3. A biological sample weighing 0.150 g was burned in a muffle furnace, the residue was dissolved in hydrochloric acid and diluted with a solution of lanthanum salt to 5.0 cm3. Equal volumes of standard calcium solutions containing 0; 0, 2.0; 4.0; 8.0 μg/cm3 were added to four equal portions of the analyzed solution and the optical density of the solutions was measured at a = 422.67 nm. Calculate the calcium concentration (mg/kg) in the sample using the addition method if the following optical density values ​​were obtained: 0.050; 0.116; 0.185; 0.320.

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