Homework Help: Chemistry

Posted by Samantha on Sunday, April 15, 2012 at 6:47pm.

I need help with this in order to correctly complete my other related problems.
Thank you so much in advance =)

Arrange the following 0.1 M solutions in order of increasing pH and state why you placed each solution in that position: NaCH3COO, HCl, HCN, NaOH, NH3, NaCN, KNO3, H2SO4, NH4Cl, H2SO3, NaHCO3, Na3PO4 and CH3COOH.

In order of increasing pH:

Chemistry - DrBob222, Sunday, April 15, 2012 at 7:15pm
I'm not about to do your homework for you but here is HOW you do it.
All of the materials are salts, bases, or acids.

The long way, but sure way, is to calculate the pH of the solns.
For strong acids such as HCl, the ionization is 100%, and pH = -log(HCl)

For strong bases, such as NaOH, pOH = -log(OH^-) and use pH + pOH = pKw = 14 to convert to pH.

For weak acids are bases, go through an ICE chart; for example, HAc for acetic acid. ........HAc ==> H^+ + Ac^-
initial.......0.1.....0......0
change.......-x........x......x
equil......0.1-x.......x......x
Substitute into the Ka expression, solve for x and convert to pH.

For weak bases, such as NH3, use tahe same process but x = OH^-; convert to pOH then to H.

For salts, the pH is determined by the hydrolysis of the salt. For sodium acetate or NaAc (a basic salt), this is
..............Ac^- + HOH ==> HAc + OH^-
initial.......0.1............0......0
change........-x.............x......x
equil......0.1-x.............x......x
Kb for Ac^- = (Kw/Ka for HAc) = (HAc)(OH^-)/(Ac^-). Substitute and solve for x = (OH^-), convert to pOH and pH.

Acidic salts do the same kind of thing; i.e., NH4^+ + H2O ==>H3O^+ + NH3
Ka for NH4^+ = (Kw/Kb for NH3) = (NH3)(H3O^+)/(NH4^+), solve for x = (H3O^+) and convert to pH.
When you have done each you can arrange them in increasing order of pH.
After you have finished and know the exact order you may wish to think about the results and come up with a shorter way to solve these kinds of problems.

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