A Level Chemistry Revision "Relative Molecular Mass and Relative Formula Mass"

Summary

This educational video from 'Three Science' teaches viewers how to calculate the relative molecular mass (Mr) and relative formula mass for different types of compounds. It explains that Mr is the weighted mean mass of a molecule, with carbon-12 as the reference. The video demonstrates the process using dichloroethane as an example, showing how to sum the relative atomic masses of its constituent atoms. It also introduces the concept of relative formula mass, which is calculated similarly but applies to empirical formulas of ionic compounds, using magnesium hydroxide as an example. The video promises further lessons on mole calculations in the next installment.

Takeaways

• 🧪 The video is about teaching the calculation of relative molecular mass (Mr) and relative formula mass.
• 🔍 Molecular formulas are used for covalent molecules like butane, but not for ionic compounds which form giant structures.
• 📊 For ionic compounds, the empirical formula is used, which is the simplest whole number ratio of atoms of each element.
• 📘 The relative molecular mass is calculated using the weighted mean mass of a molecule compared to 1/12 the mass of a carbon-12 atom.
• 📝 The symbol for relative molecular mass is Mr.
• 🔢 The calculation of Mr involves summing the relative atomic masses of all the atoms in a molecule.
• 🌐 Large numbers are not used in Mr calculations because it applies to a single molecule, regardless of the number of molecules.
• 📚 The video provides an example of calculating the Mr of dichloroethane, resulting in 99.0.
• 🔬 The relative formula mass is calculated in the same way as Mr, but it's used for ionic compounds with their empirical formula.
• 📈 An example given is the calculation of the relative formula mass of magnesium hydroxide, which is 58.3.
• 🚀 The next video will cover calculations involving the concept of the mole.

Q & A

What is the main topic of the video?

The main topic of the video is to teach viewers how to calculate relative molecular mass and relative formula mass.

Why can't molecular formulas be used for giant structures like ionic compounds?

Molecular formulas can't be used for giant structures like ionic compounds because they form crystals with a giant ionic lattice, and the number of atoms of each element is huge and varies with the size of the crystal.

What is an empirical formula and when is it used?

An empirical formula is the simplest whole number ratio of the atoms of each element in a compound and is used for giant structures where the number of atoms is too large to be practical for a molecular formula.

What is the definition of relative molecular mass?

Relative molecular mass is defined as the weighted mean mass of a molecule compared with 1/12 of the mass of an atom of carbon-12.

What symbol is used to represent relative molecular mass?

The symbol used to represent relative molecular mass is 'Mr', with a capital 'M' and a lowercase 'r'.

How is the relative molecular mass of a compound calculated?

The relative molecular mass of a compound is calculated by adding up all the relative atomic masses of the atoms in the molecule.

Why don't we use large numbers when calculating relative molecular mass?

We don't use large numbers to calculate relative molecular mass because it applies to a single molecule, not to a collection of molecules.

What is the relative atomic mass of carbon, hydrogen, and chlorine used in the example of dichloroethane?

The relative atomic masses used in the example are 12.0 for carbon, 1.0 for hydrogen, and 35.5 for chlorine.

What is the calculated relative molecular mass of dichloroethane based on the given atomic masses?

The calculated relative molecular mass of dichloroethane is 99.0.

What is the difference between calculating relative molecular mass and relative formula mass?

The difference is that relative molecular mass is used for covalent molecules, while relative formula mass is used for ionic compounds and is calculated using the empirical formula.

How is the relative formula mass of an ionic compound like magnesium hydroxide calculated?

The relative formula mass of an ionic compound like magnesium hydroxide is calculated by adding the relative atomic masses of the constituent atoms: one magnesium (24.3), two oxygens (16.0 each), and two hydrogens (1.0 each), resulting in a relative formula mass of 58.3.