Answer
$Rb^{+}$.
Work Step by Step
1. As we found in 4.85a, the molar mass of the titrated alkali metal hydroxide in the reaction, is approximately 102.6g/mol.
2. Find the molar mass of the alkali metal, knowing that the sum of the atom's molar masses equals to the molar mass of the molecule.
*The molecular formula of the hydroxide is: $X_{}O_{}H_{}$, with "X" being the alkali metal.
molar mass of (X) + molar mass of (O) + molar mass of (H) = 102.6
molar mass of (X) + 16 + 1 = 102.6
molar mass of (X) = 102.6 - 17
molar mass of (X) = 85.6