To determine the concentration of an unknown cobalt(II) stock solution

CHEMISTRY · Single correct · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures

Problem

To determine the concentration of an unknown cobalt(II) stock solution, a student performs the following serial dilutions: • Transfers 2.00 mL of the stock into a 50.0 mL volumetric flask and dilutes to the mark. • Withdraws 10.0 mL of this first dilution and dilutes it to 100.0 mL in a second volumetric flask. The absorbance of the final solution is measured to be 0.256 in a 1.00 cm cuvette at a wavelength where the molar absorptivity is 6.40×103 L mol−1 cm−16.40\times10^{3}\,\text{L}\,\text{mol}^{-1}\,\text{cm}^{-1}. What is the molar concentration of the original stock solution? (a) 1.0×10−3 M1.0\times10^{-3}\,\text{M} (b) 8.0×10−2 M8.0\times10^{-2}\,\text{M} (c) 1.0×10−2 M1.0\times10^{-2}\,\text{M} (d) 4.0×10−4 M4.0\times10^{-4}\,\text{M}

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