Equilibrium concentrartion

Question

For Part A of the Determination of an Equilibrium Constant experiment, you will be preparing several standard solutions for the Beer's Law plot. Using the information given in Table A of the lab worksheet (also below) answer the following questions.

(a) What is the initial Fe3+ concentration of all of the solutions, if each solution is composed of 1.00 mL of 0.100 M Fe(NO3)3 diluted to a total volume of 6.00 mL?

M

(b) What is the initial SCN– concentrations of Solutions 1A – 5A? Each solution is composed of a given volume of 5.00x10-4 M NaSCN diluted to a total volume of 6.00 mL.

Solution 1A of 1.00 mL NaSCN solution diluted to 6.00 mL M

Solution 2A of 1.50 mL NaSCN solution diluted to 6.00 mL M

Solution 3A of 2.00 mL NaSCN solution diluted to 6.00 mL M

Solution 4A of 2.50 mL NaSCN solution diluted to 6.00 mL M

Solution 5A of 3.00 mL NaSCN solution diluted to 6.00 mL M

(c) What is the final concentration of FeSCN2+ in Solution 1A?

M

Please just give a brief overveiw of set up/ how to/why, I can solve just dont know where to start.

Answer 1

Hi Catherine! At your request, I will not be providing final solutions in this answer. Please contact me if you have more questions after reading.

For question a), you should use the relationship M₁V₁=M₂V₂. M in this case is molarity, and V is volume. If your initial solution is 0.1 M Fe(NO₃)₃, with a volume of 1.00 mL, and you dilute it to 6.00 mL, then you have the variables M_1, V₁, and V₂. Use those to solve for M₂. Be sure to use the same units on both sides! This equation works because the units of M are moles/liter. With this equation, you are certain that if you change either molarity or volume, the other must change as well.

For each solution in b), you should use the same relationship (M₁V₁=M₂V_2). The initial volume and final volume are the same for each solution. The changing variable is the molarity of the solution.

Question c) is worded somewhat oddly. You mentioned Beer's Law in your question, so I assume you have done or will do spectrophotometry on your samples. This would give you variables to plug into Beer's Law (A=c, where A is absorbance of incident light; is the molar absorptivity coefficient of the species, which would be given; is the path length, or how long the light travels--this would also be given; and c is the concentration of the species, which is the answer you want to solve for). Without that, I believe you could solve for the concentration of both Fe³⁺ and SCN^- in the solution 1A using M₁V₁=M₂V₂ and take the lower number (as the limiting factor in formation of FeSCN²⁺ is the amount of the species present in lesser quantities).

I hope this helps!