To find mass of water that was in the compound before heating, subtract the third mass from the first mass.
Mass of water = Mass of crucible,cover, and contents before heating - Mass of crucible,cover and contents after heating to a constant mass
Mass of water = 23.4-20.94 =2.46
To find mass of the compound before heating only, subtract the 2nd mass from the first mass.
Mass of compound before heating = Mass of crucible, cover, and contents before heating - Mass of crucible and cover
Mass of compound before heating= 23.4-18.82 = 4.58
Experimental percent of water in the compound = mass of water/mass of compound before heating × 100
Experimental percent of water= 2.46/4.58 × 100
=53.7%