SHORTANSWER

Mole Concept

Short Answer

Problem:
Haemoglobin contains 0.334% of iron by weight. The molecular weight of haemoglobin is approximately 67200.
Calculate the number of iron atoms (Atomic weight of Fe is 56) present in one molecule of haemoglobin.

Solution:

Weight of Fe in haemoglobin = $\frac{0.334}{100} Ã— 67200$ = 224.48 g
Mass of one Fe atom = 56 g
Total number of Fe atom = $\frac{224.48}{56}$ = 4

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Problem:
Calculate the number of atoms in 0.1 mol of a tetraatomic gas.

Solution:

Tetraatomic means 4 atoms in one molecule.

So, total number of atoms = 0.1 Ã— 4 Ã— 6.022 Ã— 10²³ = 2.4 Ã— 10²³ atoms

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Problem:
Calculate the number of molecules in 4.25 g of NH₃.

Solution:

Moles of NH₃ = $\frac{4.25}{17}$ = 0.25 moles
Number of molecules = 0.25 Ã— 6.022 Ã— 10²³ molecule = 1.50 Ã— 10²³ molecules

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Problem:
What is the volume occupied by one molecule of water (density = 1 g cm^-³)?

Solution:

As water is liquid its density = 1 g/mL
i.e., 1 g of H₂O have volume = 1 mL
Mass of one molecule = $\frac{18}{6.02 Ã— 10^{23}} g$
$\frac{18}{6.02 Ã— 10^{23}} g$ of H₂O have volume = $\frac{18}{6.02 Ã— 10^{23}}$ mL = 3.0 Ã— 10^-²³ mL

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Problem:
What volume of oxygen gas (O₂) measured at 0Â°C and 1 atm, is needed to burn completely 1 L of propane gas (C₃H₈) measured under the same conditions?

Solution:

$C_{3} H_{8} + 5 O_{2} â†’ 3 {CO}_{2} + 4 H_{2} O$

For 1 mol propane, 5 mol O₂ gas is needed.
22.4 L propane = 5 Ã— 22.4 L of O₂ gas needed
So, 1 L propane = 5 L of O₂ gas is required

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Problem:
A sample of ammonium phosphate (NH₄)₃PO₄ contains 3.18 moles of hydrogen atoms. Calculate the number of moles of oxygen atoms in the sample?

Solution:

In (NH₄)₃PO₄, moles of 'H' are present with 4 moles of oxygen atom.

So, 3.18 moles of 'H' are present with = $\frac{4}{12} Ã— 3.18$

= 1.06 moles of oxygen atom

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