If a non-metal forms only two oxides and one oxide on hydrolysis gives an acid, then the non-metal could be: 

Antacid pair among the following is :

A metallic element is converted into a compound 'A' after a series of reactions. Compound A turns red litmus to blue. A on reaction with another compound B which turns blue litmus to red gave a compound C which responded to the litmus test like compound B. C on further reaction with A gave a compound D which did not respond to the litmus test. Identify the correct sequence of steps for the formation of A, B, C and D. 

$$\begin{gathered} \left(1\right) \mathrm{NaOH}+{\mathrm{H}}_2\mathrm{SO}4\to {\mathrm{NaHSO}}_4+{\mathrm{H}}_2\mathrm{O} \\ \left(2\right) 4\mathrm{Na}+{\mathrm{O}}_2\to 2{\mathrm{Na}}_2\mathrm{O} \\ \left(3\right) {\mathrm{NaHSO}}_4+\mathrm{NaOH}\to {\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O} \\ \left(4\right) {\mathrm{Na}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH} \end{gathered}$$

Arrange the following pairs in a sequence of decreasing order of acidity of base and increasing order of basicity of an acid and the salts formed between an acid and base with equal basicity and acidity in their increasing order, respectively. 

(l) Sodium chloride 

(2) Barium hydroxide and sulphuric acid 

(3) Ferric phosphate 

(4) Ferric hydroxide and hydrochloric acid 

(5) Barium sulphate 

(6) Sodium hydroxide and phosphoric acid 

Odd one among the following with respect to the strength of acids is: 

One molecule of an 'ic' acid of a non-metal having $5$ electrons in a valence shell reacts with a molecule of a base to form a salt $\mathrm{X}$. The base corresponds to the metal with one electron in the valence shell. If the salt so formed can react with the same base in $1:2$ ratio, predict the formula of the salt $\mathrm{X}$.