[kahynickel]

the Al2O3 in x grams of the mixture to dissolve. the reaction occurring is
Al2O3 +   2OH- + 3H2O ..2Al(OH)4-
800 cm3 of 2 M HCl(aq) is just sufficient to cause all the oxide in x grams of the mixture to dissolve.

AL2O3   +    6H+ ...2Al3+   +    3H2O
MgO      +    2H+ ...Mg2+    +  H2O
How many moles of each oxide is present in x grams of the mixture?
Moles of MgO = [Total moles of HCl] – [Moles of HCl used for Al2O3] /2 

did not get the right answer....??

[mjc123]

You've missed out a bracket there, don't know if that is the full explanation...