Question

The water sample does not contain OH- and HCO3- ions together. give reason

Answer 1

The reason that OH- and HCO3- are not found together is simple. As water absorbs CO2, it tends to form H2CO3 which can then ionize into hydrated protons and bicarbonate ions, as shown by:
(1) CO2 + H2O ←→ H2CO3 ←→ HCO3(-) + H+
and the pH of the aqueous solution is given by:
(2) pH = pKa for H2CO3 + log { |HCO3-| / |H2CO3| }
where the pKa for H2CO3 is 6.1
Below pH of 7 (neutral), any hydroxyl ions will be scarce. As pH rises to 6.1, the HCO3 ion and H2CO3 molecular concentrations become equal. As the pH rises above 6.1, the HCO3- ion concentration drops quickly. at pH above 8, 99% of the carbon is present in solution as H2CO3.
So there is only a small range of pH, between 7 and about 8.3 where there will be any overlap of measurable hydroxyl and bicarbonate ions. Outside this range, you will find only one or the other.