Calculate pH of 0.1 M (molar) sodium acetate solution.

General information:
Sodium acetate is a salt of acetic acid which has pKa value of 4.76 (Ka = 1.75×10^-5). It accepts a proton from water to produce acetic acid and hydroxyl ions in solution.

Calculation:
Firstly, Kb of sodium acetate is calculated as the following:

pKw = pKa + pKb

14.00 = 4.76 + pKb
pKb = 9.24

Kb= 10^-pKb = 10^-9.24 = 5.75×10^-10

Next, pH is calculated using online pH calculator or by applying the equation used for calculating pH of a weak base in solution as the following:

pH of 0.1 M sodium acetate solution is 8.88.

Calculate pH of 100 ml buffer solution containing 0.1 g acetic acid and 0.2 g sodium actetate.

General information:
Acetic acid ( molecular weight = 60.1 g/ mole ) has pKa value equal to 4.76 ( Ka = 1.75×10^-5) at 25 °C. Sodium acetate (molecular weight = 82.0 g/ mole) is a salt of acetic acid.

Calculation:
Firstly, molar concentrations of acetic acid and sodium acetate are calculated:

Acetic acid:
number of moles = 0.1 g/ (60.1 g/ mole) = 0.0017 mole
molar concentration = 0.0017 mole/ (0.1 liter) = 0.00017 M

Sodium acetate:
number of moles = 0.2 g/ (82.0 g/ mole) = 0.0024 mole
molar concentration = 0.0024 mole/ (0.1 liter) = 0.00024 M

Next, pH is calculated using online pH calculator or by applying the buffer equation as the following:

pH = 4.91.

Calculate pH of 250 ml solution containing 325 mg aspirin (acetylsalicylic acid).

General information:

Aspirin ( molecular weight = 180.15 g/ mole ) dissociates in water to produce acetylsalicylate and hydronium ions. It has pKa value equal to 3.49 ( Ka = 3.27 × 10^-4 ) at 25 °C.

Calculation:

Firstly, molar concentration is calculated as the following:

weight in grams = 325 mg × 1 (g /1000 mg) = 0.325 g

number of moles = 0.325 g / (180.15 g/ mole) = 0.0018 mole

volume of solution in liters = 250 ml  × 1 (liter/ 1000 ml) = 0.25 liter

molar concentration = 0.0018 mole/ 0.25 liter = 0.0072 M

Next, pH is calculated using online pH calculator or by applying the equation used for calculating pH of solutions containing weak acids as the following:

pH = 2.86.

Calculate pKb and Kb for sodium benzoate given that benzoic acid has pKa value equal to 4.20 at 25 °C.

Sodium benzoate ( molecular weight = 144.11 g/ mole ) completely dissociates in water to produce sodium and benzoate ions. Benzoate ions accept protons from water to produce benzoic acid and hydroxyl ions. Therefore, it acts as a weak base in solution. Its pKb and Kb values can be calculated from pKa of benzoic acid ( 4.20 at 25 °C) as the following:

pKw = pKa + pKb

14.00 = 4.20 + pKb

pKb = 14.00 - 4.20

pKb = 9.80

pKb = -log Kb

Kb = 10^-pKb

 Kb = 10^-9.80

Kb = 1.59×10^-10

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Calculate pH of 1 liter solution containing 0.5 g benzoic acid.

General information:

Benzoic acid has a molecular weight of 122.12 g/ mole. It dissociates in water to produce benzoate and hydronium ions with pKa value equal to 4.20 (Ka = 6.30×10^-5) at 25 °C.

Calculation:
Firstly, molar concentration  of the acid is calculated as the following:

1 liter solution contains 0.5 g

number of moles = 0.5 g/ molecular weight

number of moles = 0.5 g/ (122.12 g/ mole)

number of moles = 0.0041 moles

molar concentration = number of moles/ volume of solution

molar concentration = 0.0041 moles/ 1 liter

molar concentration = 0.0041 M

Next, pH is calculated using online pH calculator or by using the equation used for calculating pH of solutions containing weak acids as the following:

Result: pH =3.32.