![]() ![]() Thus, we can accurately represent this compound with the formula C 0.142H 0.248. ![]() The corresponding numbers of atoms (in moles) are: Consider a sample of compound determined to contain 1.71 g C and 0.287 g H. 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. To accomplish this, we can use molar masses to convert the mass of each element to a number of moles. Therefore, any experimentally derived data involving mass must be used to derive the corresponding numbers of atoms in the compound. However, we must keep in mind that chemical formulas represent the relative numbers, not masses, of atoms in the substance. As long as we know the chemical formula of the substance in question, we can easily derive percent composition from the formula mass or molar mass.Īs previously mentioned, the most common approach to determining a compound’s chemical formula is to first measure the masses of its constituent elements. The latter amount is most convenient and would simply involve the use of molar masses instead of atomic and formula masses, as demonstrated in the example problem below. This same approach may be taken considering a pair of molecules, a dozen molecules, or a mole of molecules, etc. A molecule of NH 3 contains one N atom weighing 14.01 amu and three H atoms weighing a total of (3 × 1.008 amu) = 3.024 amu The formula mass of ammonia is therefore (14.01 amu + 3.024 amu) = 17.03 amu, and its percent composition is: For these sorts of applications, the percent composition of a compound is easily derived from its formula mass and the atomic masses of its constituent elements. The element nitrogen is the active ingredient for agricultural purposes, so the mass percentage of nitrogen in the compound is a practical and economic concern for consumers choosing among these fertilizers. As one example, consider the common nitrogen-containing fertilizers ammonia (NH 3), ammonium nitrate (NH 4NO 3), and urea (CH 4N 2O). Percent composition is also useful for evaluating the relative abundance of a given element in different compounds of known formulas. \)ĭetermining Percent Composition from Formula Mass
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