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Answer 1 · 2016-09-13T21:34:09

#sf(A_r=11)#

Since the metal is trivalent we can say that the formula of the oxide will be #sf(M_2O_3)#.

This is because #sf(2M^(3+))# ions has the same charge as #sf(3O^(2-))# ions.

The percentage by mass of #sf(M=100-68=32)#

So the ratio of #sf(M:O)# by mass is:

#sf(32:68)#

To get the ratio by moles we divide by the mass of 1 mole which is the #sf(A_r)# of the element in grams:

Ratio by moles #sf(M:O)rArr#

#sf(32/A_r:68/16=2:3)#

#:.##sf(32/A_r:4.25=2:3)#

#:.##sf((32/A_r)xx3=4.25xx2)#

#:.##sf(96/A_r=8.5)#

#sf(A_r=96/8.5=11)# to 2sig fig.