Final answer:
The last measurement taken for determining the percentage by mass of a solution is the mass of the evaporating dish and residue, which represents the mass of the solute after the solvent has been evaporated.
Step-by-step explanation:
When collecting data to determine the percentage by mass of a solution, the measurement that occurs last is the mass of the evaporating dish and residue.
This is after the solvent has been evaporated and you are left with the solute (residue) in the dish.
The initial step typically involves weighing the empty evaporating dish, followed by weighing the dish and the sample (before evaporation) to get the total mass of the solution.
After evaporation, the weight of the dish and the residue gives you the mass of the solute.
The mass of the solvent can be determined by subtracting the mass of the solute from the total mass of the solution.
The percentage by mass is then found by dividing the mass of the solute by the total mass of the solution and multiplying by 100.