Answer
$0.01045\ g$
Work Step by Step
Molecular weight of $YBa_2Cu_3O_{6.5}$: $658.19\ g/mol$
Molecular weight of $YBa_2Cu_3O_{6.93}$: $665.07\ g/mol$
In 1.00 g of the first compound: $1.00/658.19=0.00152\ mol$
This number of moles corresponds to:
$0.00152\ mol\times665.07\ g/mol=1.01045\ g$ of the second compound.
The difference is the mass of oxygen added:
$0.01045\ g$
