Problem 125 - Chemistry The Molecular Science

Question

A chemist analyzed several portions taken from different parts of a sample that contained only selenium, Se, and oxygen, \(\mathrm{O}\). She reported the results in the table. Could this sample be a compound of selenium and oxygen? Explain why or why not. $$ \begin{array}{ccc} \hline \text { Portion } & \text { Total Mass }(g) & \text { Mass of Se }(g) \\\ \hline 1 & 1.518 & 1.08 \\ 2 & 2.056 & 1.46 \\ 3 & 1.873 & 1.33 \\ \hline \end{array} $$

Step-by-Step Solution

Answer

Yes, the sample could be a compound because the mass ratios are nearly constant.

Step 1: Calculate Mass of Oxygen for Each Portion

To determine the mass of oxygen in each portion, subtract the mass of selenium from the total mass for each. For Portion 1: \(1.518 - 1.08 = 0.438\), for Portion 2: \(2.056 - 1.46 = 0.596\), and for Portion 3: \(1.873 - 1.33 = 0.543\).

Step 2: Determine the Mass Ratios

Calculate the ratio of mass of selenium to mass of oxygen for each portion. For Portion 1: \(\frac{1.08}{0.438} = 2.47\), for Portion 2: \(\frac{1.46}{0.596} = 2.45\), and for Portion 3: \(\frac{1.33}{0.543} = 2.45\).

Step 3: Compare the Mass Ratios

Compare the mass ratios. If the ratios are the same or very close, the sample could be a compound because compounds have a fixed ratio of elements. Here, the ratios (2.47, 2.45, 2.45) are very close to each other, suggesting a fixed composition.

Step 4: Conclude Based on Constant Mass Ratio

Since the ratios of selenium to oxygen masses are consistent across portions, the sample could indeed be a compound of selenium and oxygen.

Key Concepts

Compound Identification

Identifying a chemical compound involves understanding whether the elements combine in a fixed and consistent ratio. In this exercise, we have selenium and oxygen, tested across different portions to see if they adhere to a specific pattern. Chemical compounds are characterized by a distinct atomic ratio that does not vary, wherever the compound is analyzed or sampled. This means that if selenium and oxygen form a compound, their ratio is a unique signature that remains perceptible, despite changes in sample size or quantity.

Mass Ratio

The concept of mass ratio in stoichiometry is an essential tool for determining the existence of a compound. It is calculated by dividing the mass of one element by the mass of another element within the substance.
In our example, the calculated mass ratio of selenium to oxygen across the three portions were approximately all around 2.45.

• Portion 1: \(rac{1.08}{0.438} = 2.47\)
• Portion 2: \(\frac{1.46}{0.596} = 2.45\)
• Portion 3: \(\frac{1.33}{0.543}= 2.45\)

These values being very close to each other indicate a possible fixed composition, leading us to suggest a chemical bond between the two elements.

Elemental Analysis

Elemental analysis is a common technique to determine the composition of a chemical substance. This process includes calculating the mass of each element present in a sample.
In this exercise, it involved determining the mass of selenium and then subtracting this from the total mass to find the mass of oxygen for each portion. This allows us to further explore the mass ratio between selenium and oxygen. Through consistent elemental analysis, chemists can confirm or refute the composition and identity of the sample in question, looking for uniform patterns that represent a true chemical compound.

Consistent Composition

Consistent composition is a hallmark of true chemical compounds. According to the law of definite proportions, compounds are made from elements in specific mass ratios that never change. In the case of the selenium and oxygen sample, the consistent mass ratios across portions strongly indicated that their composition was not random.
The close similarity in mass ratios (2.47 for Portion 1 and 2.45 for Portions 2 and 3) suggests that the different parts of the sample maintain a stable, fixed composition. This consistency in composition across all sampled portions strongly supports the hypothesis that the analyzed substance is a compound of selenium and oxygen.