87. Solving a chemistry problem

2. To prepare a series of standards from a solution containing 0.648 g of NaNO2 in 500.0 ml of water, from 0.50 to 5.00 ml were taken with an interval of 0.50 ml, α-naphthylamine and sulfanilic acid were added, the colored solutions were diluted with water to 20.00 ml. A weighed portion of sodium nitrate (0.520 g) was dissolved in 250.0 ml of water, 5.00 ml was taken and, after appropriate treatment, diluted with water to 20.00 ml. The color intensity of this solution was equal to the color intensity of the 8th standard. Determine the content of NO2- in sodium nitrate (in %).