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Moles

Masses or Volumes from Electrolysis.

Example 2.

The electrolysis of sodium chloride dissolved in water
produces hydrogen at the cathode and chlorine at the anode.

What mass and volume of hydrogen is obtained
if 142 g  of chlorine are discharged at the anode?

Method.
1) Find how many moles of chlorine are discharged at the anode.

RMM of Cl2 = 71.

     moles = mass ÷ RMM

moles = 142 ÷ 71
                            = 2 moles of chlorine.

2) Write the half equations for the electrolysis.
(i)  2H+  +    2e-   arrow       H2
(ii)  2Cl-   -    2e-   arrow       Cl2

Balance the half equations,
to give the same number of electrons on each side.
(the equations are already balanced).

Find the proportion of how many moles of product
are discharged at each electrode.
1 mole of H2 at the cathode gives 1 mole of Cl2 at the anode.
The proportion is 1 to 1.

2 moles of Cl2 gives 2 moles of H2.

3)  Find the mass and volume of 2 moles of hydrogen.
 The RMM of hydrogen is 2.

             mass = moles x RMM

mass = 2 x 2
                                   = 4 grams of hydrogen.

      volume  =  moles x 24,000 cm3

volume  =  2 x 24,000 cm3
                           = 48,000 cm3 of hydrogen.

This occupies the same volume as 142  g  of Cl2.

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