CAIE A-Level Chemistry 24.1.4 Determining the Avogadro Constant

Practise describing an electrolytic determination of the Faraday constant and hence the Avogadro constant.

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An electrolytic cell is set up to determine a value for the Avogadro constant, L. The electrolyte is dilute sulfuric acid and both electrodes are copper.

When a current of 0.600 A is passed through the acid for 30.0 minutes, the anode decreases in mass by 0.350 g .

Use the experimental information in (b) and data from the table on page 23 to calculate a value for the Avogadro constant, L.

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number of $\mathrm{Cu}^{2+}$ formed $=0.35 / 63.5=5.51 \times 10^{-3}$ $Q=I \times t=0.60 \times 30 \times 60=1080 C$

number of electrons $=1080 / 1.6 \times 10^{-19}=6.75 \times 10^{21}$ ecf

number of $\mathrm{Cu}^{2+}$ ions $=6.75 \times 10^{21} / 2=3.375 \times 10^{21}$ ecf

number of $\mathrm{Cu}^{2+}$ ions per mole $(L)=3.375 \times 10^{21} / 5.51 \times 10^{-3}=6.12 \times 10^{23}$ ecf min 2sf

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24.1.4—Determination of a value of the Avogadro question 1