Final answer:
The weight of Goliath's iron spearhead cannot be determined.
Step-by-step explanation:
The weight of Goliath's iron spearhead is not specified in any historical sources, including the Bible. It is important to note that the story of David and Goliath is a biblical narrative, and while it has significant cultural and religious importance, it is not a historical account that can be verified through archaeological or other historical evidence. Therefore, the specific weight of Goliath's spearhead cannot be determined.