Question

What is the formula for cupric cyanide?

Answer 1

Hint: Molecular formula tells us about the number of atoms present in the molecule but it does not describe the arrangement of atoms or how atoms are arranged so this concept is defined by the structural formula.

Complete answer:
Cupric ion is generally made from copper ion where cuprum is latin word which have the meaning copper and cupric ion is cation in nature i.e. those ion which carries positive charge on it and cupric consists of +2 positive charge and this can be represented by the symbol $C{{u}^{2+}}$.
Now we know that cyanide is represented by the symbol $C{{N}^{-}}$. Now we know that cupric cyanide is an ionic compound and in case of an ionic compound charges must add up in such a manner that overall charge becomes neutral. Now we know that cupric ions carry +2 charge but cyanide carries -1 charge; this defines that two cyanide ions should be present for one cupric ion.
This suggests that the formula for cupric cyanide contains one cupric ion and two cyanide ions and the formula is shown as $Cu{{(CN)}_{2}}$. This compound is also known by the name copper(II) cyanide.
 The equation can also be shown as:
$C{{u}^{2+}}+C{{N}^{-}}\to Cu{{(CN)}_{2}}$
Hence the formula for cupric cyanide $Cu{{(CN)}_{2}}$

Note:
Molecular formula displays how the atoms are organized and attached together in a molecular formula of a chemical compound while the chemical formula generally has a limited number of symbols due to which they are capable of only limited descriptive power.