State the number of molecules of water that is present in gypsum.

Among the given acids, how many acids are strong acids?

${\mathrm{H}}_2{\mathrm{SO}}_4$, ${\mathrm{H}}_2{\mathrm{CrO}}_4$, $\mathrm{HCN}$, $\mathrm{HCl}$, Phenol, ${\mathrm{HNO}}_3$

Daivik, a class 10 student studied the reaction between a carbonate and an acid in the lab. His results are shown in the given graph:
    
Which of the following experimental conditions did he use?

1.     $\mathrm{Al}(\mathrm{OH}{)}_3$
2.     $\mathrm{CuO}$
3.     ${\mathrm{K}}_2\mathrm{O}$
4.     $\mathrm{NaOH}$