51. Option 2 A standard cerium solution was prepared by

51. Option 2
A standard cerium solution was prepared by dissolving 0.0500 g in a 250.0 ml mixture of perchloric and hydrochloric acids. In flasks with a capacity of 25.00 ml were placed: 2.50; 3.00; 3.50; 4.00; 4.50; 5.00 ml of standard solution, ammonia, hydrogen peroxide, nitrilotriacetic acid was added and diluted to the mark with water. When photometry relative to the first solution received:
Vst, ml 3.00; 3.50; 4.00; 4.50; 5.00
Aot 0.135 0.270 0.405 0.545 0.680
A portion of lanthanide oxides with a mass of 0.0500 g was dissolved, the necessary reagents were added, diluted to 25.00 ml, and photometric, as in the construction of the calibration curve.
Calculate the mass fraction in the sample of lanthanide oxides using the following data.
Aoth. X = 0,440