Answer
$91.9\%$
Work Step by Step
Number of moles of water removed:
$(2.634\ g - 2.125\ g)/18.015\ g/mol=0.0282\ mol$
Number of moles of the hydrated salt:
$0.0282\ mol/2=0.0141\ mol$
Mass fraction of the hydrated salt:
$0.0141\ mol\times 170.482\ g/mol=2.42\ g$
$2.42/2.634\times100\%=91.9\%$
