68. When determining zinc in aluminum by atomic absorpt

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68. When determining zinc in aluminum by atomic absorption method, a calibration curve was constructed according to the following data:
The concentration of standard solutions of zinc, µg / ml
0.3000 0.5000 0.8000 1.0000
Atomic absorption of zinc at 213.9 nm, the number of tick marks
7.50 11.50 16.33 21.50
A portion of the analyzed sample weighing 0.5000 g dissolved in a mixture of acids and transferred to a volumetric flask with a capacity of 50.00 ml. The atomic absorption of this solution is 15.00 scale divisions. Determine the mass fraction (%) of zinc in the sample.