Question

--  A 1.0000 g sample of zinc metal is added to a solution containing 1.2500 g of an unknown compound of bismuth and chlorine. The reaction results in the formation of zinc chloride and metallic bismuth. When the reaction is complete, unreacted zinc remains, and this unreacted zinc is consumed by reaction with HCl. After washing and drying, the mass of bismuth metal recovered is 0.6763g.

Q. Write a balanced chemical equation for the reaction of zinc with the original unknown compound

Assuming that your answer to the question is correct, what is the minimum mass of zinc metal needed to react with the original bismuth chloride compound?

Answer 1

Ans. Given,

            Mass of unknown (Bismuth - Cl) compound = 1.2500 g

            Mass of Bismuth metal in unknown sample = 0.6763 g

Now,

            Mass of Cl in unknown sample = Mass of unknown sample – Mass of Bi metal

                                                            = 1.2500 g – 0.6763 g

                                                            = 0.5737 g

            Moles of Cl in sample = Mass of Cl / Atomic mass

                                                = 0.5735 g / (35.4527 g/ mol)

                                                = 0.0161821 mol

            Moles of Bi in sample = 0.6763 g / (208.98037 g/ mol) = 0.0032362 mol

Now,

            Molar ratio of Cl : Bi in sample = Moles of Cl / Moles of Bi

                                                = 0.0161821 mol / 0.0032362 mol

                                                = 5.000 : 1

Therefore, there are 5 Cl atoms per Bi atom.

Thus, stoichiometric chemical formula of unknown compound = BiCl₅

# Balanced reaction:

            2 BiCl₅(aq) + 5 Zn -----> 5 ZnCl₂ + 2 Bi(s)

# Stoichiometry of reaction: 5 mol Zn precipitates 2 mol Bi

So,

            Minimum moles of Zn required = (5 / 2) x Moles of Bi precipitated

                                                            = (5 / 2) x 0.0032362 mol

                                                            = 0.008090473 mol

And,

            Minimum mass of Zn required = Minimum moles of Zn required x Molar mass

                                                            = 0.008090473 mol x (65.39 g/ mol)

                                                            = 0.52904 g

Therefore, minimum mass of Zn required = 0.5290 g