Formula, calculating percentage

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. 

To calculate percentage survival, the following formula is used ... 

Problem 7.20 Optically pure (S)-(-i-)-CH3CHBr-n-CjH,3 has [a] = -1-36.0°. A partially racemized sample having a specific rotation of -i-30° is reacted with dilute NaOH to form (R)-(-)-CH,CH(OH)-n-CsH,3 ([ ] =-5. 97°), whose specific rotation is -10.3° when optically pure, (a) Write an equation for the reaction using projection formulas, (b) Calculate the percent optical purity of reactant and product, (c) Calculate percentages of racemization and inversion, (d) Calculate percentages of frontside and backside attack, (e) Draw a conclusion concerning the reactions of 2° alkyl halides. (/) What change in conditions would increase inversion ... 

Calculation, Percentage of moisture in wet LA may be calcd from the formula ... 

The approximate molar mass, calculated from the gas density data, is 89 g/mol. The empirical formula, calculated from the percentage composition data, is C2H3O with the empirical formula unit mass of 43.0. The exact molar mass must be (2)(43) = 86.0 g/mol since this is the only multiple of 43.0 (whole-number multiple) reasonably close to the approximate molecular formula of 89 g/mol. The molecule must be the equivalent of 2 empirical formulas CqHgO. 

Calculating Percentage Composition from a Chemical Formula... 

In the previous Practice Problems, you used mass data to calculate percentage composition. This skill is useful for interpreting experimental data when the chemical formula is unknown. Often, however, the percentage composition is calculated from a known chemical formula. This is useful when you are interested in extracting a certain element from a compound. For example, many metals, such as iron and mercury, exist in mineral form. Mercury is most often found in nature as mercury(II) sulfide, HgS. Knowing the percentage composition of HgS helps a metallurgist predict the mass of mercury that can be extracted from a sample of HgS. 

When determining the percentage composition by mass of a homogeneous sample, the size of the sample does not matter. According to the law of definite proportions, there is a fixed proportion of each element in the compound, no matter how much of the compound you have. This means that you can choose a convenient sample size when calculating percentage composition from a formula. 

To determine an empirical formula, calculate the number of moles of each element in a sample (arbitrarily choose 100 g if percentages are given), divide each of them by the smallest number of moles, and then multiply by some small integer if necessary to get integral numbers of moles. 

REVERSE CALCULATIONS PERCENTAGE COMPOSITION FROM FORMULA... 

We can turn this sort of calculation around and convert chemical formula into percentage composition. 

Calculate percentage composition of a compound from its molecular formula or formula unit. 

Chemical formulas can be used to calculate percentage composition. 

Calculation of empirical formulae from percentage data... 

The mean of row G = 1.35, which is equivalent to 0% competition (a lot of color). The mean of row H = 0.07, which is equivalent to 100% competition (little color). Subtract the mean of row H from all the values obtained. If the value is minus then call it 0. This determines the 100 C0% OD competition values (i.e., the range is from 0 to 1.29 OD). Using a simple formula, the percentage of competition of the samples can be calculated. Table 13 presents the processed data with respect to subtraction of background for all data, using the following equation... 

FIGURE 3.13 Procedure for calculating an empirical formula from percentage composition. The key step in the calculation is step 2, determining the number of moles of each element in the compound. 

Based on the following structural formulas, calculate the percentage of carbon by mass present in each compound ... 

A Figure 3.13 Procedure for calculating an empirical formula from percentage composition. 

Chemical formulas are expressed in terms of numbers of particles. In this problem, we have to start with masses and somehow infer information about the numbers of particles. We did a similar problem in this chapter the determination of an empirical formula from percentage by mass data. In this case, with the experiment described, we can determine the mass information for each element in the unknown compound—providing the same type of data we had from percentage mass calculations. Then, because we know the empirical formula and we know the molar mass of the oxygen, we can determine the molar mass of the unknown metal. 

The possibility of rotation of some groups of atoms around some chanical bonds is intuitively associated with molecular flexibility. Consequently, the number of rotatable bonds and the percentage of rotatable bonds are the simplest descriptors that measure the molecular flexibility. More complicated formulas calculate flexibility descriptors by weighting some topological descriptors therefore, they consider the shape and size of the molecules, the number of atoms, and the sum of bond orders (Tarko 2004c). 

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