Problem 123 - Chemistry The Molecular Nature of Matter

Question

What are the formulas for mercury(I) nitrate dihydrate and mercury(II) nitrate monohydrate?

Step-by-Step Solution

Answer

The formula for mercury(I) nitrate dihydrate is \( Hg_2(NO_3)_2 \cdot 2H_2O \), and the formula for mercury(II) nitrate monohydrate is \( Hg(NO_3)_2 \cdot H_2O \).

Step 1: Determine the mercury(I) ion

Mercury(I) ion is represented as \( Hg_2^{2+} \) since two mercury atoms bond together to give a +2 charge overall.

Step 2: Combine mercury(I) ion with nitrate ion

Nitrate (\( NO_3^- \) ion) combines with \( Hg_2^{2+} \) in a 2:1 ratio to form mercury(I) nitrate. The formula is \( Hg_2(NO_3)_2 \).

Step 3: Add water molecules for dihydrate

Since it's a dihydrate, we include two water molecules. The formula for mercury(I) nitrate dihydrate is \( Hg_2(NO_3)_2 \cdot 2H_2O \).

Step 4: Determine the mercury(II) ion

Mercury(II) ion is represented as \( Hg^{2+} \) for a single mercury atom with a +2 charge.

Step 5: Combine mercury(II) ion with nitrate ion

The nitrate ion \( NO_3^- \) combines with \( Hg^{2+} \) in a 1:1 ratio to form mercury(II) nitrate. The formula is \( Hg(NO_3)_2 \).

Step 6: Add water molecule for monohydrate

Since it's a monohydrate, we include one water molecule. The formula for mercury(II) nitrate monohydrate is \( Hg(NO_3)_2 \cdot H_2O \).

Key Concepts

Mercury(I) Nitrate Dihydrate

Understanding mercury(I) nitrate dihydrate requires familiarity with the oxidation state of mercury and its effects on chemical bonding. Mercury(I), also known as mercurous, is unique as it is often represented by the formula \( Hg_2^{2+} \) indicating that two mercury atoms are bonded, sharing the +2 charge. This dimeric form is what sets the groundwork for the compound's structure when combined with nitrate ions, denoted as \( NO_3^- \). Hence, the dimer interacts with nitrate in a ratio of 1:1, leading to the compound \( Hg_2(NO_3)_2 \).

When we refer to this compound as a 'dihydrate', it means that two water molecules are coordinated to this structure, a common phenomenon in inorganic chemistry to stabilize ionic compounds. The complete formula for mercury(I) nitrate dihydrate is thus \( Hg_2(NO_3)_2 \cdot 2H_2O \), which indicates the presence of these two water molecules in the crystalline network of the salt.

Mercury(II) Nitrate Monohydrate

Mercury(II) nitrate monohydrate contains mercury in a +2 oxidation state, which is also referred to as mercuric. The chemical formulation for this compound starts with the mercury(II) ion, depicted as \( Hg^{2+} \), a single mercury atom carrying the +2 charge. This ion then forms a compound with the nitrate ion in a 1:1 ratio, resulting in \( Hg(NO_3)_2 \).

When the term 'monohydrate' is attached, it specifies that there is one water molecule associated with each formula unit of the salt. This integration of water is a distinctive aspect of many hydrates and is instrumental in their crystalline structure and stability. The correct representation for mercury(II) nitrate monohydrate is \( Hg(NO_3)_2 \cdot H_2O \), which signifies its hydrated form serving a crucial role in determining the physical properties of the compound.

Chemical Formula Writing

Writing chemical formulas is a fundamental skill in inorganic chemistry, allowing students and scientists to convey complex information about a substance's composition and structure succinctly. To correctly write a chemical formula, one must first identify the ions that comprise the compound and their oxidation states, which dictate how they combine. For instance, in understanding the balance of charges, it becomes clear why the mercury(I) compound requires two nitrate ions to balance the \( Hg_2^{2+} \) ion and why mercury(II) combines with two nitrates as well. Each component stacks together following the rules of valency and stoichiometry.

Another key aspect is the representation of hydrates. The number of water molecules coordinated to the salt is indicated after a dot, known as a 'dot of crystallization', implying their inclusion in the solid structure. Mastery of chemical formula writing is essential not only for communication but also for predicting the behavior and interactions of chemical species.

Hydrates in Chemistry

Hydrates play a pivotal role in chemistry, particularly within the realm of inorganic compounds. A hydrate is a compound that includes water molecules directly bonded to its ions or molecules, an aspect crucial for crystallinity and stability. These water molecules can be held by various types of bonds, such as ionic or hydrogen bonds, and are often integral to the structure of the crystal lattice.

In the names of hydrates, such as 'dihydrate' and 'monohydrate', the prefixes 'di-' and 'mono-' indicate the number of water molecules associated with each unit of the compound. In practice, different hydrates of the same substance can exhibit remarkably different properties, including color, solubility, and melting point, underscoring the importance of precisely stating the hydrate form when describing a chemical compound. The removal of water, a process known as dehydration, can lead to a different compound with its own unique attributes.