the molar mass of the compound. The percent composition is determined by comparing the mass of each element with the molar mass of the compound. 

In each case, we first determine the molar mass of the compound, and then the mass of the indicated element in one mole of the compound. Finally, we determine the percent by mass of the indicated element to four significant figures. 

After the coordination compound was made, it was then analyzed to determine (1) the mass percent of NH3 in the compound and (2) the mass percent of Ni2+ in the compound. 

Among the congeners investigated was 2,2 ,5,5 -tetrachlorobiphenyl (TeCIBP), which was determined to be present in the Aroclor 1242 mixture at about 3.2 mass percent (i.e., mass fraction miAroclor = 0.032 g, -g jor). The measured aqueous concentrations for this compound were 1.11 pg L 1 (case a) and 0.10 pg-L"1 (case b), respectively. Are these concentrations reasonable What aqueous TeCIBP concentrations would you have predicted from the above information, when assuming that Raoult s law is valid in both cases ... 

Active" ZDDP. Differential Infrared Spectroscopy (DIR) was used to determine the concentration of ZDDP in the used oil samples by measuring the absorbance of the P-O-C band at 1,000 cm 1. The ZDDP concentrations of the used oil samples were generally less than 0.05 mass percent (as zinc), which is substantially less than the nominal 0.12 mass percent in the fresh oils. There was no correlation between camshaft and lifter valve wear and amount of ZDDP remaining in the used oil. This result supports other observations that the decomposition of ZDDP results in other compounds which may also exhibit some antiwear properties. 

The carbon-hydrogen combustion analyzer works because we know that compounds containing carbon and hydrogen will burn in a stream of pure oxygen, O2, to yield only carbon dioxide and water. If we can find the mass of the carbon dioxide and water separately, we can determine the mass percent of carbon and hydrogen in the compound. 

C2ZD A compound that contains carbon, hydrogen, chlorine, and oxygen is subjected to carbon-hydrogen analysis. Can the mass percent of oxygen in the compound be determined using this method Explain... 

Determine the mass percent of each element in a compound. 

Explain how a carbon-hydrogen combustion analyzer can be used to determine the mass percent of carbon, hydrogen, and oxygen in a compound. 

When a new compound is prepared, one of the first items of interest is its formula. The formula is often determined by taking a weighed sample of the compound and either decomposing it into its component elements or reacting it with oxygen to produce substances such as CO2, H20, and N2, which are then collected and weighed. A device for doing this type of analysis is shown in Fig. 3.5. The results of such analyses provide the mass of each type of element in the compound, which can be used to determine the mass percent of each element present. 

Using the mass percents and the molar mass, determine the mass of each element present in 1 mole of compound. 

Determine the empirical and molecular formulas for a compound from mass percent and actual mass data. 

It s the analytical chemist s job to identify the elements a compound contains and determine their percent by mass. Gravimetric and volumetric analyses are experimental procedures based on the measurement of mass for solids and liquids, respechvely. For example, a 100-g sample of a new compound contains 55 g of element X and 45 g of element Y. The percent by mass of any element in a compound can be found by dividing the mass of the element by the mass of the compound and multiplying by 100. 

The data used to determine the chemical formula for a compound may be in the form of percent composition or it may be the actual masses of the elements in a given mass of the compound. If percent composition is given, you can assume that the total mass of the compound is 100.00 g and that the percent by mass of each element is equal to the mass of that element in grams. For example, the percent composition of an oxide of sulfiir is 40.05% S and 59.95% O. Thus, as you can see in Figure 11-10, 100.00 g of the oxide contains 40.05 g S and 59.95 g O. The mass of each element can be converted to a number of moles by multiplying by the inverse of the molar mass. Recall that the number of moles of S and O are calculated in this way. 

The Statne of Liberty turns green in air because of the formation of two copper compounds, Cu3(0H)4S04 and Cu4(OH)gS04. Determine the mass percent of copper in these compounds. 

The empirical formula of a compound can be determined if the percent composition of the compound is known (Section 3.7). But where do the percent composition data come from Various methods are used, and many depend on reactions that decompose the unknown but pure compound into known products. Assuming the reaction products can be isolated in pure form, the masses and the number of moles of each can be determined. Then, the moles of each product can be related to the number of moles of each element in the original compound. One method that works well for compounds that burn in oxygen is analysis by combustion. Each element (except oxygen) in the compound combines with oxygen to produce the appropriate oxide. 

The percent composition of a compound can be determined from its formula and the molar masses of the elements that make it up. Let s do a careful analysis of the formula of N2Os to see how it is possible to calculate its percent composition is calculated. 

The procedure used in the example can be reversed if necessary. Given the percent composition by mass of a compound, we can determine the empirical formula of the compound. Since we are dealing with percentages and the sum of aU the percentages is 100 percent, it is convenient to assume that we started with 100 g of a compound, as Example 3.9 shows. 

As always, the individual mass percents of the elements in the compound must add up to 100% (within rounding). As Sample Problem 3.3 demonstrates, an important practical use of mass percent is to determine the amount of an element in any size sample of a compound. 

I Explain what is meant by the percent composition of a compound. I Determine the empirical and molecular formulas for a compound from mass percent and actual mass data. 

It is the analytical chemists job to identify the elements a compound contains and determine their percents by mass. Gravimetric and volumetric analyses are experimental procedures based on the measurement of mass for solids and liquids, respectively. 

Percent composition or masses of the elements in a given mass of compound can be used to determine the formula for the compound. If percent composition is given, assume the total mass of the compound is... 

Strategy In a chemical formula, the subscripts represent the ratio of the number of moles of each element that combine to form the compound. Therefore, we need to convert from mass percent to moles in order to determine the empirical formula. If we assume an exactly 100 g sample of the compound, do we know the mass of each element in the compound How do we then convert from grams to moles ... 

The mass percent of Cl is given. From the mass of the compound and the number of hydrogen atoms given, we can calculate the mass percent of H. The mass percent of carbon is then obtained by difference. Once the mass percentages of each element are known, the empirical formula can be determined. 

Mass percent the percent by mass of a component of a mixture (17.1) or of a given element in a compound. (3.4) Mass spectrometer an instrument used to determine the relative masses of atoms by the deflection of their ions in a magnetic field. (3.1)... 

Serotonin is a compound that conducts nerve impulses in the brain. It contains 68.2 mass percent C, 6.86 mass percent H, 15.9 mass percent N, and 9.08 mass percent O. Its molar mass is 176 g/moL Determine its molecular formula. 

Strategy This problem can be divided into two parts. First, it asks for the empirical formula of the compound from the percent by mass of Si and F. Second, the information provided enables us to calculate the molar mass of the compound and hence determine its molecular formula. What is the relationship between empirical molar mass and molar mass calculated from the molecular formula ... 

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