Empirical Formulae From Percentage Composition | Chemical Calculations | Chemistry | FuseSchool

Summary

This educational video explains the distinction between empirical and molecular formulas, demonstrating how to derive the empirical formula from percentage composition by mass and how to calculate the molecular formula from it. Using hydrazine (N2H4) as an example, the video illustrates the process of determining the simplest atomic ratio and multiplying it by a factor to match the compound's molecular mass, ultimately revealing the exact number of atoms in a molecule.

Takeaways

• 🔍 The video explains the difference between empirical and molecular formulas.
• 📚 Empirical formula shows the simplest ratio of different types of atoms in a molecule.
• 📝 Molecular formula indicates the exact number of each type of atom in a molecule.
• 🧪 The empirical formula of hydrazine (N2H4) is NH2, which simplifies the ratio of nitrogen to hydrogen to 1:2.
• 🔢 To find the empirical formula from percentage composition by mass, divide the percentages by the relative atomic masses of the elements.
• 📉 For example, with 87.5% nitrogen and 12.5% hydrogen, dividing by their respective atomic masses (14 for N, 1 for H) gives the ratio 6.25:12.5.
• 🔄 Further simplification of the ratio 6.25:12.5 by dividing by the smallest number (6.25) results in the empirical formula NH2.
• 📈 The molecular mass of a compound can be used to find the molecular formula from the empirical formula.
• 🔑 The phrase 'empirical mass' is introduced to describe the mass of the empirical formula, which is 16 for NH2.
• 🔍 The molecular mass (32 for N2H4) divided by the empirical mass gives the factor 'n' to multiply the empirical formula to get the molecular formula.
• 🧠 The molecular formula is the empirical formula multiplied by 'n', resulting in N2H4 for hydrazine.

Q & A

What is the main topic of the video?

The main topic of the video is to explain the difference between empirical and molecular formulas, and how to find them from percentage composition by mass.

What are the two things that the molecular formula of a compound tells us?

The molecular formula of a compound tells us the types of atoms in the molecule and the exact number of each type of atom present.

What is an empirical formula?

An empirical formula tells us the types of atoms in a molecule and the simplest whole number ratio of each type of atom.

What is the empirical formula of hydrazine (N2H4)?

The empirical formula of hydrazine is NH2, which represents the simplest ratio of nitrogen to hydrogen atoms.

How do you find the empirical formula from percentage composition by mass?

To find the empirical formula from percentage composition by mass, you divide the percentage of each element by its relative atomic mass, then divide by the smallest resulting number to get the simplest whole number ratio.

What is the percentage composition by mass of nitrogen and hydrogen in the given example?

In the given example, the percentage composition by mass is 87.5% nitrogen and 12.5% hydrogen.

How is the empirical formula determined from the calculated ratios of elements?

The empirical formula is determined by taking the smallest whole number ratio obtained from dividing the calculated ratios by the smallest value among them.

What is the molecular mass of the compound in the example, and how does it relate to the empirical mass?

The molecular mass of the compound in the example is 32. It is related to the empirical mass by the formula: molecular mass = n * empirical mass, where n is the multiplier needed to reach the molecular mass.

What is the term 'empirical mass' used to describe?

The term 'empirical mass' is used to describe the total mass of the empirical formula, which is the sum of the relative atomic masses of the atoms in the empirical formula.

How do you find the molecular formula from the empirical formula and the molecular mass?

You find the molecular formula by dividing the molecular mass by the empirical mass, which gives you the multiplier n. Then you multiply the empirical formula by n to get the molecular formula.

What is the final molecular formula of the compound in the example?

The final molecular formula of the compound in the example is N2H4, which is obtained by multiplying the empirical formula NH2 by the multiplier n, which is 2.