pH and Buffer questions

Chemistry
Tutor: None Selected Time limit: 1 Day

Calculate the pH of the following solutions. Show work pls

0.0893 M HBr

1.2 x 10 -4 M NaOH

buffer containing 0.150 M HC2H3O2 and 0.050 M C2H3O2-

Nov 20th, 2014

In solution,

HBr + NaOH ==== Na+ + Br- + H2O

So the 1.2 x 10 -4 M OH- in NaOH will be completely neutralized by the H+ in HBr, which makes the concentration of H+ in the solution at this point:

[H+]=0.0893 -1.2 x 10 -4 =0.08918 M

Now we can think about the buffer. 

HC2H3O2 ==== H+ + C2H3O2-           pKa=4.76

So when the above reaction reaches its balance, 

[H+][C2H3O2-]/[HC2H3O2 ]=10^-4.76=1.74 x 10 -5

Since now 

[H+][C2H3O2-]/[HC2H3O2 ]=0.08918 x 0.05 /0.15 = 0.02973  >> 1.74 x 10 -5

So the reaction will move in the left direction. Assuming a M H+ is converted to HC2H3O2  to reach the final balance, we can now get the following equation

(0.08918-a) (0.05-a) / (0.15 + a) = 1.74 x 10 -5

(0.08918-a) (0.05-a) = 1.74 x 10 -5 (0.15 + a)

a^2-0.139a+0.004456=0

Since a has to be not larger than 0.05, so after the calculation you will find that a=0.05, which means all C2H3O2- is converted to HC2H3O2 at the end. 

Therefore the final concentration of H+

[H+] = 0.08918-0.05 = 0.03918 M

pH = 1.4


Nov 20th, 2014

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