At certain temperature, two moles of A combines with five moles of B to produce two moles of C .If atomicity of A and B is 2, the formula of compound C is:

Ratio of volumes $=A:B:C=2:5:2$

Ratio of number of molecules in A, B and C $=2:5:2$ or $1:\frac{5}{2}:1$

1 molecule of C contains 1 molecule (i.e., 2 atom) of

A and $\frac{5}{2}\text{ molecules (i.e., }\frac{5}{2}\times 2\text{ atoms) }$ of B

Hence, the formula of B is $A_2{B}_5$.