Given the balanced ionic equation:
3Pb2+(aq) + 2Cr(s) ---> 3Pb(s) + 2Cr3+(aq)

what is the number of moles of electrons gained by 3.0 moles of lead ions?

1. 5.0 mol
2. 2.0 mol
3. 3.0 mol
4. 6.0 mol

Answer : The correct option is, (4) 6.0 mol

Explanation :

The given balanced chemical equation is,

In this reaction, lead undergoes reduction and chromium undergoes oxidation.

Oxidation reaction : It is the reaction in which a substance looses its electrons. In this oxidation state increases.

Reduction reaction : It is the reaction in which a substance gains electrons. In this oxidation state decreases.

Half reactions of oxidation-reduction are :

Oxidation :

Reduction :

From the reduction reaction, we conclude that 6 moles of electrons gained by the 3 moles of lead ions.

Hence, the correct option is, (4) 6.0 mole

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.