Problem 141 - Chemistry and Chemical Reactivity

Question

The formula of barium molybdate is \(\mathrm{BaMoO}_{4}\). Which of the following is the formula of sodium molybdate? (a) \(\mathrm{Na}_{4} \mathrm{MoO}\) (b) NaMoO (c) \(\mathrm{Na}_{2} \mathrm{MoO}_{3}\) (d) \(\mathrm{Na}_{2} \mathrm{MoO}_{4}\) (e) \(\mathrm{Na}_{4} \mathrm{MoO}_{4}\)

Step-by-Step Solution

Answer

The formula for sodium molybdate is \( \mathrm{Na}_{2} \mathrm{MoO}_{4} \).

Step 1: Identify the Similar Structure

The problem provides the formula for barium molybdate, \( \mathrm{BaMoO}_{4} \). Sodium molybdate will have a similar molybdate part, which is \( \mathrm{MoO}_{4} \). Hence the part after the metal, \( \mathrm{MoO}_{4} \), will remain the same.

Step 2: Determine the Cation-Substitution

Since we need to find sodium molybdate, substitute barium (\( \mathrm{Ba}^{2+} \)) with sodium (\( \mathrm{Na}^{+} \)). To neutralize the compound, it requires two sodium ions for every molybdate to balance the \( \mathrm{MoO}_{4}^{2-} \) ion, resulting in \( \mathrm{Na}_{2}\mathrm{MoO}_{4} \).

Step 3: Check Options for Correct Formula

Examine the options provided:- (a) \( \mathrm{Na}_{4} \mathrm{MoO} \)- (b) \( \mathrm{NaMoO} \)- (c) \( \mathrm{Na}_{2} \mathrm{MoO}_{3} \)- (d) \( \mathrm{Na}_{2} \mathrm{MoO}_{4} \)- (e) \( \mathrm{Na}_{4} \mathrm{MoO}_{4} \)Only option (d) \( \mathrm{Na}_{2} \mathrm{MoO}_{4} \) has the corrected ratio for charge neutrality and matches our earlier calculation.

Key Concepts

Ionic Compounds

Ionic compounds are a fundamental type of chemical compound. They are formed from the electrostatic attraction between oppositely charged ions. These ions are typically formed when atoms lose or gain electrons, leading to a charged particle called an ion.
In an ionic compound, metals, which tend to lose electrons, form positive ions (cations), while nonmetals, which tend to gain electrons, form negative ions (anions).
For example, in sodium molybdate, the sodium atom (\(\mathrm{Na}\)) becomes a cation (\(\mathrm{Na}^{+}\)), while the molybdate part (\(\mathrm{MoO}_{4}^{2-}\)) serves as the anion.Since ionic compounds are formed from ions with opposite charges, they tend to form solid crystalline structures where these ions are closely packed.
These compounds are generally characterized by high melting and boiling points, and they can usually dissolve in water, where they separate into their respective ions.

Cation-Anion Interaction

Cation-anion interaction is a critical concept in the formation of ionic compounds. This interaction is what holds ionic compounds together, providing stability to the structure.Cations and anions attract each other due to their opposite charges.

• Cations, such as \(\mathrm{Na}^{+}\), arise from metal elements, which typically have 1 to 3 valence electrons that they tend to lose in chemical reactions.
• Anions, such as \(\mathrm{MoO}_{4}^{2-}\), come from non-metals or polyatomic ions. These ions gain electrons in chemical reactions, giving them a net negative charge.

The force of attraction between cations and anions is called an electrostatic force, which results from the difference in charge and is fundamental in forming ionic bonds.
This electrostatic force is the reason why ionic compounds exhibit hardness and high melting points.
In sodium molybdate, the \(\mathrm{Na}^{+}\) ions are attracted to the \(\mathrm{MoO}_{4}^{2-}\) ions to form a stable ionic crystal.

Charge Neutrality

Charge neutrality is a key feature of ionic compounds, ensuring that the compound is electrically neutral despite being composed of charged ions.
In any stable ionic compound, the total charge from cations must balance the total charge from anions.In the case of sodium molybdate, the molybdate ion (\(\mathrm{MoO}_{4}^{2-}\)) carries a 2- charge. To achieve charge neutrality, two sodium ions (each \(\mathrm{Na}^{+}\) with a 1+ charge) must be present for each molybdate ion.
This results in the chemical formula of \(\mathrm{Na}_{2}\mathrm{MoO}_{4}\).The principle of charge neutrality guides the combination of ions in ionic compounds. It helps determine the ratios of ions needed to form a stable compound.

• For every positive charge added by cations, an equal negative charge must be supplied by anions.
• Charge neutrality is essential for the stability and formation of the lattice structure in ionic compounds.

Understanding charge neutrality is crucial for predicting the correct formulas and combinations of ionic compounds, such as selecting \(\mathrm{Na}_{2}\mathrm{MoO}_{4}\) as the correct formula for sodium molybdate.