Ascientist is combining two grey elements he thinks he will get a compound why is this prediction wrong

In a compund, the proportion must be fixed, unlike mixture. Therefore, if he just combine two elements, there must be excess elements mixed in it.
Second, how did he combined two elements? In order to create a compund, either heat or electricity must be applied.
Therefore, even if he used heat / electricity, he still hasn't got the right proportion, therefore he must have mixed some excess elements into the compund that he thought he made.

The answer is 3,5g co