This is a chemistry question dealing with pH. HELP!!?

Question

This is a long confusing problem. Mr. H has been told by his superiors that only a chemist can be trusted with the combination to a safe. The combination is the pH of solution A, described bellow, followed by the pH of solution C (for example: If the pH of solution A is 3.47 and that of solution C is 8.15, then the combination to the safe is 3-47-8-15). The chemist must determine the combination using only the information below (all solutions at 25 degrees Celcius).

1) Solution A is 50.0 mL of a .100 M solution of the weak monoprotic acid HX.
2) Solution B is a .0500 M solution of the salt NaX. It has a pH of 10.02.
3) Solution C is made by adding 15.00mL of 0.250 M KOH to solution A.

What is the combination to the safe?


This is really confusing. I have no idea where solution B even came from or what the heck I'm supposed to do with it!! HELP!!!

Answer 1

Solution B

NaX >> Na+ + X-

when X- is put in the water we have

X- + H2O <> HX + OH-

pH = 10.02 >> pOH = 3.98

[ OH- ] = 10^-3.98 = 0.000105 M = [ HX ]

[ X- ] = 0.0500 - 0.000105 = 0.0499 M

For this equilibrium

K = Kw / Ka = ( 0.000105)^2 / 0.0499 = 2.21 x 10^-7

1.00 x10^-14 / Ka = 2.21 x 10^-7

Ka = 4.5 x 10^-8


Solution A

HX <> H+ + X-

4.5 x 10^-8 = x^2 / 0.05 - x

x = 0.0000476 M = [ H+ ] pH = 4.32


Solution C

moles HX = 50 mL x 0.100 / 1000 = 0.005

moles OH- = 15 x 0.250 / 1000 = 0.00375

0.005 mole HX + 0.00375 mole OH- react to give 0.00125 mole HX + 0.00375 mole X- by the reaction HX + OH- >> H2O + X-

Total volume = 50+15 = 65 mL = 0.065 L

[ HX ] = 0.00125 / 0.065 = 0.0192 M

[ X- ] = 0.00375 / 0.065 = 0.0577 M

Ka = 4.5 x10^-8 = (x) ( 0.0192 + x ) / 0.0577-x

x = 1.35 x 10^-7 M pH =6.87


Summing from the solution B i get Ka , from the solution A I can get pH using Ka obtained from the solution B then I can work on the solution C

Answer 2

From number 2, work out the Kb of X-, and hence the Ka of HX. Then you can work out the pH of the weak acid HX and the pH of the buffer solution in 3.

Answer 3

a million)i imagine you recommend what concentration of hydroxide neutralizes a pH of 5. for this reason we opt to comprehend the way many hydroxide ions are mandatory to achieve a pH of seven. 10^-5M -[OH]=10^-7M [OH-]=9.9x10^-6M (i might want to get 2d evaluations in this) 2) a) HCN(aq) + H2O(l)--> H3O+(aq) + CN-(aq) b)Sr(OH)2(aq) + H2O(l) --> Sr2+(aq) + OH-(aq)