precipitate?

Question

i'm not sure why a certain part is included in the solution...

20mL of 1.0 x 10^-4 M Ba(OH)2
mixed with
10mL of 0.0025 M Fe(NO3)2
will a precipitate result?

Here's the solution.

[OH] -> (.02 L / .03 L) x (1.0x10^-4 M Ba(OH)2) x 2

[Fe] -> (.01 L / .03 L) x (0.0025 M Fe(NO3)2

When finding [OH]..am I multiplying by 2 because of the 2 OH?

The Ksp for Fe(OH)2 is 5.0 x 10^-17.

Usually when doing these kinds of problems..I just multiply the concentrations and see if they are larger or smaller than the Ksp. I don't understand why you would multiply by 2

Answer 1

Yes, you are multiplying by 2 because of the 2 OH. For every mole of Ba(OH)2, you will have 2 moles of OH. Think of it as:

(OH)---Ba---(OH)

If you split that up, you will get 2 OH's for every one molecule that you started with.

Answer 2

Ba(OH)2 is an ionic solid. When 1 mole of Ba(OH)2 dissociates in water, you get this:

Ba(OH)2 ==> Ba2+ + 2 OH-

There are two moles of hydroxide ions produced for every one mole of Ba(OH)2.

For example, if you had a solution that was 1.0 M Ba(OH)2, the concentration of hydroxide ions would be 2.0 M.

Answer 3

Yup! you got here Ba(OH)2, so each mole of it gives two OH- when fully dissociated.


If you use NaOH, each mole of this compound gives only one OH- and you wouldn´t have to multiply by 2

If you use NH4OH.....well I think you got it. Dont you?

This is an example of what i mean when I say they (teachers) get into the complicated things before students understand the basics... Whats the use for that? =)