##C_18H_32O_16##

since this is a dehydration reaction, it means that when two molecules are linked together, there will be one molecule of water ##H_2O## removed.

Therefore, the molecular formula will be ##C_18H_32O_16##.

The reaction will be: ##C_6H_11O_5-underbrace(OH+H)_(color(blue)(H_2O))-C_6H_10O_5-underbrace(OH+H)_(color(blue)(H_2O))-C_6H_11O_6->C_18H_32O_16+color(blue)(2H_2O)##