[daijia]

Hi everyone,

I have encountered such question on my exercice,

How many moles of Na2SO4 we have to add into a saturated 0.5 L solution of Ag2SO4 so that the concentration of Ag is 4.0 * 10^-3 mol/L?

So what I did is following:

• According to the source I am usingz the Ksp for Ag2SO4 is 1.4×10^-5. So Ksp = (Ag)^2*(SO4), 4s^3 = 1.4×10^-5, s= 0.015.
  As a result, we have [SO4]=0.015 mol/L in a normal saturated solution of Ag2SO4
• Then, I found what the concentration of SO4 suppose to be if [Ag] is 4.0 * 10 ^-3 mol/L in a saturated Ag2SO4 solution.
  1.4*10^-5 = (4.0*10^-3)^2 × (SO4), SO4 = 0.875.  So we need 0.875 mol/L of SO4 in order to have a saturated solution with 4.0*10^-3 of Ag
• SO4 needed: 0.875 - 0.015 = 0.86 mol/L. So we need to add 0.86 mol/L of SO4
• nSO4 = 0.86 mol/L × 0.5 L = 0.43 mol
  0.43 mol of SO4 is the same as the mole of Na2SO4.
  So we need to add 0.43 mol of Na2SO4
  

So the full answer is: We need to add 0.43 mol of Na2SO4 into a saturated solution of Ag2SO4 in order to bring down the concentration do Ag to 4.0*10^-3 mol/L.

So I would like to ask if my step is wrong? If yes, what are the right steps!

Thanks,
Jack

[AWK]

Students make a lot of calculation errors when calculating chemical equilibria - it's hard to guess from the description that you are doing everything right.

[daijia]

Students make a lot of calculation errors when calculating chemical equilibria - it's hard to guess from the description that you are doing everything right.

Sure yes I understand. Suppose I don't make calculation error, are my steps logic (is my method appropriate to solve this question)? Or I need to take a different approach to solve this question?

At mean time, I gonna modify my post to include my calculations, thanks for reminding me that!

Thanks!

[AWK]

This is a typical manual description, but I don't know if you understand it properly.

[daijia]

This is a typical manual description, but I don't know if you understand it properly.

Hi,

I have just updated my post with specific calculations. Would you mind taking a look?

Thanks!

[AWK]

OK (In the meantime you corrected a calculation error)

[daijia]

OK (In the meantime you corrected a calculation error)

Yes that was more of a typo😅Because I was typing on the phone, and I feel the answer doesn't seem the same as the one I got yesterday. That is when I realized I typed an extra zero

But thanks for helping me!

[AWK]

These calculations are based solely on the common ion effect. In fact, the effect of ionic strength is opposite and much greater than the effect of a common ion and the addition of sodium sulfate will increase the solubility of silver sulfate. Therefore the assumed reduced concentration of silver cation will probably never be reached.

[daijia]

Oh, okay. Thank you for your explanation!

But considering the fact that this is a high school question and the purpose is merely to practice students' knowledge on solubility, are my calculations justified in this case?

Thanks a lot!

[AWK]

In the last post, I noticed that the example is terribly chosen. You can find many chemical compounds for which this type of calculation can be made with an error of 10-30%. In this case, the error is several hundred %.

I wrote earlier: OK