Question

What experimental evidence about a new compound must be known before its formula can be determined?

Short answer

To determine the formula of a new compound, the following experimental evidence must be known: 1) the elements present in the compound, 2) the relative proportions of these elements, 3) the oxidation states of the elements, 4) the empirical formula (lowest whole-number ratio of elements), and 5) the molecular formula (actual number of atoms of each element in one molecule of the compound). This information can be obtained through various analytical techniques such as mass spectrometry, elemental analysis, X-ray diffraction, and redox titration.

Step-by-step solution

1. Determine the Elements Present in the Compound

First, the elements present in the compound need to be identified. This can be done using various analytical techniques like mass spectrometry, X-ray diffraction, or elemental analysis.

2. Calculate the Relative Proportions of the Elements

Once the elements are identified, it is necessary to determine their relative proportions in the compound. This information can be obtained using quantitative elemental analysis, such as combustion analysis for organic compounds, gravimetric analysis, or titration.

3. Determine the Oxidation States of the Elements

Depending on the elements present and their potential oxidation states, it may be necessary to determine the oxidation state of each element in the compound. This information can be obtained using techniques like X-ray photoelectron spectroscopy, electron paramagnetic resonance, or redox titration.

4. Use Data to Derive Empirical Formula

With the elemental composition, their relative proportions, and oxidation states, the empirical formula (lowest whole-number ratio of elements) can be derived. For example, if you find that the compound is composed of 40% carbon, 53.3307% oxygen, and 6.6667% hydrogen, you will first convert these percentages into moles and then calculate the lowest whole-number ratio to obtain the empirical formula.

5. Determine the Molecular Formula

Finally, once the empirical formula has been derived, additional data such as the molar mass of the compound can be used to find the actual molecular formula. The molecular formula represents the actual number of atoms of each element in one molecule of the compound. Using the molar mass, you can calculate the multiple of the empirical formula and derive the molecular formula.

Key concepts

Elemental Composition

To determine the formula of a compound, the elemental composition must be analyzed first. This involves identifying which elements are present in the compound. Various analytical techniques are used for this purpose, such as:

• Mass spectrometry, which helps identify elements based on their mass-to-charge ratio.
• X-ray diffraction, revealing the structural arrangement and presence of elements.
• Elemental analysis providing percentages of different elements in the compound.

Understanding the elemental composition is crucial. It lays the groundwork for determining how these elements combine to form the compound.

Empirical Formula

Once the elemental composition is clear, the next step is to find the empirical formula. This represents the simplest whole-number ratio of elements in a compound. Calculating this involves:

• Converting the mass or percentage of each element into moles.
• Finding the smallest mole ratio by dividing each element's mole quantity by the smallest number of moles present among the elements.

The empirical formula does not depict the actual numbers in a molecule, but it gives an essential insight into the compound's composition. For example, a compound with 40% carbon, 6.67% hydrogen, and 53.33% oxygen has an empirical formula of CH₂O.

Molecular Formula

After obtaining the empirical formula, the molecular formula must be determined. This defines the exact number of atoms of each element in a molecule. Identifying the molecular formula requires knowing the molar mass of the compound. Use the empirical formula's mass to find how many times it fits into the molar mass of the compound. Multiply this ratio by the subscripts in the empirical formula to find the molecular formula. For example, if the empirical formula is CH₂ and the molecular mass is found to be 28 g/mol, the molecular formula would be C₂H₄ (since CH₂ has a molar mass of 14 g/mol).

Oxidation States

Determining the oxidation states of elements in a compound is essential for understanding chemical bonding and reactivity. The oxidation state, or oxidation number, indicates the degree of oxidation of an atom. This can be determined using analytical methods such as:

• X-ray photoelectron spectroscopy, analyzing the energy levels of electrons.
• Electron paramagnetic resonance, used for elements that exhibit paramagnetism.
• Redox titration, a chemical technique to measure oxidation numbers.

Knowing the oxidation states helps in predicting chemical behavior and reaction pathways. It ensures that the derived formula is chemically plausible and accurate.

Analytical Techniques

Various analytical techniques are employed to determine the composition and structure of a compound. These techniques help identify elements, their amounts, and other chemical properties crucial for accurate formula determination. Some key techniques include:

• Mass Spectrometry: Provides detailed elemental composition by measuring ion mass-to-charge ratios.
• X-ray Diffraction: Offers insights into crystal structures, aiding in identifying elemental arrangements.
• Combustion Analysis: Commonly used for organic compounds to determine the elemental composition by burning the sample and analyzing the resultant gases.

These methods are essential for the chemist to gather experimental evidence, laying foundational data necessary for deriving empirical and molecular formulas.