ANS:B - 15-25

The yield of elemental phosphorus from rock phosphate is about 40-45 percent. The production of elemental phosphorus (P4) from rock phosphate involves several steps, including:

1. Phosphate Rock Processing: Rock phosphate is processed to remove impurities and obtain a purified form of phosphate ore.
2. Phosphate Rock Reduction: The purified phosphate ore is then heated in a furnace with carbon (usually coke) and silica to reduce it to elemental phosphorus.
3. Formation of Phosphorus Vapor: The reduction process produces phosphorus vapor, which is collected and condensed into liquid elemental phosphorus.
4. Refining and Purification: The condensed phosphorus undergoes further refining and purification to remove any remaining impurities.

The overall yield of elemental phosphorus from rock phosphate depends on various factors, including the quality of the rock phosphate, the efficiency of the reduction process, and the purity requirements of the final product. On average, the yield is approximately 40-45 percent, meaning that about 40-45 percent of the phosphorus content in the rock phosphate is converted into elemental phosphorus during the production process.