Question

Predict the chemical formula for each of the following ternary oxyacids given the formula of phosphoric acid, \(\mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})\) (a) arsenic acid (b) phosphorous acid

Short answer

(a) Arsenic acid: \( \mathrm{H}_{3} \mathrm{AsO}_{4} \); (b) Phosphorous acid: \( \mathrm{H}_{3} \mathrm{PO}_{3} \).

Step-by-step solution

Understanding Ternary Oxyacids

Ternary oxyacids are acids that consist of hydrogen, non-metal (often from Group 15 or 16 of the periodic table), and oxygen. They typically follow the formula: \\( ext{H}_{ ext{n}} ext{A} ext{O}_{ ext{m}} \\), where \( ext{A} \) is a non-metal.

Identify the Non-metal

For arsenic acid and phosphorous acid, identify the respective central elements: arsenic (\( ext{As} \)) and phosphorus (\( ext{P} \)). Arsenic is in the same group as phosphorus, so they form similar acids.

Predict Arsenic Acid Formula

Arsenic acid is analogous to phosphoric acid, \( ext{H}_{3} ext{PO}_{4} \), with arsenic replacing phosphorus. Therefore, the formula should be \( ext{H}_{3} ext{AsO}_{4} \). Ensure the valency is consistent with phosphoric acid.

Predict Phosphorous Acid Formula

Phosphorous acid has one less oxygen than phosphoric acid. It follows the pattern similar to phosphoric acid, but reduced by one oxygen: \( ext{H}_{3} ext{PO}_{3} \). Verify that the chemical formula balance complies with known polyatomic ions.

Review and Confirm

Check both formulas: \Arsenic acid: \( ext{H}_{3} ext{AsO}_{4} \) \Phosphorous acid: \( ext{H}_{3} ext{PO}_{3} \). \Compare with known chemical standards to ensure accuracy.

Key concepts

Ternary Oxyacids

Ternary oxyacids are a fascinating class of acids in chemistry. These acids consist of three types of elements: hydrogen, a non-metal, and oxygen. The central component, the non-metal, is often found near the middle or the right side of the periodic table, specifically in groups 15 or 16. This positioning allows them to bond with oxygen to form a backbone structure by which the acid gains additional properties. These acids are commonly represented by the formula \( \text{H}_n\text{A}\text{O}_m \), where \( \text{A} \) symbolizes the non-metal atom.

The characteristic feature of ternary oxyacids is their ability to release hydrogen ions \( (\text{H}^+) \) into solutions, making them acidic. The strength of these acids can vary significantly based on the number of oxygen atoms bonded within the molecule's structure, which, in some cases, influences the acidity. For example, acids with more oxygens are generally stronger because they stabilize the negative charge better after donating a hydrogen ion.

This property of differing oxygen atoms leading to varying properties is key when examining structurally similar oxyacids, like phosphoric and arsenic acids.

Phosphoric Acid

Phosphoric acid, represented by the formula \( \text{H}_3\text{PO}_4 \), is one of the more well-known ternary oxyacids. In its structure, three hydrogen atoms are bonded to a central phosphorus atom that also bonds with four oxygen atoms. This composition helps to illustrate the typical formula used for ternary oxyacids.

Phosphoric acid is a classic example, not just as a laboratory staple, but also due to its prevalence in various industrial and food applications, such as serving as an acidic additive in cola drinks.

The phosphorus atom in phosphoric acid has the ability to bond with multiple oxygen atoms simultaneously, maintaining a neutral charge overall. This ability highlights the flexibility within the group 15 elements to form stable bonds with oxygen. When studying phosphoric acid, one also sees how variations in structure can lead to different acids, as seen in the comparison with phosphorous acid, \( \text{H}_3\text{PO}_3 \). This variation is crucial for understanding how changes in oxygen content result in different acid characteristics.

Chemical Nomenclature

Chemical nomenclature is the systematic method used to name chemical compounds, ensuring consistent communication across the global scientific community. In the realm of ternary oxyacids, nomenclature follows specific patterns that reflect the structure and composition of the compound.

The name of a ternary oxyacid typically derives from the central non-metal. For acids using elements in the same group on the periodic table as phosphorus, such as arsenic, they often share a similar name format, replacing the ending 'ic' with 'ic' and adding the term 'acid'. For instance, phosphoric acid becomes arsenic acid or nitric acid becomes nitrous acid for lesser oxygenated forms. This systematic approach allows scientists to easily identify critical differences in elemental composition and predict chemical behavior.

Understanding chemical nomenclature also enables quick adaptation in naming acids when different numbers of oxygen atoms are involved, as is the case with phosphorous acid \( \text{H}_3\text{PO}_3 \), which has one fewer oxygen atom than phosphoric acid. Easy adaptation of names depending on a compound's composition serves as an essential tool in communicating chemical identities clearly and efficiently.