The molal freezing point depression constant for benzene ( C ₆ H ₆ ) is 4.90 ~K ~kg mol ⁻¹ . Selenium exists as a polymer of the type Se _x . When 3.26 g of selenium is dissolved in 226 g of benzene, the observed freezing point is 0.112^ C lower than that of pure benzene. The molecular formula of selenium is (atomic mass of Se =78.8 ~g ~mol ⁻¹ )

Step-by-step solution

To calculate the molar mass of selenium (Se) M_B= K_f W_B T_f W_A Mas of selenium (W_B )=3.26 ~g Mass of benzene (W_A )=226 ~g =0.226 ~kg Depression in freezing point ( T_f )=0.112^ C Molar depression constant (K_f )=4.9 ~K ~kg ~mol ⁻¹ M_B= (4.9 ~K ~kg ~mol ⁻¹ )(3.26 ~g ) (0.112 ~K )(0.226 ~kg ) =631.08 ~g ~mol ⁻¹ Given, gram atomic mass of selenium =78.8 ~g / mol For Se _x (78.8 ~g ~mol ⁻¹ )=631.08 ~g ~mol ⁻¹ or, x= 631.08 ~g ~mol ⁻¹ 78.8 ~g ~mol ⁻¹ =8 Thus, molecular formula of selenium = Se ₈