45. Solving a chemistry problem

130. A weighing of 0.2000 g of silicate was dissolved in a mixture of H2SO4 and HF, the solution was evaporated, the residue was treated with HCl and transferred to a 200 ml volumetric flask. Determination of sodium was carried out by the method of calibration curve. Before measurement, the analyzed solutions were diluted 2 times. Photometry data are given in Table. 25.
Table 25
Standard solutions Analyzed solutions
CNa, mg/l 2.5 5.0 7.5 10.0 ?
I, μA 10 20 30 40 41