Which process is used for the removal of hardness of water?

Which of the following is the strongest Lewis acid?

If the electron of a hydrogen atom is present in the first orbit,the total energy of the electron is

$100 \ mL$ of $0.6 \ M$ acetic acid is shaken with $2 \ g$ of activated charcoal. After adsorption,the final concentration of the solution becomes $0.5 \ M$. The amount of acetic acid adsorbed per gram of carbon is .......... $g$.

If $\alpha$ and $\beta$ are the roots of $x^2-2x+4=0$,then the value of $\alpha^6+\beta^6$ is

If $x + \sqrt{x^2 + 1} = a$,then $x =$