Question

Selenium, an element used in the manufacture of photoelectric cells and solar energy devices, forms two oxides. One has \(28.8 \%\) O, by mass, and the other, 37.8\% O. What are the formulas of these oxides? Propose acceptable names for them.

Short answer

The formulas are Selenium Dioxide (\(SeO_2\)) and Selenium Trioxide (\(SeO_3\)).

Step-by-step solution

Convert Percentage Composition to Masses

Assume a 100g sample of each compound. Hence, one compound will have \(28.8g\) of Oxygen and \(71.2g\) of Selenium (since \(100g - 28.8g = 71.2g\) of Selenium). The other compound will have \(37.8g\) of Oxygen and \(62.2g\) of Selenium (since \(100g - 37.8g = 62.2g\) of Selenium).

Convert to Moles

The molar mass of Oxygen (\(O\)) is \(16g/mol\) and Selenium (\(Se\)) is \(78.96 g/mol\). For the first oxide, the moles of \(O = \frac{28.8}{16} = 1.8 mol\) and moles of \(Se = \frac{71.2}{78.96} = 0.9 mol\). For the second oxide, the moles of \(O = \frac{37.8}{16} = 2.3625 mol\) and moles of \(Se = \frac{62.2}{78.96} = 0.7875 mol\).

Find Empirical Formula

The ratio of \(O:Se\) for the first compound is \(1.8:0.9 = 2:1\) so the empirical formula is \(SeO_2\). The ratio of \(O:Se\) for the second compound is \(2.3625:0.7875 = 3:1\) so the empirical formula is \(SeO_3\).

Determine the Names of Compounds

The compound with empirical formula \(SeO_2\) is known as Selenium Dioxide, and the one with empirical formula \(SeO_3\) is often referred to as Selenium Trioxide.

Key concepts

Percentage Composition

Percentage composition helps determine the amount of each element in a compound by its mass relative to the entire compound. This is crucial when analyzing the makeup of compounds like the oxides of selenium.
For instance, in our problem, one oxide of selenium consists of \(28.8\%\) oxygen. The calculation for this is based on assuming a 100g sample, which simplifies the math because it directly translates to 28.8 grams of oxygen.
By subtracting this from 100 grams, you find the mass of selenium in the compound, which is crucial for further calculations.

• In a 100g sample:
• Selenium's mass for the first oxide: \(100g - 28.8g = 71.2g\)
• For the second oxide: \(100g - 37.8g = 62.2g\)

This step precedes converting these masses to moles, to engage in deeper analysis of the compound's structure.

Molecular Composition

Molecular composition involves understanding which atoms and how many of them make up a compound. In any chemical compound, each element is present in a certain fixed ratio.
The initial step is finding out the composition by percentage, as seen with selenium's oxides. The percentage values translate to mass quantities, which then can be evaluated in terms of moles.
This calculation involves dividing the mass of each element by its atomic molar mass, giving the number of moles.

• Moles of Oxygen in first oxide: \( \frac{28.8}{16} = 1.8 \) moles
• Moles of Selenium in first oxide: \( \frac{71.2}{78.96} \approx 0.9 \) moles

The ratio of these moles informs us about the molecular structure or composition, allowing for empirical formula determination.

Empirical Formula

The empirical formula reflects the simplest whole number ratio of atoms in a compound, and it is crucial in chemical identification and naming.
For the selenium oxides, after calculating the moles of selenium and oxygen, you find their ratio:
First oxide:

• Oxygen to Selenium: \(1.8:0.9 = 2:1\)

Thus, the empirical formula is \(SeO_2\).
In the second oxide:

• Oxygen to Selenium: \(2.3625:0.7875 = 3:1\)

This gives us \(SeO_3\).
Empirical formulas help in discerning different compounds even with similar elemental compositions.

Molar Mass Calculation

Calculating molar mass is pivotal in transitioning from a percentage composition to understanding a molecule's structure.
The molar mass of an element is the mass of one mole of its atoms. Oxygen has a molar mass of \(16g/mol\), and selenium's molar mass is \(78.96 g/mol\).
This information allows chemists to convert from grams to moles, a standard step in chemical analysis.

• The formula includes dividing the mass of an element by its molar mass, thereby finding the number of moles.

With moles, you can explore empirical formulas and gain deeper insights into the compound's chemical behavior.

Chemical Naming Conventions

Chemical naming conventions, or nomenclature, give each chemical compound a specific and universally understood name.
Once the empirical formula of a compound is known, naming it is simpler. For selenium's oxides, the names are derived from their formulas:

• \(SeO_2\) is called Selenium Dioxide because it has two oxygen atoms for every Selenium atom.
• \(SeO_3\) becomes Selenium Trioxide, indicating three oxygen atom presence.

These conventions ensure clarity and consistency in chemical communication, allowing chemists worldwide to understand essential information about the compounds.