Identify the correct chemical formula for sodium tripolyphosphate (STPP), a common builder in detergents and a phosphate fertiliser intermediate.
-
ANa5P3O10
-
BNa4P3O8
-
CNa3P4O6
-
DNa2PO4
-
ENa3PO4
Answer
Correct Answer: Na5P3O10
Explanation
Introduction / Context:Sodium tripolyphosphate (STPP) is widely used as a sequestrant in detergents and appears in phosphate fertiliser process streams. Correct stoichiometry helps with mass balances and environmental compliance calculations (e.g., phosphorus loading).
Given Data / Assumptions:
- Compound name: sodium tripolyphosphate.
- We seek the empirical formula reflecting three phosphate units and five sodium cations.
Concept / Approach:“Tripolyphosphate” indicates three tetrahedral phosphate units linked via two P–O–P bridges, carrying an overall charge of 5− balanced by five Na+. Hence the formula Na5P3O10 is standard in handbooks and labels for STPP (often as the hexahydrate in commerce, but the anhydrous formula is Na5P3O10).
Step-by-Step Solution:
Interpret the name: tri- (three) phosphate units linked → P3 backbone.Count oxygens: 4 per PO4 minus 2 bridging oxygens shared → total 10.Balance charge: 5− anion requires 5 Na+ → Na5P3O10.Confirm with standard references.Verification / Alternative check:Material safety data sheets and chemical catalogs list STPP as Na5P3O10 (with hydrates noted separately), verifying the formula.
Why Other Options Are Wrong:
- Na4P3O8 and Na3P4O6: incorrect stoichiometry and phosphorus count.
- Na2PO4 or Na3PO4: monophosphates, not tripolyphosphate.
Common Pitfalls:Confusing the anhydrous formula with hydrate compositions; hydrates add water molecules but do not alter the Na:P:O ratio of the anion.
Final Answer:Na5P3O10