Q 2)

In which of the following reaction H2PO4− acts as an acid:

(i) H3PO4 + H2O → H3O+ + H2PO4−

(ii) H2PO4−+ H2O → HPO42− + H3O+

(iii) H2PO4− + OH−→ H3PO4 + O2−

• A)(i) only B)(ii) only
• C)(iii) only
• D)(i) and (
  ii)

•  

Dear student

According to the Arhenius concept of acids and bases, acids are proton donors. So for H2PO4− to act as an acid, it has to loose a proton. The reactions in which H2PO4− will loose a proton, will be found to behave as acids. In the first reaction, H2PO4− is gaining a proton so it is not acting as an acid. 
In the second reaction, H2PO4− looses a proton to water and thus can be acting as acid. Again in the third reaction,   H2PO4− 
gains a proton and thus again can not act as acid.
so the correct answer will be (b)

Regards