A student mixes \(5.0 \mathrm{mL} 2.00 \times 10^{-3} \mathrm{M}
\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}\) with \(5.0 \mathrm{mL} 2.00 \times
10^{-3} \mathrm{M} \mathrm{KSCN}\). She finds that in the equilibrium mixture
the concentration of \(\mathrm{FeSCN}^{2+}\) is \(1.2 \times 10^{-4} \mathrm{M}\).
Find \(K_{\mathrm{c}}\) for the reaction \(\mathrm{Fe}^{3+}(\mathrm{aq})+\)
SCN-(aq) \(=\) FeSCN \(^{2+}\) (aq).
Step 1 Find the number of moles \(\mathrm{Fe}^{3+}\) and \(\mathrm{SCN}^{-}\)
initially present. (Use Eq. 3.)
Step 2 How many moles of FeSCN \(^{2+}\) are in the mixture at equilibrium? What
is the volume of the equilibrium mixture? (Use Eq. 3.) How many moles of
\(\mathrm{Fe}^{3+}\) and \(\mathrm{SCN}^{-}\) are used up in making the
\(\mathrm{FeSCN}^{2+} ?\)
Step 3 How many moles of \(\mathrm{Fe}^{3+}\) and \(\mathrm{SCN}^{-}\) remain in
the solution at equilibrium? (Usc Eq. 4 and the results of Steps 1 and 2 .)
Step 4 What are the concentrations of \(\mathrm{Fe}^{3+}, \mathrm{SCN}^{-}\),
and \(\mathrm{FeSCN}^{2+}\) at equilibrium? What is the volume of the
equilibrium mixture? (Use Eq. 3 and the results of Step 3.)
Step 5 What is the value of \(K_{c}\) for the reaction? (Use Eq. 2 and the
results of Step 4.)
