Will rusting increase the weight of iron?

Yes, rusting will increase the mass of iron.

The empirical formula of rust is Fe­2O3•nH2O.
Molar mass of Fe = 56 g mol-1
Molar mass of Fe­2O3•nH2O = 56x2 + 16x3 + n(1x2+16) = (160 + 18n) g

Consider 1 mol of rust (Fe­2O3•nH2O).
Mass of 1 mol of rust = (160+18n) g
Mass of Fe in 1 mol of rust = 56 x 2 = 112 g

In other words, 112 g of Fe can form (160+18n) g of rust.
Obviously, the mass increases.

2008-01-13 00:30:18 補充：
有兩個講法：(1) The total mass of solid increases in rusting.(2) In rusting, the mass of the Fe decreases, but the mass of the rust increases. This is because Fe changes to rust in rusting.