Atomic And Molecular Masses

Student can calculate average atomic mass and molecular/formula mass using unified mass units.

7 lessons

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Concept

Atomic Mass and 'u'

Define atomic mass unit based on Carbon-12.

Atoms are extremely small, making it impractical to weigh them in grams for daily chemical calculations. Instead, chemists use relative masses based on a standard reference atom.

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Atomic and Molecular Masses Visual

Showing the 3D lattice of NaCl clarifies why formula mass is used instead of molecular mass.

clean scientific diagram, 3D lattice structure of solid sodium chloride showing positive sodium ions and negative chloride ions arranged in an alternating tightly packed pattern, pastel color palette, elegant typography, translucent layers for cross-sections, white background, soft shadows

Click to zoom

In solid sodium chloride, discrete molecules do not exist. Instead, ions are packed in a 3D lattice, which is why we calculate formula mass instead of molecular mass.

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Average Atomic Mass

Formula for weighted average of isotopes.

Used because most elements naturally exist as a mixture of multiple isotopes.

Avg. Mass = \sum (Fractional Abundance \times Isotopic Mass)

Example: Carbon Isotopes

C-12: 98.892% abundance, 12 u
C-13: 1.108% abundance, 13.00335 u
C-14: 0.0000000002% abundance, 14.00317 u

= (0.98892 \times 12) + (0.01108 \times 13.00335) + (2 \times 1 0^{- 12} \times 14.00317)

= 12.011 u

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reading

Calculate Average Atomic Mass

Exercise 1.9: Calculate average mass of Chlorine.

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To calculate the average atomic mass of chlorine, we use the fractional abundance and the molar mass of its naturally occurring isotopes. The data provided for chlorine indicates two isotopes: Cl-35 with a natural abundance of 75.77% (mass = 34.9689 u) and Cl-37 with a natural abundance of 24.23% (mass = 36.9659 u).

First, convert the percentage abundances into fractional abundances by dividing by 100. This gives a fractional abundance of for Cl-35 and 0.2423 for Cl-37.

Next, multiply the fractional abundance of each isotope by its respective molar mass and add the results together. The calculation setup is: $(0.7577 \times 34.9689$ u) + $(0.2423 \times$ ).

Calculating these individual terms yields approximately $26.4959$ u + $8.9568$ u. Adding these values together gives the final average atomic mass of chlorine as .

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Learn

Molecular and Formula Mass

Distinguish between molecular mass and formula mass (for ionic compounds).

Molecular mass is simply the sum of the atomic masses of all the elements present in a single molecule. For example, a discrete water molecule ( $H_2O$ ) has a molecular mass of:

$2 \times (1.008 \text{ u}) + 16.00 \text{ u} = 18.02 \text{ u}$ .

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Example

Calculating Molecular Mass

Problem 1.1: Glucose.

Problem

Statement: Calculate the molecular mass of a glucose ( $C_6H_{12}O_6$ ) molecule.

Given Data:

Formula: $C_6H_{12}O_6$
Atomic mass of C = $12.011 \text{ u}$
Atomic mass of H = $1.008 \text{ u}$
Atomic mass of O = $16.00 \text{ u}$

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Practice: Molar Masses

Exercise 1.1: Calculate molar masses of water, CO2, and methane.

Use the following average atomic masses: $H = 1.008 \text{ g/mol}$ , $C = 12.011 \text{ g/mol}$ , and $O = 16.00 \text{ g/mol}$ . Follow the steps to show your working.

Given Formulas & Atomic Masses*

List the target molecules and the atomic masses you will use.

Governing Principle*

State the rule used to calculate molar mass.

Substitution*

Substitute the numbers into the formula for each molecule.

Calculation Steps*

Show the intermediate addition steps.

Final Answer (with units)*

Provide the final values rounded appropriately, including units.

← previousLaws Of Chemical Combinations And Dalton...next →The Mole Concept And Molar Mass

reading

Calculate Average Atomic Mass

Exercise 1.9: Calculate average mass of Chlorine.

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First, convert the percentage abundances into fractional abundances by dividing by 100. This gives a fractional abundance of for Cl-35 and 0.2423 for Cl-37.

Next, multiply the fractional abundance of each isotope by its respective molar mass and add the results together. The calculation setup is: $(0.7577 \times 34.9689$ u) + $(0.2423 \times$ ).

Calculating these individual terms yields approximately $26.4959$ u + $8.9568$ u. Adding these values together gives the final average atomic mass of chlorine as .

Learn

Molecular and Formula Mass

Distinguish between molecular mass and formula mass (for ionic compounds).

Molecular mass is simply the sum of the atomic masses of all the elements present in a single molecule. For example, a discrete water molecule ( $H_2O$ ) has a molecular mass of:

$2 \times (1.008 \text{ u}) + 16.00 \text{ u} = 18.02 \text{ u}$ .

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Example

Calculating Molecular Mass

Problem 1.1: Glucose.

Problem

Statement: Calculate the molecular mass of a glucose ( $C_6H_{12}O_6$ ) molecule.

Given Data:

Formula: $C_6H_{12}O_6$
Atomic mass of C = $12.011 \text{ u}$
Atomic mass of H = $1.008 \text{ u}$
Atomic mass of O = $16.00 \text{ u}$

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feedback

Practice: Molar Masses

Exercise 1.1: Calculate molar masses of water, CO2, and methane.

Use the following average atomic masses: $H = 1.008 \text{ g/mol}$ , $C = 12.011 \text{ g/mol}$ , and $O = 16.00 \text{ g/mol}$ . Follow the steps to show your working.

Given Formulas & Atomic Masses*

List the target molecules and the atomic masses you will use.

Governing Principle*

State the rule used to calculate molar mass.

Substitution*

Substitute the numbers into the formula for each molecule.

Calculation Steps*

Show the intermediate addition steps.

Final Answer (with units)*

Provide the final values rounded appropriately, including units.