Question

A current of electricity is passed through copper voltameter and a water voltameter connected in series. If the copper of the copper voltameter now weighs $16$ mg less, hydrogen liberated at the cathode of the water voltameter measures about (at STP)

• A. $4.0$ ml
• B. $5.6$ ml
• C. $6.4$ ml
• D. $8.4$ ml

Answer 1

The correct option is B $5.6$ ml

It is given that copper of the copper voltameter weighs $16$ mg less after electricity is passed.
Moles of $C{u}^{2+}$ used $=\frac{16\times {10}^{-3}}{64}=0.25\times {10}^{-3}$

Number of equivalents of $C{u}^{2+}=2\times 0.25\times {10}^{-3}=0.5\times {10}^{-3}$

Number of equivalents of electrons $=0.5\times {10}^{-3}$

$2H^{+}+2e^{-}\to H_2$

$2\times$ Moles of $H_2=0.5\times {10}^{-3}$

$\therefore$ Moles of $H_2=\frac{0.5}{2}\times {10}^{-3}$ moles

$1$ mole of $H_2$ at STP has volume of $22.4$ L.

$0.25\times {10}^{-3}$ moles has volume $=22.4\times 0.25\times {10}^{-3}$ L $=5.6$ mL