This is because we can divide each number in C 6 H 12 O 6 by 6 to make a simpler whole number ratio. You can use information about reacting masses to calculate the formula of a compound.
Here is an example:. What is the formula of the oxide? The action at Step 5 usually gives you the simplest whole number ratio straightaway. Sometimes it does not, so you might get 1 and 1. In this example, you would multiply both numbers by 2, giving 2 and 3 instead of rounding 1. In order to determine the Empirical formula for a compound or molecule, we need to know the mass percentages of the the elements in the compound.
Once we have this information we can convert it to moles to determine the ratios between the elements. NutraSweet is Calculate the empirical formula of NutraSweet and find the molecular formula. The molar mass of NutraSweet is Convert the mass of each element to moles using the molar mass from the periodic table.
Check Your Learning To three significant digits, what is the mass percentage of iron in the compound Fe 2 O 3? However, we must keep in mind that chemical formulas represent the relative numbers , not masses, of atoms in the substance. Therefore, any experimentally derived data involving mass must be used to derive the corresponding numbers of atoms in the compound. To accomplish this, we can use molar masses to convert the mass of each element to a number of moles.
We then consider the moles of each element relative to each other, converting these numbers into a whole-number ratio that can be used to derive the empirical formula of the substance. Consider a sample of compound determined to contain 1. The corresponding numbers of atoms in moles are:.
Thus, we can accurately represent this compound with the formula C 0. Of course, per accepted convention, formulas contain whole-number subscripts, which can be achieved by dividing each subscript by the smaller subscript:. The empirical formula for this compound is thus CH 2. Consider as another example a sample of compound determined to contain 5. Following the same approach yields a tentative empirical formula of:.
In this case, dividing by the smallest subscript still leaves us with a decimal subscript in the empirical formula. To convert this into a whole number, we must multiply each of the subscripts by two, retaining the same atom ratio and yielding Cl 2 O 7 as the final empirical formula.
In summary, empirical formulas are derived from experimentally measured element masses by:. Figure 1 outlines this procedure in flow chart fashion for a substance containing elements A and X. What is the empirical formula of hematite? Solution For this problem, we are given the mass in grams of each element.
Begin by finding the moles of each:. Next, derive the iron-to-oxygen molar ratio by dividing by the lesser number of moles:. The ratio is 1. Finally, multiply the ratio by two to get the smallest possible whole number subscripts while still maintaining the correct iron-to-oxygen ratio:. Check Your Learning What is the empirical formula of a compound if a sample contains 0. For additional worked examples illustrating the derivation of empirical formulas, watch the brief video clip.
In such cases, the percent composition can be used to calculate the masses of elements present in any convenient mass of compound; these masses can then be used to derive the empirical formula in the usual fashion. The bacterial fermentation of grain to produce ethanol forms a gas with a percent composition of What is the empirical formula for this gas? Solution Since the scale for percentages is , it is most convenient to calculate the mass of elements present in a sample weighing g.
Coefficients for the tentative empirical formula are derived by dividing each molar amount by the lesser of the two:. Since the resulting ratio is one carbon to two oxygen atoms, the empirical formula is CO 2. Check Your Learning What is the empirical formula of a compound containing Determining the absolute numbers of atoms that compose a single molecule of a covalent compound requires knowledge of both its empirical formula and its molecular mass or molar mass.
These quantities may be determined experimentally by various measurement techniques. Molecular mass, for example, is often derived from the mass spectrum of the compound see discussion of this technique in the previous chapter on atoms and molecules.
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