Question

2. Would the following procedural changes cause the experimentally determined mass percent of NH₃ in Ni(NH₃)_nCl₂ to be too high, too low, or unchanged. Briefly explain each answer.

(a). After dissolving a known mass of Ni(NH₃)_nCl_2, a student directly titrated the NH₃ with HCl solution, using a mixed bromocresol green – methyl red indicator.

(b). A student added excess standard HCl solution to a known mass of dissolved Ni(NH₃)_nCl₂ and back titrated the excess HCl with standard NaOH solution, using phenolphthalein indicator solution.

3. Would the following procedural changes cause the experimentally determined mass percent Ni²⁺ ion in NH₃ in Ni(NH₃)_nCl₂ to be too high, too low, or unchanged. Briefly explain each answer.

(a). In Part III of the procedure, a student omitted adding concentrated NH₃ solution to the dissolved Ni(NH₃)_nCl₂ sample before analysis using the spectrophotometer, thinking that the additional NH₃ was unnecessary because the Ni²⁺ ion was already complexed [Ni(NH₃)_n]²⁺ ion.

(b). The student measured the %T of the standard and unknown solutions at a wavelength that was 20 nm lower than the actual analytical wavelength.

Answer 1

2. In the titration of NH3 present in the complex with HCl

(a). If after dissolving a known mass of Ni(NH₃)_nCl_2, a student directly titrated the NH₃ with HCl solution, using a mixed bromocresol green – methyl red indicator, the mass percentage of NH3 is complex estimated would be too high, as the end point would not be accurately measured in this case.

(b). When a student added excess standard HCl solution to a known mass of dissolved Ni(NH₃)_nCl₂ and back titrated the excess HCl with standard NaOH solution, using phenolphthalein indicator solution, the mass percentage of NH3 measured will be unchanged and will give accurate result from adding the amount of HCl used for NH4+ formation and NaOH titration.

3. Pprocedural changes and effect on the experimentally determined mass percent Ni²⁺ ion in NH₃ in Ni(NH₃)_nCl₂

(a). In Part III of the procedure, a student omitted adding concentrated NH₃ solution to the dissolved Ni(NH₃)_nCl₂ sample before analysis using the spectrophotometer, thinking that the additional NH₃ was unnecessary because the Ni²⁺ ion was already complexed [Ni(NH₃)_n]²⁺ ion, the mass percentage of Ni2+ measured would be low as some of the NH3 will escape from the complex before the titration begun and lowers the final concentration.

(b). When the student measured the %T of the standard and unknown solutions at a wavelength that was 20 nm lower than the actual analytical wavelength, the mass percentage of Ni2+ measured would be unchanged.