need help!! physical chem

• jaydunkfull

  2 Oct 09, 13:02

  hello, need help solving a few chem qns. thank you very much! 1)A 0.1246g sample of a compound of Cr and Cl was dissolved in water. All of the chloride ion was then captured by silver ion in the formof AgCl. A mass of 0.3383g of Ag Cl was obtained. Determine the empirical formula of the compound of Cr and Cl. 2)Consider te eledctrochemical cell operating at 25 degree celsius. Cu(s) |Cu2+(0.1M)||Ag2CrO4(saturated aq solution)|Ag(s) if the cell is allowed to operate spontaneously, will Ecell increase, decrease or remain constant? explain. Ksp of Ag2CrO4=1.1x10^-12 3)A handbook lists the following formula for the percent ionization of a weak acid, HA: %ionised=100/(1+10^(pka-pH)) Derive this equatioon. State the assumption made.
• UltimaOnline

  2 Oct 09, 14:27

  I'll only comment on Qns 1 & 2, since only these two are relevant to 'A' levels (and my primary interest here on the forum is to help those doing 'A' levels and 'O' levels). The 3rd qn I'll leave it to dckx, who's doing undergrad Chem like jaydunkfull. For Qn 1, since the valency of chlorine when ionically bonded with metals is already known, let the cationic charge of Cr be x, ie. empirical formula be CrClx. Calculate moles of CrClx in terms of x. Calculate moles of Cl- present (precipitated as AgCl). Form an equation in x, solve for x. For Qn 2, use Le Chatelier's principle to consider the change in oxidation potential at anode, and for reduction potential at cathode, as time progresses. The question is ambiguous in that while it gave the Ksp value, it did not state whether additional solid Ag2CrO4 was present in the electrolyte. Can you see how that would make a difference, at least as far as the reduction half-cell was concerned?
• dkcx

  2 Oct 09, 22:49

  Originally posted by UltimaOnline:

  I’ll only comment on Qns 1 & 2, since only these two are relevant to ‘A’ levels (and my primary interest here on the forum is to help those doing ‘A’ levels and ‘O’ levels).

  The 3rd qn I’ll leave it to dckx, who’s doing undergrad Chem like jaydunkfull.

  For Qn 1, since the valency of chlorine when ionically bonded with metals is already known, let the cationic charge of Cr be x, ie. empirical formula be CrClx. Calculate moles of CrClx in terms of x. Calculate moles of Cl- present (precipitated as AgCl). Form an equation in x, solve for x.

  For Qn 2, use Le Chatelier’s principle to consider the change in oxidation potential at anode, and for reduction potential at cathode, as time progresses. The question is ambiguous in that while it gave the Ksp value, it did not state whether additional solid Ag2CrO4 was present in the electrolyte. Can you see how that would make a difference, at least as far as the reduction half-cell was concerned?

  Oi i hate physical chem leh still ask me do physical chem qns. I don't remember this equation and at the moment i'm too busy trying to figure out quantum to check my notes etc.

  UltimaOnline u the mod leh n i'm the 1 who is supposed to leave you questions to answer not you leave for me :p

• jaydunkfull

  4 Oct 09, 17:14

  okok ): *slouches away helplessly*