The number of sodium atoms in \[2{\text{ }}moles\] of sodium ferrocyanide is:
  A.\;12{\text{ }}X{\text{ }}{10^{23}} \\
  B.\;26{\text{ }}X{\text{ }}{10^{23}} \\
  C.\;34{\text{ }}X{\text{ }}{10^{23}} \\
  D.\;48{\text{ }}X{\text{ }}{10^{23}} \\

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Hint: We must remember the molecular formula for Sodium ferrocyanide is \[N{a_4}\left[ {Fe{{\left( {CN} \right)}_6}} \right]\]. \[1{\text{ }}mole\] of the compound is having \[4{\text{ }}moles\]of\[Na\]. Generally one mole of a substance is equal to \[6.022 \times {10^{23}}\]. The number \[6.022 \times {10^{23}}\] is known as Avogadro number.

Complete step by step answer:
Let’s start with writing the molecular formula for sodium ferrocyanide. As the name suggests it is a complex compound having 2 components which are metal and a complex. The complex consists of iron \[\left( {Fe} \right)\] and cyanide \[\left( {CN} \right)\]. As we can see the iron is in a ferrous state means having 2+ oxidation states. Also 6 \[CN\] are connected with iron so the overall oxidation state of the complex is -4. Hence, 4 \[N{a^ + }\] will be attached with the complex and the compounds molecular formula will be \[N{a_4}\left[ {Fe{{\left( {CN} \right)}_6}} \right]\].
\[1{\text{ }}mole\] of \[N{a_4}\left[ {Fe{{\left( {CN} \right)}_6}} \right]\] is having \[4{\text{ }}moles\] of sodium, and one mole of compound is having \[6.022{\text{ }} \times {\text{ }}{10^{23}}\] times the atom. So, \[4{\text{ }}moles\] of sodium will be having \[4{\text{ }} \times {\text{ }}6.022{\text{ }} \times {\text{ }}{10^{23}}\] atoms.
Similarly, in case of \[2{\text{ }}moles\], 2 times the atom in \[1{\text{ }}mole\] will be present so, \[8{\text{ }}moles\] of sodium will be there and hence \[8{\text{ }} \times {\text{ }}6.022{\text{ }} \times {\text{ }}{10^{23}}\] atoms which will be equal to \[48{\text{ }} \times {\text{ }}{10^{23}}\] atoms.

So, the answer to this question is D. \[48{\text{ }} \times {\text{ }}{10^{23}}\] atoms.

Note: We must know that the Avogadro’s number is being a boon for scientists as it helps in calculating, discussing and comparing very high numbers of atomic and subatomic particles. Avogadro’s number is \[6.022{\text{ }} \times {\text{ }}{10^{23}}\]. Avogadro’s number becomes very useful because in everyday life the substances contain a large number of atoms and molecules.
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