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Questions and Answers
The relative atomic mass is a weigthed__ average of the atomic masses of the isotopes of an element, weighted by their natural abundances.
The relative atomic mass is a weigthed__ average of the atomic masses of the isotopes of an element, weighted by their natural abundances.
weighted
Relative atomic mass is measured in atomic mass units, where one atomic mass unit (amu) is defined as 1/12 of the mass of a single atom of carbon12____.
Relative atomic mass is measured in atomic mass units, where one atomic mass unit (amu) is defined as 1/12 of the mass of a single atom of carbon12____.
carbon-12
To calculate the relative atomic mass of an element, you must know the mass and relative abundance of each isotope__ of that element.
To calculate the relative atomic mass of an element, you must know the mass and relative abundance of each isotope__ of that element.
isotope
The relative atomic mass of an element is a ______ number as it is relative to the mass of carbon-12.
The relative atomic mass of an element is a ______ number as it is relative to the mass of carbon-12.
The formula for calculating relative atomic mass is $\sum$ (isotope mass $\times$ fractional ______) for all the isotopes of the element.
The formula for calculating relative atomic mass is $\sum$ (isotope mass $\times$ fractional ______) for all the isotopes of the element.
If an element has two isotopes with equal abundance, its relative atomic mass will be the ______ of the masses of the two isotopes.
If an element has two isotopes with equal abundance, its relative atomic mass will be the ______ of the masses of the two isotopes.
The relative atomic mass of an element is typically found on the ______ table.
The relative atomic mass of an element is typically found on the ______ table.
The relative atomic mass is useful in ______ calculations, such as determining the molar mass of a compound.
The relative atomic mass is useful in ______ calculations, such as determining the molar mass of a compound.
The actual mass of an individual atom is extremely small, so relative atomic mass provides a more practical scale for ______ masses.
The actual mass of an individual atom is extremely small, so relative atomic mass provides a more practical scale for ______ masses.
If an element has one isotope that is significantly more abundant than others, its relative atomic mass will be close to the mass of the most abundant ______.
If an element has one isotope that is significantly more abundant than others, its relative atomic mass will be close to the mass of the most abundant ______.
The mass spectrometer is an instrument used to determine the relative ______ and masses of different isotopes in a sample.
The mass spectrometer is an instrument used to determine the relative ______ and masses of different isotopes in a sample.
The term 'relative atomic mass' is ______ from 'atomic weight,' although the terms are often used interchangeably.
The term 'relative atomic mass' is ______ from 'atomic weight,' although the terms are often used interchangeably.
The relative atomic mass is a ______ property, meaning it is intrinsic to the element and does not depend on the size of the sample.
The relative atomic mass is a ______ property, meaning it is intrinsic to the element and does not depend on the size of the sample.
Knowing the relative atomic mass allows chemists to accurately measure and ______ substances in chemical reactions.
Knowing the relative atomic mass allows chemists to accurately measure and ______ substances in chemical reactions.
The accuracy of relative atomic mass values is crucial for precise chemical ______ and analysis.
The accuracy of relative atomic mass values is crucial for precise chemical ______ and analysis.
The concept of relative atomic mass addresses the challenge of working with the incredibly small masses of individual ______.
The concept of relative atomic mass addresses the challenge of working with the incredibly small masses of individual ______.
If a new isotope of an element is discovered, the element's relative atomic mass may need to be ______.
If a new isotope of an element is discovered, the element's relative atomic mass may need to be ______.
The relative atomic mass takes into account the natural isotopic ______ variations that exist in different geographical locations.
The relative atomic mass takes into account the natural isotopic ______ variations that exist in different geographical locations.
The relative atomic mass of an element is used to determine the number of ______ in a given mass of the element.
The relative atomic mass of an element is used to determine the number of ______ in a given mass of the element.
The discovery and understanding of isotopes by scientists led to the development of the concept of relative ______ mass.
The discovery and understanding of isotopes by scientists led to the development of the concept of relative ______ mass.
Flashcards
Relative Atomic Mass
Relative Atomic Mass
The weighted average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.
Study Notes
- Relative atomic mass is a dimensionless physical quantity
- It is the ratio of the average mass of atoms of an element to 1/12 of the mass of a carbon-12 atom
- The relative atomic mass is a weighted average that takes into account the relative abundance of each isotope of the element
- It is commonly referred to as atomic weight
How to Calculate Relative Atomic Mass
- Identify all isotopes of the element
- Determine the atomic mass of each isotope (usually very close to the mass number)
- Establish the relative abundance of each isotope (usually given as a percentage)
- Multiply the atomic mass of each isotope by its relative abundance (as a decimal)
- Sum the results to obtain the relative atomic mass
Formula
- The formula for calculating relative atomic mass (Ar) is:
- Ar = Σ (isotope mass × fractional abundance)
- Where Σ means 'sum of' and fractional abundance is the percentage abundance divided by 100
Example Calculation
- Consider chlorine (Cl), which has two isotopes: chlorine-35 (35Cl) and chlorine-37 (37Cl)
- Chlorine-35 has an atomic mass of 34.969 u and a relative abundance of 75.77%
- Chlorine-37 has an atomic mass of 36.966 u and a relative abundance of 24.23%
- Ar(Cl) = (34.969 u × 0.7577) + (36.966 u × 0.2423)
- Ar(Cl) = 26.496 u + 8.957 u
- Ar(Cl) = 35.453 u
- Therefore, the relative atomic mass of chlorine is approximately 35.45 u
Importance of Isotopes
- Isotopes are different forms of the same element, which have the same number of protons, but different numbers of neutrons
- The existence of isotopes means that the atoms of an element do not all have the same mass
- This is why we use the term 'relative atomic mass' rather than simply 'atomic mass', as it takes into account the different masses and abundances of the isotopes
Standard Atomic Weights
- IUPAC maintains a table of standard atomic weights
- These are the values that should be used in most calculations
- Standard atomic weights are periodically reviewed and adjusted
- For some elements, the isotopic composition varies depending on the source
- In these cases, the standard atomic weight is given as an interval
- For example, the standard atomic weight of lithium is given as [6.938, 6.997]
Applications
- Relative atomic mass is used in various calculations:
- Determining the molar mass of compounds
- Stoichiometry calculations in chemical reactions
- Quantitative analysis to determine the composition of substances
- Understanding isotopic variations in geological and environmental samples
Measurement Techniques
- Mass spectrometry is the primary technique to measure atomic masses and isotopic abundances
- In mass spectrometry, a substance is ionized and the ions are separated based on their mass-to-charge ratio
- This allows for accurate determination of the masses of different isotopes and their relative amounts in a sample
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