Molecular mass calculations, Mole-mass calculations

How to calculate mass, molar mass and number of moles of a substance?

Molar mass calculations

To calculate the molar mass of an atom, molecule, or formula unit, we need to find atomic mass, molecular or formula mass, respectively. Then express that mass into grams.

Example 1.5: Calculate mass of one mole of following substances

(a) Na

(b) Nitrogen

Solution:

a) Atomic mass of Na = 23 amu

Mass of 1 mole of Na = 23g

b) Nitrogen occurs as diatomic molecules.

Molecular mass of N₂= 14 × 2

= 28 amu

Mass of 1 mole of N₂ = 28 g

c) Molecular mass of C₁₂H₂₂O₁₁ = 12 × 12 + 1 × 22 + 16 × 11

= 144 + 22 + 176

Mass of 1mole of sucrose = 342g

Self-Assessment

Calculate mass of one mole of following substances

(a) Copper

(b) Iodine

(d) Oxygen

Mole-Mass Calculations

To calculate the mass of given number of moles of substance, we need to calculate:

Atomic mass, molecular mass or formula mass

Mass of 1 mole of substance

Mass of given number of moles

Example: Oxygen is converted to ozone (O3) during thunder storms. Calculate the mass of ozone if 9.05 moles of ozone is formed in a storm?

Solution:

Molecular mass of O3 = 3 × 16

= 48 amu

Mass of 1 mole of O3 = 48 g

Mass of 9.05 moles of O3 = 9.05 × 48 g

= 434.4g

So, mas of 9.05 moles of O3 is 434.4 g

Example: When natural gas burns, CO₂ is formed. If 0.25 moles of CO₂ is formed, what mass of CO₂ is produced?

Solution:

Molecular mass of CO₂ = 12 + 2 (16)

= 12 + 32

= 44 amu

Mass of 1 mole of CO₂ = 44g

Mass of 0.25 moles of CO₂ = 44 × 0.25 g

= 11g

Mass of CO₂ is 11 g.

Also Read: Tactics to solve mole-mass, mole-particle calculations

Example: Converting grams to moles

How many moles of each of the following substance are present?

(a) A balloon filled with 5g of hydrogen.

(b) A block of ice that weighs 100g.

Solution:

(a)

Molecular mass of H₂ = 1.008 × 2

= 2.016 amu

Mass of 1 mole of H₂ = 2.016 g

Moles of 2.016 g of H₂ = 1 mole

Moles of 1 g of H₂ = 1/2.016 mole

Moles of 5 g of H₂ = 5 × 1/2.016 mole

(b)

Molecular mass of ice (H₂O) = 1.008 × 2 + 16

= 2.016 + 16 amu

= 18.016 amu

Mass of 1 mole of H₂O = 18.016 g

Moles of 18.016 g of H₂O = 1 mole

Moles of 1 g of H₂O = 1/(18.016 ) mole

Moles of 100 g of H₂O = 100 × 1/(18.016 ) mole

= 5.55 moles

So, number of moles of 100 g of H₂O is 5.55 moles

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