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Understanding the concept of moles is crucial in chemistry, as it serves as a bridge between the atomic and macroscopic worlds. A mole is a unit that defines a specific number of atoms, molecules, or ions in a given substance. The number, known as Avogadro's number, is approximately \(6.022 \times 10^{23}\) particles per mole.
To find the number of molecules, we first determine how many moles of a substance we have. This is calculated by dividing the mass of the substance by its molar mass (the mass of one mole of that substance).
In the exercise given, we calculated the moles for different substances like \(\mathrm{H}_2\), \(\mathrm{O}_2\), and \(\mathrm{CO}_2\). For instance, in \(10 \mathrm{~g}\) of \(\mathrm{H}_2\) molecules, we calculated \(5\) moles, which equates to \(5 \times 6.022 \times 10^{23}\) molecules, demonstrating that it's the most on the list. This highlights how moles facilitate comparisons based on mass.

Chemical reactions involve transformations where substances, called reactants, undergo changes to become new substances known as products. In the example provided, we observe a reaction between \(\mathrm{NaOH}\) and \(\mathrm{H}_3\mathrm{PO}_4\).
Each reaction can be represented by a balanced chemical equation that reflects the conservation of mass and atoms. Here, \(\mathrm{NaOH}\) reacts with \(\mathrm{H}_3\mathrm{PO}_4\) to form \(\mathrm{NaH}_2\mathrm{PO}_4\) and \(\mathrm{H}_2\mathrm{O}\).
A vital concept in such reactions is the "n-factor," or the number of electrons exchanged or atoms replaced in the reaction. For phosphoric acid, the reaction demonstrates that only one hydrogen ion is replaced, affecting its equivalent weight.

Equivalent weight is a critical concept for understanding stoichiometry in chemical reactions. It refers to the mass of a substance that will react with or displace one mole of hydrogen atoms, electrons, or ions.
To find the equivalent weight, you divide the molar mass of a compound by its n-factor, which indicates the number of electrons transferred or hydrogen ions replaced in the reaction.
In the exercise, the equivalent weight of \(\mathrm{H}_3\mathrm{PO}_4\) was calculated by dividing its molar mass \(98\, \mathrm{g/mol}\) by its n-factor, which in this case was one. Thus, the equivalent weight remained \(98\), corresponding to option (b) in the question. This calculation exemplifies how understanding equivalent weight aids in determining how substances interact in reactions.

Haemoglobin is a vital protein in red blood cells responsible for oxygen transport throughout the human body. Its molecular structure includes iron atoms, essential for binding oxygen.
In the given problem, haemoglobin's iron content was specified as \(0.33\%\) by weight. To determine how many iron atoms are present per molecule of haemoglobin, first calculate the total iron mass in one mole of haemoglobin, which has a molecular weight of approximately 67200 g/mol.
Moles of iron in haemoglobin can be found by dividing the iron weight \(221.76\, \mathrm{g}\) by its atomic weight \(56\). This calculation results in approximately \(3.96\) moles, rounded to \(4\) iron atoms per molecule of haemoglobin, confirming that it allows effective oxygen transport capacity for each molecule. Understanding the content and role of iron in haemoglobin is crucial for grasping how our body sustains vital biological processes.