Answer
$Sr^{2+}$.
Work Step by Step
As we found in 4.86a, the molecular formula of the hydroxide is:
$X_{1}(O_{}H_{})_2$
*Being "X" a Group 2 Metal.
And the molar mass is 121.62g/mol.
1. We can find the identity of the ion by his mass.
Knowing that the sum of the atom's masses equals the molecular mass:
mass(X) + 2*( mass(O) + mass(H)) = mass(Hydroxide)
mass(X) + 2*(16 + 1) = 121.62
mass(X) + 34 = 121.62
mass(X) = 121.62 - 34
mass(X) = 87.62 g/mol
2. If we consult a Periodic Table, we will find that 87.62 is the mass of Sr.
