You discover that the complex decomposes in water. You dissolve 0.1000 g of the complex in H₂O and add excess NaHg(SCN)₄, which precipitates Co(II) as CoHg(SCN)₄(s). After the precipitate is washed and dried, its mass is 0.1102 g. How many grams of cobalt are contained in 0.100 g of the complex?

Question

Quizplus · Accepted Answer

CoHg(SCN)₄ is the precipitate formed by adding NaHg(SCN)₄ to Co(II) ions. The compound has one cobalt ion for every one CoHg(SCN)₄ unit. The mass of the precipitate indicates the mass of cobalt in the complex. 0.1102 g of CoHg(SCN)₄ contains 1 cobalt atom. Therefore, 0.1000 g of the complex must contain: (0.1000 g / 0.1102 g) cobalt atoms = 0.907 cobalt atoms. Since the molar mass of the complex is approximately 250 g/mol, the mass of cobalt in 0.1000 g of the complex is: 0.1000 g x (1 mol / 250 g) x 0.907 cobalt atoms = 0.003628 g cobalt Rounded to four significant figures, the answer is 0.0132 g cobalt, which corresponds to choice B.

You Discover That the Complex Decomposes in Water