Percentage composition and empirical formulas

What is the percentage composition and empirical formula of magnesium oxide ... 

George L. Gilbert, "Percentage Composition and Empirical Formula—A New View," /. Chem. Educ, Vol. 75,1998, 851. 

Experiment 14 Composition 1 Percentage Composition and Empirical Formula of Magnesium Oxide... 

The empirical formula of a compound is determined from the mass percentage composition and the molar masses of the elements present. 

A 0.539 g sample of a compound that contained only carbon and hydrogen was subjected to combustion analysis. The combustion produced 1.64 g of carbon dioxide and 0.807 g of water. Calculate the percentage composition and the empirical formula of the sample. 

Example of the Calculation of the Percentage Composition and the Empirical Formula, from the. Data of Analysis... 

Percentage Composition, and using the Law of Constant Composition to find the Empirical and Chemical Formula of a Compound... 

Problem 2.7 Calculate the percentage composition and then the empirical formula for each of the following compounds (a) Combustion of a 3.02-mg sample of a compound gave 8.86 mg of carbon dioxide and 5.43 mg of water, (b) Combustion of an 8.23-mg sample of a compound gave 9.62 mg of carbon dioxide and 3.94 mg of water. Analysis of a 5.32-mg sample of the same compound by the Carius method gave 13.49 mg of silver chloride. 

In this experiment you will determine the empirical formula of a compound composed of lead and iodine. A weighed quantity of lead is reacted with nitric acid, HNO3, solution. The resulting lead nitrate solution is then reacted with sodium iodide, Nal, solution to form insoluble lead iodide, which is filtered, dried, and weighed. From your experimental data you can calculate the percentage composition and the ratio of moles of lead to moles of iodine in the compound, and then write the empirical formula. 

Calculate the empirical and molecular formulas of a compound from percentage composition and molecular weight. (Section 3.5) Calculate amounts, in grams or moles, of reactants and products for a reaction. (Section 3.6)... 

Calculate the empirical and molecular formulas of a compound from percentage composition and molecular... 

The sum of the percentages is 5.926% + 94.06% = 99.99%. The small discrepancy from 100 percent is due to the way we rounded off the molar masses of the elements. If we had used the empirical formula HO for the calculation, we would have obtained the same percentages. This is so because both the molecular formula and empirical formula tell us the percent composition by mass of the compound. 

Once you determine the empirical formula for a compound, you can calculate its empirical formula weight. If you have an experimental determination of its molecular weight, you can calculate n and then the molecular formula. The next example illustrates how you use percentage composition and molecular weight to determine the molecular formula of acetic acid. 

Determining the molecular formula from percentage composition and molecular weight Given the empirical formula and molecular weight of a substance, obtain its molecular formula. (EXAMPLE 3.12)... 

The mass percentage composition of a compound that assists in the coagulation of blood is 76.71 % C, 7.02% H, and 16.27% N. Determine the empirical formula of the compound. 

Self-TfsT F.4B The mass percentage composition of the compound thionyl difluoride is 18.59% O, 37.25% S, and 44.16% F. Calculate its empirical formula. 

F.14 Paclitaxel, which is extracted from the Pacific yew tree Taxus brevifolia, has antitumor activity for ovarian and breast cancer. It is sold under the trade name Taxol. On analysis, its mass percentage composition is 66.11% C, 6.02% H, and 1.64% N, with the balance being oxygen. What is the empirical formula of paclitaxel ... 

J.9 You are asked to identify compound X, which was extracted from a plant seized by customs inspectors. You run a number of tests and collect the following data. Compound X is a white, crystalline solid. An aqueous solution of X turns litmus red and conducts electricity poorly, even when X is present at appreciable concentrations. When you add sodium hydroxide to the solution a reaction takes place. A solution of the products of the reaction conducts electricity well. An elemental analysis of X shows that the mass percentage composition of the compound is 26.68% C and 2.239% H, with the remainder being oxygen. A mass spectrum of X yields a molar mass of 90.0 g-moF. (a) Write the empirical formula of X. (b) Write... 

In the problem above, we determined the percentage data from the chemical formula. We can determine the empirical formula if we know the percent compositions of the various elements. The empirical formula tells us what elements are present in the compound and the simplest whole-number ratio of elements. The data may be in terms of percentage, or mass or even moles. However, the procedure is still the same—convert each element to moles, divide each by the smallest, and then use an appropriate multiplier if necessary. We can then determine the empirical formula mass. If we know the actual molecular mass, dividing the molecular formula mass by the empirical formula mass, gives an integer (rounded if needed) that we can multiply each of the subscripts in the empirical formula. This gives the molecular (actual) formula, which tells what elements are in the compound and the actual number of each. 

After we receive the results of a combustion analysis from the laboratory, we need to convert the mass percentage composition to an empirical formula. For this step, we need to determine the relative number of moles of each type of atom. The simplest procedure is to imagine that we have a sample of mass 100 g exactly. That way, the mass percentage composition tells us the mass in grams of each element. Then we can use the molar mass of each element to convert these masses into moles and go on to find the relative numbers of moles of each type of atom. Let s do that for vitamin C, which was once identified in this way, and suppose that the laboratory has reported that the sample you supplied is 40.9% carbon, 4.58% hydrogen, and 54.5% oxygen. 

Derive the empirical formula of a hydrocarbon that on analysis gave the following percentage composition C = 85.63% and H = 14.37%. 

Answer The first step is to calculate the empirical formula. To begin, we need to determine the number of moles of each element. However, the percentage composition does not tell us a mass. To get around this, the commonly used technique is to assume that you have a 100.0-gram sample of the substance. Because 100.0 grams is equal to 100%, you can simply take the percentages and say that the masses are equal to that same amount in grams. From there, the problem proceeds just like the previous example ... 

Since you know the percentage composition, it is convenient to assume that you have 100 g of the compound. This means that you have 85.6 g of carbon and 14.4 g of hydrogen. Convert each mass to moles. The number of moles can then be converted into a lowest terms ratio of the elements to get the empirical formula. 

The percentage composition of a fuel is 81.7% carbon and 18.3% hydrogen. Find the empirical formula of the fuel. 

In practice, you can determine a compound s empirical formula by analyzing its percentage composition. There are several different ways to do this. One way is to use a synthesis reaction in which a sample of an element with a known mass reacts with another element to form a compound. Since you know the mass of one of the elements and you can measure the mass of the compound produced, you can calculate the percentage composition. 

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