The given question states that a liquid with the formula A3B has a specific gravity of 1.50, with the atomic masses of A and B being 33 and 21 respectively. When 20 cm3 of A3B is mixed with water to a final volume of 500 cm3, the resulting solution will have a concentration of how many mol/dm3.
To find the concentration, we need to calculate the number of moles of A3B in the solution. The specific gravity of 1.50 indicates that the density of A3B is 1.50 times that of water. Since the volume of A3B is 20 cm3, the mass of A3B is (20 cm3) * (1.50 g/cm3) = 30 g.
The molar mass of A3B is (33 g/mol) * 3 + (21 g/mol) = 150 g/mol. Therefore, the number of moles of A3B is (30 g) / (150 g/mol) = 0.20 mol.
The final volume of the solution is 500 cm3, which is equal to 0.500 dm3.
Therefore, the concentration of the solution is (0.20 mol) / (0.500 dm3) = 0.40 mol/dm3.
Since the answer choices are not provided in the question, we cannot determine the correct answer based on the given information.