JEE MAIN - Physics (2003 - No. 11)
The nagative $$Zn$$ pole of a Daniell cell, sending a constant current through a circuit, decreases in mass by $$0.13g$$ in $$30$$ minutes. If the electrochemical equivalent of $$Zn$$ and $$Cu$$ are $$32.5$$ and $$31.5$$ respectively, the increase in the mass of the positive $$Cu$$ pole in this time is
$$0.180$$ $$g$$
$$0.141$$ $$g$$
$$0.126$$ $$g$$
$$0.242$$ $$g$$
Explanation
According to Faraday's first law of electrolysis $$m = z \times q$$
For same $$q,$$ $$\,\,\,\,\,\,\,\,\,$$ $$m \propto Z$$
$$\therefore$$ $${{{m_{Cn}}} \over {{m_{Zn}}}} = {{{Z_{Cu}}} \over {{Z_{Zn}}}}$$
$$ \Rightarrow {m_{Cu}} = {{{Z_{Cu}}} \over {{Z_{Zn}}}} \times {m_{Zn}}$$
$$ = {{31.5} \over {32.5}} \times 0.13 = 0.126\,g$$
For same $$q,$$ $$\,\,\,\,\,\,\,\,\,$$ $$m \propto Z$$
$$\therefore$$ $${{{m_{Cn}}} \over {{m_{Zn}}}} = {{{Z_{Cu}}} \over {{Z_{Zn}}}}$$
$$ \Rightarrow {m_{Cu}} = {{{Z_{Cu}}} \over {{Z_{Zn}}}} \times {m_{Zn}}$$
$$ = {{31.5} \over {32.5}} \times 0.13 = 0.126\,g$$
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