A standard solution of FeSCN2+ is prepared by combining 9.00 mL of 0.200 M Fe(NO3)3 w/1.00 mL of 0.0020 M KSCN. The equilibrium concentration of FeSCN2+ ([FeSCN2+]std) for this standard solution is assumed to be ____ M.
I tried this problem by adding the moles of Fe(NO3)3 and KSCN and dividing that by 0.01 L. I got 0.1802 but it's wrong.
Write the equation to see what is going on.
Fe(NO3)3 + KSCN ==> FeSCN^+2 + 3NO3^-
mols Fe(NO3)3 = M x L = ??
mols KSCN = M x L = ??
Use the equation to make an educated guess about the FeSCN^+2. Without doing any of the math it is obvious that the KSCN is there is MUCH lesser quantity (said another way the KSCN is the limiting reagent); therefore, mols FeSCN^+2 = mols KSCN. then (FeSCN^+2) = mols FeSCN^+2/liters. Note that the final volume is 10 mL = 0.010 Liters and mols/L = M.
Thanks!!!
actually when you calculate the equilibrium concentrations of each using the 0.010 L you find that the concentration of KSCN is the limiting reagent. so [FeSCN]^2 = mols KSCN/0.010L
To find the equilibrium concentration of FeSCN2+ ([FeSCN2+]std) for this standard solution, we need to use the concept of stoichiometry and the balanced chemical equation that relates the reactants and products.
The balanced chemical equation for the reaction between Fe(NO3)3 and KSCN to form FeSCN2+ is:
Fe(NO3)3 + 3KSCN → Fe(SCN)3 + 3KNO3
From the balanced equation, we can see that one mole of Fe(NO3)3 reacts with three moles of KSCN to produce one mole of Fe(SCN)3 (which dissociates into Fe3+ and SCN- ions in solution) and three moles of KNO3. The Fe(SCN)3 then reacts further to form FeSCN2+.
Now let's calculate the moles of Fe(NO3)3 and KSCN used in this standard solution:
Moles of Fe(NO3)3:
Given volume of Fe(NO3)3 = 9.00 mL = 0.00900 L
Given concentration of Fe(NO3)3 = 0.200 M
Moles of Fe(NO3)3 = concentration × volume = 0.200 M × 0.00900 L = 0.0018 moles
Moles of KSCN:
Given volume of KSCN = 1.00 mL = 0.00100 L
Given concentration of KSCN = 0.0020 M
Moles of KSCN = concentration × volume = 0.0020 M × 0.00100 L = 0.000002 moles
Now, we need to consider the limiting reactant. Since we have more moles of Fe(NO3)3 than KSCN, KSCN is the limiting reactant. This means that all the KSCN will react, and only as much Fe(NO3)3 as necessary will be used.
We can now use the stoichiometry of the reaction to determine the moles of FeSCN2+ formed:
From the balanced equation, we know that for every one mole of KSCN, one mole of FeSCN2+ is formed.
Therefore, the moles of FeSCN2+ formed = moles of KSCN used = 0.000002 moles
Finally, we can calculate the concentration of FeSCN2+ ([FeSCN2+]std) in the standard solution:
Volume of standard solution = volume of Fe(NO3)3 + volume of KSCN = 9.00 mL + 1.00 mL = 0.01 L
[FeSCN2+]std = moles of FeSCN2+ formed / Volume of standard solution
= 0.000002 moles / 0.01 L
= 0.0002 M
Therefore, the equilibrium concentration of FeSCN2+ ([FeSCN2+]std) for this standard solution is 0.0002 M.