Ionization Constant for Acid Problems, 5

Category: Chemical Engineering Math

"Published in Newark, California, USA"

When 0.02 moles of a monoprotic acid is dissolved in 350 mL of water, the pH is 3.05. What is the ionization constant of this acid?

Solution:

Monoprotic acid is an acid which donates one electron during the ionization process. Monoprotic acid can be an inorganic acid or organic acid as long as one electron is lost during the ionization process.

If the pH of an acid is 3.05, then the concentration of hydrogen ion is

Let's assume that the volume of water is equal to the volume of a solution because the amount of a monoprotic acid is very small or negligible. The concentration of a monoprotic acid at the start of ionization process is

Since the concentration of [H⁺] is very small compared with [HA], then HA is a weak acid. The ionization of HA is written as follows
 

Therefore, the ionization constant of a monoprotic acid is
 

 

 

 

Solving for pH of a Mixture of Acid and Base Problems

Category: Chemical Engineering Math

"Published in Newark, California, USA"

What is the pH of the resulting solution made by mixing 25 mL of 0.1 M HCl and 15 mL of 0.1 M NaOH?

Solution:

The given word problem is about the mixing of an acid and a base. If you mix an acid and a base, their products are salt and water. The chemical reaction for the given mixture is written as follows

In order to convert the HCl and NaOH completely into NaCl and water, you must have equal moles at each reactants which are HCl and NaOH. Let's see if the two reactants will be used completely as follows

Since the number of moles of HCl is greater than the number of moles of NaOH, then the resulting solution is an acid. 

Hence, the molarity of the resulting solution which is HCl is

              

Since HCl is a strong acid, then it is completely ionized as follows

Therefore, the pH of the resulting solution which is HCl solution is

 

Solving for pH of an Acid Problems, 2

Category: Chemical Engineering Math

"Published in Newark, California, USA"

Calculate the pH of 3.5 x 10^-3 M HNO₃.

Solution:

Let's consider the ionization of HNO₃ as follows 

Since HNO₃ is  a strong acid, then it is ionized completely into H⁺ and NO3^-. 

Therefore, the pH of a given solution is

Stoichiometry Problem - Decomposition, 2

Category: Chemical Engineering Math

"Published in Newark, California, USA"

A 250 mg sample containing 45% MgCO₃ and 55% CaCO₃ was ignited producing CO₂ as decomposition product. Assuming that the decomposition reaction is complete, what is the change in weight of a NaOH solution used to absorb CO₂?

Solution:

From the description of a given problem, it is about decomposition reaction. The mixture of MgCO₃ and CaCO₃ is decomposed by heat as follows

and then NaOH solution is used to absorb CO₂ as follows
 

By gravimetric analysis, the weight of CO₂ from MgCO₃ is 
 

 

By gravimetric analysis, the weight of CO₂ from CaCO₃ is 
 

 

Therefore, the change of weight of a NaOH solution used to absorb CO₂ which is also equal to the total weight of CO₂ is

  

Gravimetric Analysis Problems, 4

Category: Chemical Engineering Math

"Published in Newark, California, USA"

What weight of an impure NaCl sample must be taken for analysis so that the weight of AgCl precipitate obtained in mg will be equal to the % Cl in the sample?

Solution:

The description of a given problem is about gravimetric analysis in which the amount of an impure NaCl is unknown. If the amount of impure NaCl sample is dissolved in water and then add with AgNO₃ solution, then AgCl precipitate will be formed as follows

By using gravimetric analysis, if x is the weight of AgCl crystals in mg, then the weight of Cl in AgCl is

If the weight of AgCl precipitate is equal to the % Cl in the sample, therefore the weight of impure NaCl sample is
 

 

 

 

 

Gravimetric Analysis Problems, 3

Category: Chemical Engineering Math

"Published in Newark, California, USA"

The aluminum in a 759.08 mg of impure ammonium aluminum sulfate sample was precipitated as Al(OH)₃ and ignited at 1100°C to yield a precipitate of Al₂O₃ weighing 387.953 mg. Express the result of analysis in terms of % Al.

Solution:

From the description of a given problem, it is about gravimetric analysis because it involves the analysis of a certain element or a compound in a sample. Most of the sample is either impure or a mixture of two or more compounds. In this case, we will use the molecular weights as their factors to solve for the weight of a pure substance or a compound. 

The weight of Al in Al₂O₃, we have 

Therefore, the % Al in a sample is