Calculate the pH of a solution that contains 7.8 times 10^-6 M OH at 25 degree C

Question

Calculate the pH of a solution that contains 7.8 times 10^-6 M OH at 25 degree C. 1 28 5.11 12.72 8 89 9 64 A heptanoic acid buffer has a concentration of 0 400 M and a pH of 4 41 If 1.00L of buffer solution has 100.0 mL of 1.00 M HCI added, what is the new pH? The Ka for heptanoic acid is 6.5 times 10^-5. 4.19 5.03 4.41 3.34 3.97 Consider a reaction that has a positive Delta H and a positive Delta S. Which of the following statements is TRUE? This reaction will be spontaneous only at high temperatures This reaction will be spontaneous at all temperatures. This reaction will be nonspontaneous at all temperatures This reaction will be nonspontaneous only at high temperatures It is not possible to determine without more information A cysteine residue in a protein active site has a pKa of 7 6 (referring to deprotonation of the SH) What percent of the cysteines are deprotonated (-S') at pH 8? 39 8 % 28.5 % 60.2 % 71.5 % None of the above

Explanation / Answer

Q14.

pH of OH- = 7.8*10^-6

recall that

pOH = -log(OH-) = -log(7.8*10^-6) = 5.1079

and

pH = 14-pOH = 14-5.1079 = 8.8921

choose D

Q15.

Apply Henderson hasselbach equation:

pH = pKa + log(A-/HA)

pKa = -log(Ka) = - log(6.5*10^-5) = 4.18708

initially:

pH = pKa + log(A-/HA)

4.41 = 4.18708 + log(A-/HA)

A- + HA = 0.4

A-/HA = 10^(4.41-4.187) = 1.67

A- + HA = 0.4 --> 1.67*HA + HA = 0.4

2.67HA = 0.4

HA = 0.4/2.67 = 0.1498 M = 0.1498*¨1 = 0.1498 mol

A-/HA = 1.67

A- = 0.1498 *1.67 = 0.250166 M --> 0.250166*1 = 0.250166 mol

then... after mol of HCL = MV = 0.100*0.1 = 0.01

after addition:

pH = 4.18708 + log((0.250166 -0.01)/(0.1498 +0.01))

pH = 4.3640

choose C which is the nearest

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