Solution :
Assuming all Al will be consumed in the reaction.
From the given equation, 2 mole of Al produce 2 mole of [tex]Na_3AlO_3[/tex].
So, 1 mole of Al produce 1 mole of [tex]Na_3AlO_3[/tex]. .....statement 1)
Number of moles of Al in 258 gram of Al.
[tex]n = \dfrac{mass}{molar\ mass}\\\\n = \dfrac{258}{27}\ moles\\\\n = 9.556 \ moles[/tex]
Therefore, from statement 1) number of moles of [tex]Na_3AlO_3[/tex] produced is also 9.556 moles.