Problem: A 30.0 ml sample of 0.165 M propanoic acid is titrated with 0.300 M KOH.a. Calculate the pH at 0 mL of added base.b. Calculate the pH at 5 mL of added base.c. Calculate the pH at 10 mL of added base.d. Calculate the pH at the equivalent point.e. Calculate the pH at one-half of the equivalence point.f. Calculate the pH at 20 mL of added base.g. Calculate the pH at 25 mL of added base.

🤓 Based on our data, we think this question is relevant for Professor Nishida's class at WSU.

FREE Expert Solution

The dissociation of propanoic acid (CH3CH2COOH) is as follows:


CH3CH2COOH(aq) + H2O(l)  CH3CH2COO(aq) + H3O+(l);   Ka = 1.3x10-5


a) Before adding KOH

We can construct an ICE table. Remember that liquids are ignored in the ICE table.



The Ka expression for CH3CH2COOH is:


Ka=reactantsproducts=[CH3CH3COO-][H3O+][CH3CH2COOH]

**Liquids are ignored in the Ka expression

Note that each concentration is raised by the stoichiometric coefficient: [CH3CH2COOH], [H3O+] and [CH3CH2COO] are raised to 1.


Plugging in the equilibrium concentrations from the ICE table into the Ka expression:


1.3×10-5=(x)(x)(0.165-x)1.3×10-5=x20.165-x


Now, we need to determine if we can remove (–x) from the equation. To do so, we need to determine the ratio of the initial concentration and Ka:


[HC3H5O3]inititalKa=0.1651.3×10-5=>>>500


Since the ratio is greater than 500, we can remove (–x) from the equation. the equation becomes:


(0.165) 1.3×10-5=x2(0.165)(0.165)2.145×10-6=x2


x = 1.46×10-3


This means [H3O+] = [CH3CH2COO] = x = 1.76x10-3 M. From [H3O+], we can calculate for pH:


pH=-log [H3O+]pH=-log [1.46×10-3 M]


pH = 2.83→before KOH is added (0 mL)


b) After adding 5 mL KOH

 Calculate the initial amounts of CH3CH2COOH and KOH in moles before the reaction happens.

Recall that molarity is given by:

Molarity = moles of soluteLiters of solvent

molarity (volume) → moles

*convert volumes from mL to L → 1 mL = 10-3 L


30 mL of 0.165 M propanoic acid


moles CH3CH3COOH = (30 mL ×10-3 L1 mL)×0.165 mol CH3CH3COOH L


moles CH3CH2COOH = 0.00495 mol 


5 mL of 0.300 M KOH


moles KOH = (5 mL ×10-3 L1 mL)×0.300 mol KOH L


moles KOH = 0.0015 mol


Construct an ICF Chart.


After the reaction is complete the solution contains:

▪ 0.00345 mol CH3CH2COOH → weak acid

▪ 0.0015 mol CH3CH2COO-  conjugate base

whenever we have a conjugate base and a weak acid, we have a buffer


Since we have a buffer, we can calculate the pH of the solution using the Henderson-Hasselbalch equation:

pH = pKa + log(conjugate baseweak acid)

pKa can be calculated from Ka:

pKa = -log Ka

*substituting in the Henderson-Hasselbalch equation:

pH = -log Ka + log(conjugate baseweak acid)


Calculate pH:

pH = -log Ka + log(CH3CH2COO-CH3CH2COOH)pH = -log (1.3×10-5) + log(0.0015 mol0.00495mol)

*we can use moles of the conjugate base and the conjugate acid because we’re calculating the ratio

pH = 4.37→After adding KOH (5 mL)

c) After adding 10 mL KOH

 Calculate the initial amounts of CH3CH2COOH and KOH in moles before the reaction happens.

Recall that molarity is given by:

Molarity = moles of soluteLiters of solvent

molarity (volume) → moles

*convert volumes from mL to L → 1 mL = 10-3 L

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Problem Details

A 30.0 ml sample of 0.165 M propanoic acid is titrated with 0.300 M KOH.

a. Calculate the pH at 0 mL of added base.

b. Calculate the pH at 5 mL of added base.

c. Calculate the pH at 10 mL of added base.

d. Calculate the pH at the equivalent point.

e. Calculate the pH at one-half of the equivalence point.

f. Calculate the pH at 20 mL of added base.

g. Calculate the pH at 25 mL of added base.

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Weak Acid Strong Base Titrations concept. You can view video lessons to learn Weak Acid Strong Base Titrations. Or if you need more Weak Acid Strong Base Titrations practice, you can also practice Weak Acid Strong Base Titrations practice problems.

What professor is this problem relevant for?

Based on our data, we think this problem is relevant for Professor Nishida's class at WSU.