Question

A student has 500.0 mL of a 0.1133 M aqueous solution of BaI2 to use in an experiment. She accidentally leaves the container uncovered and comes back the next week to find only a solid residue. The mass of the residue is 28.28 g. Determine the chemical formula of this residue.

Answer 1

Answer:

$BaI_2\ ^.6H_2O$

Explanation:

Hello,

In this case, by knowing the volume and the molarity of the barium iodide, is it possible to compute the residue's mass as shown below:

$m_{BaI_2}=0.500L*0.1133\frac{molBaI_2}{L}*\frac{391.136gBaI_2}{1molBaI_2} =22.16gBaI_2$

Nevertheless, the obtained value is lower than the obtained by 6.133 g which means that mass corresponds to water forming a hydrate. In such a way, one could know how many waters in form of hydrate remain with the residue by a trial-error procedure as shown below:

$m=28.28g=0.500L*0.1133\frac{molBaI_2}{L}*\frac{(391.136+18)gBaI_2\ ^.H_2O}{1molBaI_2} =23.17g\rightarrow No\\m=28.28g=0.500L*0.1133\frac{molBaI_2}{L}*\frac{(391.136+2*18)gBaI_2\ ^.2H_2O}{1molBaI_2} =24.20g\rightarrow No\\m=28.28g=0.500L*0.1133\frac{molBaI_2}{L}*\frac{(391.136+3*18)gBaI_2\ ^.3H_2O}{1molBaI_2} =25.22g\rightarrow No\\m=28.28g=0.500L*0.1133\frac{molBaI_2}{L}*\frac{(391.136+4*18)gBaI_2\ ^.4H_2O}{1molBaI_2} =26.24g\rightarrow No$

$m=28.28g=0.500L*0.1133\frac{molBaI_2}{L}*\frac{(391.136+5*18)gBaI_2\ ^.5H_2O}{1molBaI_2} =27.26g\rightarrow No\\m=28.28g=0.500L*0.1133\frac{molBaI_2}{L}*\frac{(391.136+6*18)gBaI_2\ ^.6H_2O}{1molBaI_2} =28.28g\rightarrow Yes$

Therefore, the formula is barium iodide hexahydrate:

$BaI_2\ ^.6H_2O$

Best regards.