Ch.2 - Atoms, Molecules, and Ions

Problem 62e

Chapter 2, Problem 62e

Predict the chemical formulas of the compounds formed by the following pairs of ions: (e) NH4+ and PO43-.

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Identify the charges on each ion: \( \text{NH}_4^+ \) has a charge of +1 and \( \text{PO}_4^{3-} \) has a charge of -3.

Determine the ratio of ions needed to balance the charges. You need enough \( \text{NH}_4^+ \) ions to balance the -3 charge of \( \text{PO}_4^{3-} \).

To balance the charges, use 3 \( \text{NH}_4^+ \) ions for every 1 \( \text{PO}_4^{3-} \) ion, resulting in a neutral compound.

Write the chemical formula by placing the number of each ion as a subscript: \( (\text{NH}_4)_3\text{PO}_4 \).

Verify that the total positive charge from the \( \text{NH}_4^+ \) ions equals the total negative charge from the \( \text{PO}_4^{3-} \) ion, ensuring the compound is neutral.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionic Compounds

Ionic compounds are formed when positively charged ions (cations) and negatively charged ions (anions) combine. The overall charge of the compound must be neutral, meaning the total positive charge must balance the total negative charge. Understanding how to combine these ions based on their charges is essential for predicting the correct chemical formula.

Polyatomic Ions

Polyatomic ions are ions that consist of two or more atoms bonded together, carrying a net charge. In this case, ammonium (NH4+) is a common polyatomic cation, while phosphate (PO43-) is a polyatomic anion. Recognizing these ions and their charges is crucial for determining how they interact to form compounds.

Charge Balance

Charge balance is the principle that the total positive charge in a compound must equal the total negative charge. For the ions NH4+ and PO43-, the ammonium ion has a +1 charge and the phosphate ion has a -3 charge. To achieve charge neutrality, three ammonium ions would be needed to balance one phosphate ion, leading to the formula (NH4)3PO4.

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