Question

What information must a chemist obtain in order to determine the empirical formula of an unknown compound?

Short answer

To determine the empirical formula of an unknown compound, a chemist must obtain the following information: the elements present in the compound, the mass or percentage composition of each element, convert these masses to moles, find the simplest whole number mole ratio of elements, and finally write the empirical formula using subscript numbers to indicate the atom count for each element. Techniques like qualitative and quantitative analysis, gravimetric analysis, and spectrometric methods can be used to obtain the required information.

Step-by-step solution

Understand the elements present in the unknown compound

The chemist needs to know what elements are present in the unknown compound to determine its empirical formula. They can obtain this information through qualitative analysis or using a technique like mass spectrometry.

Obtain the mass or percentage composition of each element

The chemist must obtain the mass or percentage composition of each element present in the compound. This can be determined through quantitative analysis, gravimetric analysis, or spectrometric techniques. If the percentage composition is known, the mass of the element can be calculated on the basis of a fixed sample mass (e.g., 100 grams).

Convert the mass or percentage of each element to moles

The chemist should then convert the mass or percentage of each element to moles. To do this, divide the mass of each element by its respective molar mass (from the periodic table). This will give the moles of each element present in the compound.

Determine the mole ratio of elements

The chemist should then find the mole ratio of each element. To do this, divide the moles of each element by the smallest number of moles among the elements. This will simplify the mole ratios to a whole number ratio.

Write the empirical formula

After obtaining the simplest whole number ratios for each element, the chemist can now write the empirical formula by placing subscript numbers next to the appropriate element symbols to indicate how many atoms of each element are present in the compound. The empirical formula represents the simplest ratio of atoms in a compound.

Key concepts

Qualitative Analysis

Qualitative analysis is an essential process in chemistry that allows a chemist to determine what elements are present in an unknown compound. This type of analysis focuses on the identification of different substances without quantifying them. It answers the question: "Which elements are there?" rather than "How much of each is there?"
For instance, if you have a mysterious powder, qualitative analysis might establish that it contains carbon, hydrogen, and oxygen. This can be achieved using various techniques, among which mass spectrometry is a common one. Mass spectrometry can be used not only to identify the elements but also to reveal structural or isotopic information through the mass-to-charge ratios of ionized particles.
Qualitative analysis lays the foundation for understanding the chemical makeup of a compound, which is critical for further analysis and determining empirical formulas.

Quantitative Analysis

Quantitative analysis complements qualitative analysis by determining the amount of each element present in the compound. This type of analysis is concerned with measuring and reporting the proportions of substances within a sample.
To realize this, a chemist often relies on tools such as gravimetric analysis, which involves measuring the mass of substances, or spectrometric techniques, which can quantify concentrations by measuring how much light a substance absorbs. By obtaining this data, a chemist can convert these measurements into a percentage composition or into grams for further calculations.
Thus, quantitative analysis provides the numerical data necessary to compute the empirical formula of the compound by comparing the relative amounts of each element.

Mole Ratio

The mole ratio is a critical concept in chemistry used to describe the proportional relationship between the elements in a compound. Once a chemist has determined the number of moles of each element from their masses or percentage compositions, the next step is to find the simplest ratio among them.
To calculate the mole ratio, divide the number of moles of each element by the smallest number of moles that have been calculated among the elements. This step is about simplifying these ratios to the smallest whole numbers. For example, if you have 2 moles of hydrogen and 1 mole of oxygen, the mole ratio is 2:1.
This simplification helps in determining the empirical formula, which represents the simplest integer ratio of the elements in a compound, leading to a better understanding of the compound's basic composition.

Mass Spectrometry

Mass spectrometry is a powerful analytical tool used for qualitative and quantitative analysis, and is especially useful in determining the empirical formula of a compound.
This technique involves ionizing chemical compounds to generate charged particles or ions and then measuring their mass-to-charge ratio. Each element produces a unique mass spectrum, like a fingerprint, which helps in identifying the elements present in a sample qualitatively.
Quantitative data can also be derived from mass spectrometry by evaluating the abundance of ions, providing insights into the proportion of different elements within the compound. Unlike qualitative methods, it combines both elemental detection and quantification, making it an invaluable technique in modern chemistry to derive empirical formulas accurately.