S1.4.2 Relative atomic mass and molecular mass

Summary

This video by MSJ Chem explores the concepts of relative atomic mass (RAM) and relative molecular mass (RMM). RAM is the weighted average mass of an element's isotopes, referenced to carbon-12, and is dimensionless. Elements' RAMs, like hydrogen (1.01) and iron (55.85), are compared to carbon-12 to understand their mass relationships. The video demonstrates calculating RAM using isotopes' abundances, such as for magnesium, yielding 24.32. RMM is the sum of an element's RAMs in a molecule, like H2O's 18.02. The script also introduces relative formula mass, used for ionic compounds like NaCl, calculated at 58.44, emphasizing the importance of these concepts in chemistry.

Takeaways

• 🌟 The relative atomic mass is the weighted average mass of an element's naturally occurring isotopes, compared to 1/12 the mass of a carbon-12 atom.
• 📊 Relative atomic masses are dimensionless and are based on the carbon-12 isotope, which is exactly 12 atomic mass units (amu).
• 🔍 Elements' relative atomic masses can be found on the periodic table, with examples given such as hydrogen (1.01), magnesium (24.31), chlorine (35.45), and iron (55.85).
• 🌐 Relative atomic masses are not whole numbers due to being a weighted average of isotopes, reflecting the natural isotopic distribution of elements.
• ⚖️ Relative atomic mass values allow for the comparison of atomic masses to carbon-12 and to each other, as demonstrated with hydrogen, helium, magnesium, phosphorus, and chlorine.
• 🧪 The calculation of an element's relative atomic mass involves multiplying each isotope's mass by its relative abundance, then summing and normalizing these values.
• 🔢 For magnesium, the relative atomic mass is calculated using the abundances of its isotopes, resulting in a value of 24.32 amu.
• 💧 Relative molecular mass is the sum of the relative atomic masses of the atoms within a molecule, such as H2 (2.02 amu), H2O (18.02 amu), and C2H6 (30.08 amu).
• 🌐 Similar to relative atomic mass, relative molecular mass is also a dimensionless quantity and does not have units.
• 🧩 The term 'relative formula mass' is used for ionic compounds and other substances that do not form discrete molecules, such as sodium chloride (NaCl) with a relative formula mass of 58.44 amu.
• 📚 The video concludes with an introduction to the concept of molar mass, which will be the focus of the next video.

Q & A

What is the definition of relative atomic mass?

Relative atomic mass is the weighted average mass of the naturally occurring isotopes of an element, relative to 1/12 the mass of an atom of carbon-12.

Why is carbon-12 used as the reference for relative atomic mass?

Carbon-12 is used as the reference because it has a mass of exactly 12 atomic mass units (amu), providing a convenient and precise standard for comparison.

What is the relative atomic mass of hydrogen?

The relative atomic mass of hydrogen is 1.01.

How do relative atomic masses help in comparing the masses of different atoms?

Relative atomic masses allow us to compare the masses of different atoms by providing a dimensionless scale where each element's mass is relative to 1/12 the mass of a carbon-12 atom.

Why are relative atomic masses not usually whole numbers?

Relative atomic masses are not usually whole numbers because they represent a weighted average of the naturally occurring isotopes of an element.

How can you calculate the relative atomic mass of an element like magnesium?

You calculate the relative atomic mass of an element by multiplying the mass of each isotope by its relative abundance, summing these values, and then dividing by 100.

What is the relative atomic mass of chlorine?

The relative atomic mass of chlorine is 35.45.

What is the concept of relative molecular mass?

Relative molecular mass is the weighted average mass of a molecule, relative to 1/12 the mass of an atom of carbon-12, and is calculated by summing the relative atomic masses of the atoms in the molecule.

How is the relative molecular mass of a molecule like H2O calculated?

The relative molecular mass of H2O is calculated by adding the relative atomic masses of two hydrogen atoms (1.01 each) and one oxygen atom (approximately 16.00), resulting in a total of 18.02.

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

Relative molecular mass is used for molecules, while relative formula mass is used for compounds that do not form molecules, such as ionic compounds.

Can you provide an example of calculating the relative formula mass of an ionic compound?

Yes, the relative formula mass of sodium chloride (NaCl) is calculated by adding the relative atomic masses of sodium (approximately 22.99) and chlorine (35.45), resulting in 58.44.