What was the calculation of what the copper was used for?
He assumes that each copper socket weighed a Kikar, just like those of silver. Copper is about 5% less dense than silver; perhaps their cavities for Yados (or their hollows - refer to 26:17:152:1) were slightly smaller. Rav Sadya Gaon always translates 'Kikar' of gold or silver as 'Badrah', and regarding copper, 'Kantar'. R. Kapach says that Kantar is much heavier (Meleches ha'Mishkan v'Kelav p.424). If so, the sockets were bigger, or much less was hollow. (PF)
Chizkuni: The remaining five Kikar were also for the pegs and Kelim. (It is difficult to say that it was all for the Mizbe'ach, for it was merely plated. According to the opinion that it was three Amos tall, the plating would have to be about a finger thick! - PF)
Why does the Torah not mention that some of the copper was used for clasps to connect the two sets of goats' hair curtains (26:11)?
Refer to 38:24:1:2*.