Final answer:
The mass of the rubber stopper is found by subtracting the mass of the flask and water from the total mass. It is calculated to be 23.888 g and rounded to 23.9 g to match the number of significant figures in the least precise measurement.
Step-by-step explanation:
To find the mass of the rubber stopper, we subtract the mass of the empty flask and the water from the total mass of the flask, water, and the rubber stopper.
The mass of the flask and water is given as 35.4 g (water) + 87.432 g (flask) = 122.832 g.
The total mass of the flask, water, and stopper is 146.72 g. So to find the mass of the stopper, we subtract the mass of the flask and water from this:
Mass of stopper = 146.72 g (total) - 122.832 g (flask + water)
= 23.888 g
The mass of the stopper should be expressed to the correct number of significant figures. Since the flask mass was given to five significant figures and the water to three, the least precise measurement, which is the mass of water, dictates that the mass of the stopper should also be rounded to three significant figures.
Therefore, the correct mass of the stopper expressed to three significant figures is 23.9 g.