What Are Atomic and Molecular Masses Definition Formula and Example Problems
We characterize the matter as anything that has mass and occupies some space. Since matter is characterized as whatever has mass and occupies room, it ought not to be astonishing to discover that atoms and atoms have mass. Singular particles and atoms, be that as it may, are exceptionally little, and the masses of individual particles and atoms are additionally little. For plainly visible items, we use units (for example, grams and kilograms to express their masses). However, these units are excessively huge to serenely portray the masses of individual particles and atoms. Another scale is required.
Different elements were contrasted and the atomic mass of hydrogen and their overall masses were acquired. The current situation is unique and now the standard utilized for atomic masses is carbon 12, an isotope of carbon. This standardization has been acknowledged everywhere on the globe. The mass of 12C is 12 atomic mass units and all the elements are doled out their particular masses as indicated by this norm. One atomic mass unit is equal to 112th of the mass of a carbon-12 molecule. The word amu that is atomic mass unit has been supplanted by 'u' which means bringing together mass.
If the element contains isotopes, the atomic mass of that element is the sum total of the total elements multiplied by the atomic mass of the individual isotopes. On the off chance that the elements have isotopes, at that point, the atomic mass of the element is the summation of the general plenitude of the element in multiplication with an atomic mass of the separate isotopes. In this article, we will learn about the atomic and molecular masses and the relative molecular mass definition chemistry.
Atomic Mass
The atomic mass of an element is the number of times a molecule of that element is heavier than an atom of carbon taken as 12. One atomic mass unit is equal to one-twelfth of the mass of a particle of carbon 12 isotope. The atomic mass of an element is the normal relative mass of its particles when contrasted with a molecule of carbon 12 taken as 12.
Fractional bounty of an isotope is the fraction of the absolute number of particles that are included in that specific isotope. The atomic mass of an element = (Fractional plentitude of isotope 1 × mass of isotope 1) + (Fractional plentitude of isotope 2 × mass of isotope 2).
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Gram Atomic Masses
The atomic masses of elements that are expressed in grams are their gram atomic masses. For eg: the atomic mass of an oxygen molecule is 16 amu.
Hence, the gram atomic mass of oxygen is 16 g.
Molecular Mass
The sub-atomic mass of a substance is the number of times the particle of the substance is heavier than one-twelfth the mass of an atom of carbon - 12. Or on the other hand, the sub-atomic mass is equal to the whole of its atomic masses of the apparent multitude of particles present in one particle of a substance. For eg: water
The atomic mass of H= 1 unit
The atomic mass of O =16 units
The sub-atomic mass of water = 2 × atomic mass of H + 1 × atomic mass of O
= 2 × 1 + 16 × 1
= 18 units
Gram Molecular Mass
The sub-atomic mass of a substance expressed in grams is the gram sub-atomic mass. For eg: Molecular mass of oxygen = 32u
∴ Gram sub-atomic mass of oxygen = 32 g
FAQs on Atomic and Molecular Masses Explained with Definitions and Calculations
1. What is atomic mass in chemistry?
The atomic mass of an element is the weighted average mass of all its naturally occurring isotopes, expressed in atomic mass units (u).
- It is measured relative to 1/12 of the mass of a carbon-12 atom.
- Unit: atomic mass unit (u) or unified mass unit.
- Example: The atomic mass of chlorine is about 35.5 u because it is a mixture of 35Cl and 37Cl isotopes.
2. What is the difference between atomic mass and mass number?
The atomic mass is the weighted average mass of an element’s isotopes, while the mass number is the total number of protons and neutrons in a single atom.
- Atomic mass: Average value (can be decimal), e.g., chlorine ≈ 35.5 u.
- Mass number (A): Whole number for a specific isotope.
- Formula: Mass number = protons + neutrons.
- Example: 23Na has mass number 23.
3. What is molecular mass?
The molecular mass is the sum of the atomic masses of all atoms present in a molecule.
- It is expressed in atomic mass units (u).
- Formula: Molecular mass = Σ (atomic masses of all atoms in the molecule).
- Example: For H2O, molecular mass = (2 × 1 u) + 16 u = 18 u.
4. How do you calculate molecular mass of a compound?
To calculate molecular mass, add the atomic masses of all atoms in the chemical formula.
- Step 1: Write the correct chemical formula.
- Step 2: Note atomic masses from the periodic table.
- Step 3: Multiply each atomic mass by the number of atoms present.
- Step 4: Add the values.
5. What is the formula for calculating relative atomic mass?
The relative atomic mass (Ar) is calculated as the weighted average of isotopic masses using their natural abundances.
- Formula: Ar = (m1 × %abundance1 + m2 × %abundance2 + …) / 100
- Where m represents isotopic mass.
- Example for chlorine:
- (35 × 75) + (37 × 25) / 100 = 35.5 u
6. What is the difference between molecular mass and molar mass?
The molecular mass is the mass of one molecule in atomic mass units (u), while the molar mass is the mass of one mole of substance in grams per mole (g mol-1).
- Molecular mass unit: u.
- Molar mass unit: g mol-1.
- Numerically, they are equal.
- Example: Molecular mass of H2O = 18 u, molar mass = 18 g mol-1.
7. Why is atomic mass not a whole number for most elements?
Atomic mass is usually not a whole number because it is a weighted average of different isotopes of an element.
- Most elements exist as a mixture of isotopes.
- Each isotope has a different mass number.
- The average depends on natural abundance.
8. How do you calculate the atomic mass of an element from isotopic abundance?
The atomic mass is calculated by multiplying each isotope’s mass by its fractional abundance and adding the results.
- Step 1: Convert percentage abundance to decimal.
- Step 2: Multiply isotopic mass by its decimal abundance.
- Step 3: Add all values.
- (10 × 0.20) + (11 × 0.80) = 2 + 8.8 = 10.8 u
9. What is formula mass and how is it different from molecular mass?
The formula mass is the sum of atomic masses in an ionic compound’s formula unit, while molecular mass applies to covalent molecules.
- Used for ionic compounds like NaCl.
- Formula mass of NaCl = 23 u + 35.5 u = 58.5 u.
- Molecular mass applies to molecules like CH4.
10. How are atomic and molecular masses used in mole calculations?
Atomic and molecular masses are used to determine molar mass, which connects mass and number of moles using the formula n = mass / molar mass.
- Step 1: Calculate molar mass from atomic or molecular mass.
- Step 2: Use n = m / M.
- n = 88 / 44 = 2 moles
