Assume that 30.0 mL of a 0.10 M solution of a weak base B that accepts one proton is titrated with a 0.10 M solution of a monoprotic strong acid HX. (a) How many moles of HX have been added at the equivalence point? (b) What is the predominant form of B at the equivalence point? (c) What factor determines the pH at the equivalence point? (d) Which indicator, phenolphthalein or methyl red, is likely to be the better choice for this titration?
c. is actually methyl red
a. moles HX added = M x L.
b. B + HX ==> BH^+ + X^-
c. The hydrolysis of the salt BH^+. Said another way, the strength of BH^+ as an acid.
d. Phenolphthalein changes color at pH about 9. Methyl red changes color about 5. Which do you think?
For c it is both the Ka for BH^+ and concentration of BH^+ to determine pH at the equivalence point.
(a) Well, let's calculate how many moles of HX have been added at the equivalence point. We know the volume of the HX solution is 30.0 mL, so converting that to liters gives us 0.030 L. The concentration of HX is 0.10 M, so we can multiply the volume by the concentration to get the number of moles. Okay, I'm done with my calculations, but I forgot my calculator at home. Let me just pretend I have one... *nervous laughter* The number of moles of HX is 0.003 moles!
(b) Ah, the predominant form of B at the equivalence point is like a surprise party, it's basic! *ba dum tss* Since it's a weak base, it accepts one proton, and at the equivalence point, it has reacted with an equal number of protons from the strong acid, leaving it in its basic form.
(c) The factor that determines the pH at the equivalence point is... *drumroll* the nature of the conjugate acid-base pair formed! Isn't it exciting how a little chemistry can have such a big impact?
(d) Ah, the choice of indicator! Phenolphthalein or methyl red? Well, let me tell you, choosing the right indicator is key in avoiding a "colorful" situation. Since the weak base is accepting protons and turning basic at the equivalence point, phenolphthalein, which changes color in the pH range of 8.2 to 10, would be the better choice. Methyl red, on the other hand, changes color at a lower pH, around 4.4 to 6, and would probably start partying too early in this case. Gotta keep the indicators on their toes, you know! *wink*
To find the answers to the questions, we need to understand the concept of titration and the reaction that takes place between the weak base B and the strong acid HX.
(a) To calculate the number of moles of HX added at the equivalence point, we can use the equation:
Molarity (M) = Moles (mol) / Volume (L)
Given that the volume of the weak base solution is 30.0 mL (or 0.030 L) and the molarity of the strong acid solution is 0.10 M, we can calculate the moles of HX added:
Moles of HX = Molarity of HX x Volume of HX
Moles of HX = 0.10 M x 0.030 L
(b) At the equivalence point, the weak base B is completely neutralized by the strong acid HX. Since B is a weak base that accepts one proton, it will have accepted one proton from the HX and become its conjugate acid, BH+.
(c) The factor that determines the pH at the equivalence point is the concentration of the resulting acid, which in this case is BH+. The pH is determined by the concentration of hydrogen ions (H+). Since B accepts a proton from HX and becomes BH+, the concentration of H+ ions will be determined by the concentration of BH+.
(d) The choice of indicator for the titration depends on the pH range in which the indicator changes color. Phenolphthalein changes color in a pH range of approximately 8.2 to 10. Methyl red, on the other hand, changes color in a pH range of approximately 4.4 to 6.2.
Since we are titrating a weak base with a strong acid, the equivalence point will be in the acidic pH range. Therefore, methyl red, which changes color in a lower pH range, would be a better choice for this titration compared to phenolphthalein.
hi to you. is this right?
a. 2.7 x 10 -3 moles or 0.0027
b. BH + (aq)
c. Already given.
d. phenolphthalein