Question

Considering your answer to Exercise \(81,\) which type of formula, empirical or molecular, can be obtained from elemental analysis that gives percent composition?

Short answer

Only an empirical formula can be obtained from elemental analysis that gives percent composition, as it provides the simplest whole number ratio of elements present in a compound. The molecular formula cannot be directly determined from percent composition data as it requires information on the actual number of atoms of each element in the compound.

Step-by-step solution

Understand empirical and molecular formulas

Empirical formulas are the simplest whole number ratio of elements present in a compound, while molecular formulas are the actual number of atoms of each element present in a compound. The molecular formula is always a multiple of the empirical formula. For example, the empirical formula of hydrogen peroxide is HO while its molecular formula is H2O2 (which is made up of two empirical formula units).

Understand elemental analysis and percent composition data

Elemental analysis is a method used to determine the relative amounts of different elements in a compound. The results are usually given in the form of a percent composition, which indicates the percentage of each element present. Percent composition can be determined using the mass of each element present in the compound and the total mass of the compound. The formula to find the percent composition of an element in a compound is: Percent composition =\(\frac{mass\,of\,element\phantom{}in\phantom{}the\phantom{}compound }{total\,mass\,of\,the\,compound}\) x 100

Determine which formula can be obtained from percent composition data

With percent composition data, one can initially determine the simplest whole number ratio of elements present in a compound, which is the empirical formula. However, since the molecular formula is a multiple of the empirical formula and we don't have information on the actual number of atoms of each element in the compound, we cannot directly determine the molecular formula from percent composition data. Therefore, only an empirical formula can be obtained from elemental analysis that gives percent composition.

Key concepts

Molecular Formula

A molecular formula represents the actual number of atoms of each element in a compound. It is like a detailed map that shows the complete details of the structure of the compound. This means it provides the exact count of every type of atom present in a molecule. The molecular formula might look complex at first. But, it is just an extension of the simplest form of the compound you may encounter, the empirical formula.

To understand it better, think about water. While the empirical formula gives the simplest atom ratio, the molecular formula for water is \( H_2O \) indicating there are two hydrogen atoms bonded to a single oxygen atom.

Sometimes, the molecular formula and empirical formula are the same, like in the case of water. However, for compounds like glucose \( C_6H_{12}O_6 \), the molecular formula includes the exact number of atoms, whereas the empirical formula would be \( CH_2O \), representing the simplest ratio by dividing each atom count by 6.

It's key to remember: without additional data, like molar mass, direct determination of the molecular formula from percentage composition isn't possible. Yet, it helps kickstart the process in identifying possibilities.

Elemental Analysis

Elemental analysis is an essential analytical technique used in chemistry to determine the elemental composition of a compound. Essentially, it tells us what mix of elements is there in the compound and their relative amounts.

This is done by measuring the mass of every element in the compound and comparing this to the total mass. The analysis provides crucial insights, helping chemists to understand and confirm the composition before moving to more involved analyses, like structural determination.

In practical terms, elemental analysis results are often given as percent composition. This breaks down into the percentage of each different element within the compound. For example, a common elemental analysis might reveal that a sample consists of 12% carbon, 88% oxygen, indicating high levels of oxygen relative to carbon.

While elemental analysis is thorough, the process itself only unveils an empirical formula, not a molecular one. This limitation stems from lacking the mass needed to precisely determine how many times the empirical formula unit repeats within the molecule.

Percent Composition

Percent composition is a handy way to express the amount of each element in a compound, stated as a percentage of the total mass. This concept reflects the mass ratio of each element to the overall mass of the compound, providing a straightforward understanding of its makeup.

Calculating percent composition involves a simple formula: first, determine the mass of each individual element in the compound, then divide by the total compound mass and multiply by 100. For example:

• Mass of the element = 16 grams
• Total mass of the compound = 50 grams

The percent composition would be \( \left(\frac{16}{50}\right) \times 100 \approx 32\% \).

This method serves as a foundation for deducing an empirical formula—the simplest ratio of elements in the compound. However, keep in mind that percent composition lacks the specific numerical detail required for the molecular formula, as it does not account for how these ratios play out in larger molecule structures. Still, it provides an essential stepping stone for theoretical deductions and further empirical analyses.