a student separated an unknown mixture of ions using the procedure of part a. when the spots were made…

a student separated an unknown mixture of ions using the procedure of part a. when the spots were made visible, a faint spot corresponding to cu²⁺ was seen. however, the student later learned that the unknown did not contain cu²⁺. which of the following could cause this result? a. the same spotting capillary was used to spot the cu²⁺ sample and the unknown sample. b. the cu²⁺ sample was contaminated with co²⁺ and fe³⁺. c. the chromatography paper was not trimmed to the correct dimensions. d. the position of the mobile - phase front was incorrectly marked. explain your answer.

a student separated an unknown mixture of ions using the procedure of part a. when the spots were made visible, a faint spot corresponding to cu²⁺ was seen. however, the student later learned that the unknown did not contain cu²⁺. which of the following could cause this result? a. the same spotting capillary was used to spot the cu²⁺ sample and the unknown sample. b. the cu²⁺ sample was contaminated with co²⁺ and fe³⁺. c. the chromatography paper was not trimmed to the correct dimensions. d. the position of the mobile - phase front was incorrectly marked. explain your answer.

Answer

Explanation:

Step1: Analyze chromatography process

In chromatography, correct spotting and sample - handling are crucial. If the sample was contaminated with other ions like $\text{Co}^{2 +}$ and $\text{Fe}^{3+}$, it could affect the results. But the lack of a $\text{Cu}^{2+}$ spot in the unknown when it was expected indicates an issue with the spotting or sample application.

Step2: Evaluate each option

  • Option a: Incorrect marking of the mobile - phase front would not directly cause the absence of a $\text{Cu}^{2+}$ spot in the unknown if the $\text{Cu}^{2+}$ was present in the sample.
  • Option b: If the capillary was used to spot the $\text{Cu}^{2+}$ sample and then contaminated with $\text{Co}^{2+}$ and $\text{Fe}^{3+}$, it could lead to inaccurate results for the unknown. But the main issue here is the non - appearance of the $\text{Cu}^{2+}$ spot in the unknown. If the capillary was contaminated during spotting of the $\text{Cu}^{2+}$ standard, it might affect the standard's spot but not necessarily the absence of $\text{Cu}^{2+}$ in the unknown.
  • Option c: If the chromatography paper was not trimmed to the correct dimensions, it would not directly cause the absence of a $\text{Cu}^{2+}$ spot in the unknown. It might affect the overall chromatography pattern but not specifically the presence or absence of a particular ion spot.
  • Option d: If the capillary was used to spot the $\text{Cu}^{2+}$ sample and then not properly cleaned before spotting the unknown, it could introduce contaminants that interfere with the detection of $\text{Cu}^{2+}$ in the unknown or could have caused the $\text{Cu}^{2+}$ in the unknown to be masked or not properly spotted. This is the most likely cause for the absence of the $\text{Cu}^{2+}$ spot in the unknown.

Answer:

d. The capillary was used to spot the $\text{Cu}^{2+}$ sample and then not properly cleaned before spotting the unknown.