Baking powder used to make cake is a mixture of starch,$NaHCO_3$,and $Ca(H_2PO_4)_2$. The function of $Ca(H_2PO_4)_2$ is

The dissociation constant of a weak acid $HA$ is $1.8 \times 10^{-4}$. Calculate the dissociation constant of its conjugate base $A^{-}$.

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A :$ The amphoteric nature of water is explained by using Lewis acid/base concept.
Reason $R :$ Water acts as an acid with $NH_{3}$ and as a base with $H_{2}S$.
In the light of the above statements,choose the correct answer from the options given below:

Conjugate acid of $NH_2^{-}$ and $NH_3$ are respectively

$A$ solution of ammonia in water contains

$H^{+}$ is a