Predict whether a precipitate will form upon mixing 75.0 mL of NaOH solution with pOH of 2.58 with 125.0 mL of a 0.018 M MgCl2 solution.

Question

I know how to tell whether a precipitate will form, and there is one for this specific reaction. Yet, I am having trouble calculating the Q and Ksp value in order to compare them to each other. I think I keep making mistakes in my math as my professor has given us the answer key and Q= 1.1x10^-8 and Ksp= 8.9x10^-12, but instead I have calculated Ksp= 4.68x10^-5.

Answer 1

I agree with answer provided by @Jacques D. You shouldn't calculate Ksp. It should be provided, or look it up. Then simply calculate Q = [Mg²⁺][OH^-]² and compare it to the Ksp to determine if a ppt will form.

Answer 2

The question is how and why you are calculating K_SP = 5.6 x 10^-12 .(I see values ranging from 1.1 x 10 ^-13 to 1.3 x 10^-11 ), but this value is prevalent and you can obtain it using gibbs energies of formation at 25 degrees. If that was how you calculated K_SP.- I get 2(-157.24) +(-454.8) - (-833.7) kJ/mole

K = e^ΔG0/RT (remember to multiply ΔG by a 1000 and use R = 8.314, T = 298.15)

Please consider a tutor. Take care.