Dear student
In parallel combination of capacitors, each capacitor has the potential difference V.
When these capacitors connected in the series combination, total potential difference across the combination is the sum up of each capacitor so the potential difference across the combination is .
Mathematically
when these capacitors of capacitance C, each have voltage V are connected in series, the net capacitance of the of the circuit is C'=C/n and in series combination the charge on each capacitor is same and equal to the charge on net capacitance. So
where V' is the voltage difference across the series combination.