A.The compound m-nitrophenol is a weak acid that is also an acid-base indicator.
ID: 515894 • Letter: A
Question
A.The compound m-nitrophenol is a weak acid that is also an acid-base indicator. It changes color from clear to yellow at a pH of 7.70, when the acidic and basic forms of the indicator have equal concentrations. Determine the pKa of m-nitrophenol.
B. Suppose we take 0.150 mol of an unknown weak acid with formula HA, and combine it with 0.100 mol of NaOH. We take 1.00 L of this solution and measure the pH.
a. If the pH is 5.20, what is the pKa of the unknown acid?
b. How many more moles of NaOH should we add if we want the pH to be 5.50?
Explanation / Answer
a)
moles of weak acid = 0.150 mol
moles of NaOH = 0.1 mol
HA + NaOH ---------------> A- + H2O
0.150 0.1 0 0
0.05 0 0.1 0.1
pH = pKa + log [salt / acid]
5.20 = pKa + log [0.1 /0.05]
pKa = 4.90
b)
pH = 5.50
pH = pKa + log [A- / HA]
5.50 = 4.90 + log [A- / HA]
[A- / HA] = 3.981
[A-] = 0.150 x 3.981
[A-] = 0.597
moles of NaOH = moles of A-
moles of NaOH added = 0.597 mol
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