Question

Kilogram has been defined as the Pt-Fe cylinder stored in an airtight jar. Pt-Fe was chosen for this standard because
(i) it is highly resistant to chemical attack and its mass will not change for an an extremely long time
(ii) both Pt and Fe are easily available and in abundance
(iii) any change in its mass is easily detectable
(iv)both (ii) and (iii)

Answer 1

The question contains a false presumption. In fact, the kg standard is an alloy of platinum and iridium, not platinum and iron. Given the ease with which iron oxidizes to rust, no one in his right mind would use an iron alloy for a weight standard.

One reason for using a Pt/Ir allow is certainly its resistance to oxidation -- both metals are unusually unreactive, so that argues for (i). Both metals are rare and expensive, which argues against (II). (III) is a priori silly, since "any" change in mass can mean anything from it loses ten atoms to it loses half its mass. Obviously not all such changes are, or could be made, easily detectable.

It seems likely another reason for picking Pt/Ir is that both metals are very dense -- indeed, Ir is the densest metal there is -- so that the object will be very small, and more easily kept in a controlled environment.