7.4 Determining Chemical Formulas

Summary

This video explores the determination of chemical formulas, focusing on empirical and molecular formulas through the example of diborane (B2H6). It explains how to calculate the empirical formula by analyzing the percentage composition of elements, converting to moles, and simplifying ratios. The video illustrates that while the empirical formula shows the simplest ratio (BH3), the molecular formula reveals the actual structure (B2H6). Viewers will learn the mathematical relationships between empirical and molecular formulas, enabling a deeper understanding of chemical compositions.

Takeaways

• 😀 Empirical formulas represent the simplest whole number ratio of elements in a compound.
• 😀 Molecular formulas show the actual number of atoms in a compound, which may differ from the empirical formula.
• 😀 The formula unit is used for ionic compounds, indicating the ratio of ions present.
• 😀 Diborane (B₂H₆) has an empirical formula of BH₃ but a molecular structure that reflects two boron and six hydrogen atoms.
• 😀 To determine the empirical formula, start with a 100 g sample to simplify calculations of percent composition.
• 😀 The percentage composition allows conversion to grams, facilitating the calculation of moles for each element.
• 😀 Moles are calculated by dividing the mass of each element by its molar mass.
• 😀 The smaller mole value indicates the limiting reactant in determining the empirical formula ratio.
• 😀 A coefficient (x) is used to relate empirical formulas to molecular formulas when necessary.
• 😀 The empirical formula mass can be compared to the molecular weight to calculate the coefficient x, which helps derive the molecular formula.

Q & A

What is the purpose of determining empirical formulas in chemistry?

Empirical formulas are determined to represent the simplest whole-number ratio of elements in a compound, based on the percentage composition of the compound.

How do empirical formulas differ from molecular formulas?

Empirical formulas show the lowest whole-number ratio of elements, while molecular formulas show the actual number of atoms of each element in a molecule.

What is an example of a compound that has a different empirical formula than its molecular formula?

Diborane is an example, where the empirical formula is BH₃ and the molecular formula is B₂H₆.

How is the empirical formula of a compound calculated from a sample?

To calculate the empirical formula, the percentage composition of each element is used, and the masses are converted to moles, followed by finding the simplest ratio of these moles.

In the example of diborane, what percentages were found for boron and hydrogen?

In diborane, boron composes 78.1% and hydrogen composes 21.9% of the mass.

What is the significance of the coefficient 'x' in determining molecular formulas?

The coefficient 'x' helps to convert the empirical formula to the molecular formula by indicating how many times the empirical formula is repeated to achieve the molecular structure.

How can you determine the ratio of elements in a compound?

The ratio of elements can be determined by dividing the number of moles of each element by the smallest number of moles calculated among them.

Why might two compounds have the same empirical formula but different molecular formulas?

Two compounds can have the same empirical formula yet differ in molecular formulas due to variations in the actual number of atoms, leading to different chemical properties, as seen with ethene and propene.

What steps are involved in calculating the empirical formula for diborane from a 100g sample?

First, find the mass of each element from the percentage composition, then convert these masses to moles, and finally determine the simplest ratio of the moles.

What is the calculated empirical formula for diborane based on the example provided?

The empirical formula calculated for diborane is BH₃, indicating a 1:3 ratio of boron to hydrogen.