Problem 22 - Chemistry and Chemical Reactivity

Question

Give the formula for each of the following ionic compounds: (a) calcium hydrogen carbonate (b) potassium permanganate (c) magnesium perchlorate (d) potassium hydrogen phosphate (e) sodium sulfite

Step-by-Step Solution

Answer

(a) \( \text{Ca(HCO}_3\text{)}_2 \), (b) \( \text{KMnO}_4 \), (c) \( \text{Mg(ClO}_4\text{)}_2 \), (d) \( \text{K}_2\text{HPO}_4 \), (e) \( \text{Na}_2\text{SO}_3 \).

Step 1: Understanding Ionic Compounds

Ionic compounds consist of positively charged ions (cations) and negatively charged ions (anions). The overall charge of an ionic compound is neutral. The cation is always written first, followed by the anion.

Step 2: Determine Common Cations and Anions

- **Calcium** ion is Ca²⁺, **potassium** ion is K⁺, **magnesium** ion is Mg²⁺, and **sodium** ion is Na⁺. - **Hydrogen carbonate** (bicarbonate) is HCO₃⁻, **permanganate** is MnO₄⁻, **perchlorate** is ClO₄⁻, **hydrogen phosphate** is HPO₄²⁻, and **sulfite** is SO₃²⁻.

Step 3: Write the Formula for Calcium Hydrogen Carbonate

Combine the calcium ion (Ca²⁺) with the hydrogen carbonate ion (HCO₃⁻). Since the calcium ion has a 2+ charge and the hydrogen carbonate ion has a 1- charge, you will need two hydrogen carbonate ions to balance the charge: The formula is \[\text{Ca(HCO}_3\text{)}_2\].

Step 4: Write the Formula for Potassium Permanganate

Combine the potassium ion (K⁺) with the permanganate ion (MnO₄⁻). Both ions have equal but opposite charges so only one of each is needed: The formula is \[\text{KMnO}_4\].

Step 5: Write the Formula for Magnesium Perchlorate

Combine the magnesium ion (Mg²⁺) with the perchlorate ion (ClO₄⁻). Since the magnesium ion has a 2+ charge and the perchlorate ion has a 1- charge, you will need two perchlorate ions to balance the charge: The formula is \[\text{Mg(ClO}_4\text{)}_2\].

Step 6: Write the Formula for Potassium Hydrogen Phosphate

Combine the potassium ion (K⁺) with the hydrogen phosphate ion (HPO₄²⁻). You need two potassium ions to balance with one hydrogen phosphate ion (2+ from K⁺ and 2- from HPO₄²⁻): The formula is \[\text{K}_2\text{HPO}_4\].

Step 7: Write the Formula for Sodium Sulfite

Combine the sodium ion (Na⁺) with the sulfite ion (SO₃²⁻). You'll need two sodium ions to balance the sulfite ion's 2- charge: The formula is \[\text{Na}_2\text{SO}_3\].

Key Concepts

Cations and Anions

In ionic compounds, ions are the building blocks. These ions are split into two categories: cations and anions. Cations are positively charged ions. They lose electrons. Metals like sodium (Na⁺) and calcium (Ca²⁺) are common cations. Anions are negatively charged ions. They gain electrons. Examples include chloride (Cl⁻) and sulfate (SO₄²⁻). To form a neutral compound, these two types of ions must balance each other out.

It's important to always write the cation first in the chemical formula. This helps clarify the compound structure and gives it a standardized form. Understanding the charges helps us predict how many ions are needed for the compound to be stable and neutral overall. This brings us directly into the next topic.

Chemical Formulas

A chemical formula tells us the types and numbers of atoms in a compound. For ionic compounds, the formula must reflect the balance of charges. Here are some examples to guide your understanding:

• Calcium hydrogen carbonate: The calcium ion (Ca²⁺) pairs with two hydrogen carbonate ions (HCO₃⁻). The formula is written as \[\text{Ca(HCO}_3\text{)}_2\].


• Potassium permanganate: Potassium (K⁺) and permanganate (MnO₄⁻) have equal but opposite charges, so the formula is simply \[\text{KMnO}_4\].

These formulas are crucial as they provide vital information on the composition of the compounds. They help chemists understand how elements interact to form stable structures.

Balancing Charges

Balancing charges is a critical concept when writing ionic compound formulas. The total positive charges from the cations must equal the total negative charges from the anions, resulting in a neutral compound. Let's look at a couple of examples:

• Magnesium perchlorate: Magnesium ion (Mg²⁺) needs two perchlorate ions (ClO₄⁻) for the charges to balance. This results in the formula \[\text{Mg(ClO}_4\text{)}_2\].


• Sodium sulfite: Sodium ion (Na⁺) needs two ions to balance with one sulfite ion (SO₃²⁻). The balanced formula is \[\text{Na}_2\text{SO}_3\].

Whenever you combine ions, use their charges to determine the correct ratio. This ensures the compound is neutral and satisfies ionic stability principles.