I am conducting an experiment where I have to record the rates at witch Alka seltzer tablets dissolve at different surface areas.

I am conducting an experiment where I have to record the rates at witch Alka seltzer tablets dissolve at different surface areas. I am dissolving one whole, one cut in half, one in eights, and the last one I will grind it to powder. The first three I just add into their individual cups filled with water (all cups have the same amount of water). To have accurate results, should I put the powder into a cup and then add the water or should I put the powder into a cup already filled with water?