Final answer:
The 200-mL and 400-mL samples of a salt solution are identical in concentration (molarity) but different in volume. The same concentration means the proportion of salt per unit volume is equal, but the larger volume sample contains more total salt.
Step-by-step explanation:
A 200-mL sample and a 400-mL sample of a solution of salt have the same molarity. This means that these two samples are identical in concentration but different in volume. The molarity of a solution indicates the number of moles of solute per liter of solution, hence, if two solutions have the same molarity, they contain the same proportion of solute to solution volume.
The 200-mL sample has half the volume of the 400-mL sample, but because they have the same molarity, the amount of salt per unit volume is the same. However, the total amount of salt in the 400-mL sample is double that of the 200-mL sample, due to its larger volume. So, although they are the same concentration, the total quantities of solute and solvent are different.
In both dilution and concentration, the amount of solute stays the same. This gives us a way to calculate what the new solution volume must be for the desired concentration of solute. From the definition of molarity, moles1 = moles2.
By adding more water, we changed the volume of the solution. Doing so also changed its concentration. However, the number of moles of solute did not change. So, the two samples are different in volume, but identical in concentration. Therefore, the correct answer is Identical in concentration, different in volume.