Massa Molekul Relatif (Mr) dan Massa Atom Relatif (Ar): Materi Kimia Kelas X

Summary

In this educational video, Kak Tiara, a chemistry tutor, explains key concepts of atomic mass (Ar) and molecular mass (Mr). The video covers the definition of atomic mass relative to the carbon-12 isotope, and how to calculate it using examples. Additionally, Tiara explains how isotopes influence atomic mass calculations and demonstrates the process for determining the molecular mass of compounds like H2SO4 and NH3. Viewers are guided through step-by-step solutions, making complex topics like stoichiometry more accessible and engaging. The video concludes with an encouragement to keep learning and explore further educational resources.

Takeaways

• 😀 The video covers topics on atomic mass, relative atomic mass (Ar), and relative molecular mass (Mr).
• 😀 Ar (atomic relative mass) represents the comparison of the mass of one atom of an element to the standard of 1/12 the mass of a carbon-12 atom.
• 😀 Carbon-12 is chosen as the standard because it has a stable nucleus, making it a reliable reference point for atomic mass.
• 😀 To calculate the atomic mass (Ar) of an atom X, divide the average mass of one atom of X by 1/12 the mass of a carbon-12 atom.
• 😀 An example is provided to calculate the relative atomic mass of oxygen (Ar), using known values of oxygen's atomic mass and the mass of carbon-12.
• 😀 Isotopes are atoms with the same number of protons but different numbers of neutrons, which affects their mass.
• 😀 To calculate Ar for an element with multiple isotopes, multiply the percent abundance of each isotope by its respective Ar and sum the results.
• 😀 An example of boron, which has two isotopes (B10 and B11), demonstrates how to calculate the atomic mass based on isotope abundance.
• 😀 Mr (molecular relative mass) refers to the relative mass of a molecule, calculated by adding up the Ar values of the atoms that make up the molecule.
• 😀 A practical example of calculating Mr is given for sulfuric acid (H2SO4), where the sum of the atomic masses of hydrogen, sulfur, and oxygen yields 98.
• 😀 Another example involves calculating the Mr for ammonia (NH3), resulting in 17 by adding the atomic masses of nitrogen and hydrogen.

Q & A

What is the relative atomic mass (Ar)?

The relative atomic mass (Ar) is a measure of the relative mass of an atom compared to one-twelfth of the mass of a carbon-12 atom. It expresses the weight of a single atom of an element relative to the standard mass of carbon-12.

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

Carbon-12 is used as the standard for relative atomic mass because it is a stable isotope with a well-defined mass that makes it a reliable benchmark for comparison with other elements.

How do you calculate the relative atomic mass (Ar) of an element?

To calculate the relative atomic mass (Ar) of an element, divide the mass of one atom of the element by one-twelfth of the mass of a carbon-12 atom. For example, if the mass of an oxygen atom is known, divide it by one-twelfth of the mass of carbon-12.

What is the formula for finding the relative atomic mass (Ar) of an element?

The formula for calculating the relative atomic mass (Ar) is: Ar = (mass of one atom of the element) / (1/12 * mass of one carbon-12 atom).

What is an isotope, and how does it relate to the calculation of Ar?

An isotope is an atom of the same element that has the same number of protons but a different number of neutrons, resulting in different atomic masses. The relative atomic mass (Ar) can be calculated by considering the abundances and masses of the isotopes of an element.

How is the relative atomic mass (Ar) of an element with multiple isotopes calculated?

To calculate the relative atomic mass of an element with multiple isotopes, you multiply the relative atomic mass of each isotope by its abundance (expressed as a percentage), sum the results, and then divide by 100.

What is the relative molecular mass (Mr), and how is it different from Ar?

The relative molecular mass (Mr) is the sum of the relative atomic masses of all the atoms in a molecule, while Ar refers to the relative mass of a single atom. Mr is used for molecules, whereas Ar is used for individual atoms.

How do you calculate the relative molecular mass (Mr) of a compound?

To calculate the relative molecular mass (Mr) of a compound, sum the relative atomic masses (Ar) of each element in the molecule, multiplying each by the number of atoms of that element in the molecule. For example, for H2SO4, you would calculate (2 * Ar of H) + (1 * Ar of S) + (4 * Ar of O).

What is an example of calculating Mr for a compound?

For sulfuric acid (H2SO4), the relative atomic masses are: H = 1, S = 32, and O = 16. The calculation would be: (2 * 1) + (1 * 32) + (4 * 16) = 2 + 32 + 64 = 98, so the relative molecular mass (Mr) of H2SO4 is 98.

How do you determine the Mr for ammonia (NH3)?

For ammonia (NH3), the relative atomic masses are: N = 14 and H = 1. The calculation is: (1 * 14) + (3 * 1) = 14 + 3 = 17, so the relative molecular mass (Mr) of NH3 is 17.