Homework Help: Chemistry- please check my work

Posted by eliz on Wednesday, February 25, 2009 at 9:54pm.

Question: The following equilibrium was studied by analyzing the equilibrium mixture for the amount of HCL produced. LaCl3(s)+ H2O(g) goes to LaOCl(s)+2HCl(g). A vessel whose volume was 1.25L was filled with 0.0125 mol of lanthanum (III)chloride and 0.0250 mol H2O. After the mixture came to equilibrium in the closed vessel at 619 degree C the gaseous mixture was removed and dissolved in more water sufficent silver (I)ion was added to precipitate the chloride ion completely as silver chloride. If 3.59g AgCl was obtained, what is the value of Kc at 619 degree C?

I worked out the problem could you please show me how you would work it so i can check my answer

Chemistry- please check my work - DrBob222, Wednesday, February 25, 2009 at 10:04pm
We would much prefer you to show your work and let us check your answer. We can help better that way.
Chemistry- please check my work - eliz, Wednesday, February 25, 2009 at 10:19pm
I first changed the 3.59g of AgCl into moles and than divided by liters given to get .0200M of Agcl. I also changed the mol of H20 and LaCl3 to Molarity as well. I got .01M for LaCl3 and.02 for H20. Than I plugged into equation Kc=(.0200)/(.010)(.02). To give me the Kc value as 200
Chemistry- please check my work - DrBob222, Wednesday, February 25, 2009 at 10:36pm
I wouldn't do it that way. First, the expression for Kc does not involve AgCl, LaCl3, or LaOCl because all three are solids and a concn of a solid is 1 by definition. (You can't get the concn to increase by adding another speck of a solid to a solid).
LaCl3(s) + H2O(g) ==> LaOCl(s) + 2HCl(g)

I would convert grams AgCl to moles as you did, which is 3.59/143 = approximately 0.025 which is moles HCl. Then moles H2O will be 1/2 that or 0.0125. Since you started with 0.0250, there will be 0.250-0.0125 = 0.0125 moles H2O at equilibrium.
Then (H2O) = 0.0125 moles/1.25L = ?? M
(HCl) = 0.0250/1.25 = ?? M.

Kc = (HCl)^2/(H2O) = ??
Note that the solids don't show up in Keq. You need to go through and change the numbers a little since I used approximations for the molar mass AgCl. I obtained 0.04 for Kc. Check my work.
Chemistry- please check my work - Anonymous, Wednesday, February 25, 2009 at 10:58pm
thanks! Once you mentioned about solids i totally remembered they don't play apart when your solving somthing.
Chemistry- please check my work - DrBob222, Wednesday, February 25, 2009 at 11:08pm
That's why we like to see your work. Seeing your work made me put those remarks in about the solids.

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