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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Kaleyrvt on January 16, 2007, 12:21:53 AM

Title: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 12:21:53 AM

The solubility product of calcium hydroxide, Ca(OH)2, is 7.9 x 10-6 at 25ºC. Will a precipitate form when 100mL of 0.10 mol/L of CaCl2 solution and 50.0 mL of 0.70 mol/L of NaOH solution are combined? Show your work.

This is my work, but I dont know if I am even close-can someone check it to ensure I did it right?

[CaCl2] = 0.10 mol/L x 100 ml(0.1 L)   =  0.066 mol/L
                               150 mL(0.15 L)

[NaOH] = 0.70 mol/L x 50 ml(0.05 L)    =  0.233 mol/L
       150 mL(0.15 L)

Ksp = [CaCl2] [NaOH] = 0.015 => 1.5 x 10 -2

Since Ca(OH)2 = 7.9 x 10-6   which is < 1.5 x 10 -2 then no precipitate will form.

Thanks all ! hope to hear form you soon!

Title: Re: will a precipitate form?
Post by: AWK on January 16, 2007, 02:32:49 AM

Ksp = [Ca²⁺] [OH^-]² = ?

A final answer is correct, but your calculations are wrong

syntax corrected

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 03:58:24 AM

Ksp = [Ca²⁺][OH^-]²

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 11:01:12 AM

so then where did I go wrong??All of it??? How do I find values for [Ca2+][OH-]2 ???
Sorry, just don't quite understand where to fix it...:(

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 12:17:21 PM

You mix dissociated salt with dissociated base - you have to calculate concentrations of ions and put them into Ksp formula.

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 01:19:52 PM

I am sorry i am completely lost--god I feel like an idiot..:(

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 01:25:23 PM

You have 0.233M solution of NaOH - what ions and in what concentrations does it contain?

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 01:56:31 PM

The NaOH contains [Na+] and [OH-] ions...how do I figure out the concentrations?
I really am trying----I am even looking through my notes :(

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 02:07:43 PM

If there is 1 mole of NaOH dissolved - how many moles of Na⁺ appeared in the solution?

Note: [Na⁺] usually means concentration of Na⁺, you don't need square brackets around ions when you refer to them.

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 02:17:14 PM

0.5 moles of Na+?

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 02:24:04 PM

How many Na atoms per NaOH molecule?

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 02:28:40 PM

1 Na atom in NaOH

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 03:36:19 PM

So how 1 mole of NaOH could produce 0.5 mole of Na⁺?

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 04:44:35 PM

divide 0.233M by 2= # of moles of Na+???

Title: Re: will a precipitate form?
Post by: Dan on January 16, 2007, 05:25:37 PM

Think about this:

If every apple has one stalk, how many stalks do a dozen apples have?

Similarly if one NaOH unit has one Na, how many Na⁺ ions will 1 mol of NaOH units form?

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 05:36:16 PM

1??? Man either I am just completely losing it or I am thinking way too hard!! It shouyldnt be this difficult!! BLAH! I am so sorry!

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 05:49:30 PM

1 - you got it. Now, what the concentration of OH^- in 0.233M NaOH is?

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 06:19:59 PM

0.233 as well?

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 07:01:23 PM

Exactly. Now, what about [Ca²⁺]?

And just to check if you really understand what is going on - what will be concentration of Cl^-?

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 07:17:05 PM

well the ration of CaCl2 is Ca:Cl2 = 1:2
Therefore Ca=0.066 and Cl= 2 x 0.066=0.132      ????
I hope this is right! ???

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 07:20:27 PM

Quote from: Kaleyrvt on January 16, 2007, 07:17:05 PM

Therefore Ca=0.066 and Cl= 2 x 0.066=0.132

Good :)

Now plug concentrations into Ksp formula - if the result will be greater than the Ksp given for the particular compound, solid will precipitate.

Title: Re: will a precipitate form?
Post by: Kaleyrvt on January 16, 2007, 07:28:22 PM

so....
Ksp= [Ca2+][OH-]2
=[0.066][0.233]2
=3.58 x 10^-3

are these calcs right???
If theya re, then Ca(OH)2, is 7.9 x 10-6  which is < Ksp= [Ca2+][OH-]2, which is 3.58 x 10^-3 thus a precipitate will form????

cross my fingers?

Title: Re: will a precipitate form?
Post by: Borek on January 16, 2007, 08:14:10 PM

Don't call result of your calculations Ksp - Ksp is constant that is characteristic for every substance. You do calculate a value of Ksp formula using  known concentrations - and then you compare it with known Ksp. In this case calculated value is 3.58*10^-3 (you are OK), it is larger then Ksp for Ca(OH)₂ - so the precipitate forms.

In short: you are right this time :)