The bulk mass of a substance is related to the number of atoms in it.
We were able to determine the amount of the substance from its mass using the chemical formula.
In this section, we will look at how to apply the same principles in order to derive the chemical formulas of unknown substances.
Chemical formulas are the most succinct way of representing the makeup of a compound.
When a compound's formula is unknown, measuring the mass of each of its elements is the first step in the process of figuring it out.
Consider a compound composed of carbon and hydrogen.
The article was accessed on January 14, 2015.
The compound is 61.0% C, 15.4% H, and 23.7% N by mass, according to the analysis results.
A sample of a compound containing only carbon, oxygen, and chlorine has been determined to contain 3.01 g C, 4.00 g O, and 17.81 g Cl.
The relative abundance of a given element in different compounds of known formulas can be evaluated using percent composition.
Consider the common nitrogen-containedfertilizer ammonia (NH3), ammonium nitrate (NH4NO3), and urea.
The mass percentage of nitrogen in the compound is a practical and economic concern for consumers choosing among these fertilizers.
The percent composition of a compound can be derived from its formula mass and atomic mass.
A molecule of NH3 has one N atom weighing 14.01 amu and three H atoms weighing a total of 3 x 1.008 amu.
This same approach can be used to consider a pair of molecules, a dozen molecule, or a mole of molecule.
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.
There is a compound with the formula C9H8O4.
To calculate the percent composition, we need to know the mass of C, H, and O.
When rounded, these percentages equal 100.00%.
The most common approach to determining a compound's chemical formula is to first measure the mass of its elements.
The corresponding numbers of atoms in the compound must be derived from any data involving mass.
The mass of each element can be converted to a number of moles using the molar mass method.
The moles of each element relative to each other are converted into a whole-number ratio that can be used to derive the empirical formula of the substance.
A sample of compound contains 1.71 g C and 0.287 g H.
The formula for this compound is CH2.
A sample of compound is determined to contain 5.31 g Cl and 8.40 g O.
If we divide by the smallest subscript, we still have a decimal subscript in the empirical formula.
To convert this into a whole number, we must use the same atom ratio and formula as the final empirical formula.
To ensure that the smallest whole-number ratio of subscripts is obtained, the procedure is outlined in Figure 3.11.
The sample can be used to derive the empirical formula of a compound.
A sample of the black mineral hematite an oxide of iron found in many iron ores has 34.97 g of iron and 15.3 g of oxygen.
Hematite is used in jewelry.
Each element has a mass in grams.
The iron to oxygen ratio is 1.000 mol to 1.500 mol.
The formula is Fe2O3.
There are more examples of the derivation of empirical formulas that you can watch.
Consider instances in which a compound's percent composition is available rather than the absolute mass of the compound's elements.
In such cases, the percent composition can be used to calculate the mass of elements present in any convenient mass of compound; these mass can then be used to derive the empirical formula in the usual fashion.
The copper pipes at the top of the fermentation tanks are used to remove the oxide of carbon from the tanks.
Since the scale for percentages is 100, it is most convenient to calculate the mass of elements present in a sample weighing 100 g.
The empirical formula is CO2 since the resulting ratio is one carbon to two oxygen atoms.
Various measurement techniques can be used to determine these quantities.
The mass spectrum of the compound can be used to derive the molecule mass.
Many of the methods used to measure molar mass will be introduced in later chapters of this text.
An empirical formula mass is the average atomic mass of all the atoms in an empirical formula.
Consider a compound with an empirical formula that is determined to be CH2O.
The formula mass for this compound is 30 amu, which is the sum of 12 amu for one C atom, 2 amu for two H atoms, and 16 amu for one O atom.
The molecule's subscripts are six times greater than those in the empirical formula, which is why this approach is used.
We are only considering one mole of empirical formula units and molecule, as opposed to single units and molecule.