Finding pH and molarities

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M In summary, we calculated the pH of a 0.1M NaHSO_4 to be 1.21 and the molarities of H^+, HSO_4^-, SO_4^{2-} to be 0.099M, 0.099M, and 0.001M respectively in 0.1M H_2SO_4.
  • #1
Clari
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Assuming the first ionization of sulphuric acid is 100% and the [tex]K_{a2}[/tex] of sulphuric acid is 0.10M at 25 degree celsius.(Ka is the acid disocciation constant)
a. Find the pH of a 0.1M [tex]NaHSO_4[/tex]
b. Calculate the molarities of [tex]H^+[/tex], [tex]HSO_4^-[/tex], [tex]SO_4^{2-}[/tex] respectively in 0.1M [tex]H_2SO_4[/tex]

Here are my steps:
a. Let a be the degree of dissociation.
[tex]K_{a2} = \frac{a^2}{0.1-a} = 0.1[/tex]
a = 0.0618
pH = - log 0.0618 = 1.21

I have no idea of how to do part b. Please help me with it and tell me whether I have done right in part a.
 
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  • #2
I've already explained this problem in multiple threads, nevertheless,

a)

[tex]Ka_2= \frac{[SO_4^{-2}][H_3O^+]}{[HSO_4^-]} [/tex]

[tex]Ka_2= \frac{[x][x]}{[.1M-x]} [/tex]

b) The first acid will dissociate completely, thus the initial concentration of both the HSO4- and H3O+ will be .1M. Thus

[tex]Ka_2= \frac{[SO_4^{-2}][H_3O^+]}{[HSO_4^-]} [/tex]

[tex]Ka_2= \frac{[x][.1M+x]}{[.1M-x]} [/tex]
 
  • #3


In part a, you have correctly calculated the degree of dissociation and the pH of a 0.1M NaHSO4 solution. Your calculation is correct, and the pH of the solution is indeed 1.21.

For part b, we can use the Henderson-Hasselbalch equation to calculate the molarities of H+, HSO4-, and SO42- in a 0.1M H2SO4 solution.

[H+] = 10^(pH) = 10^(1.21) = 0.0794 M

[HSO4-] = [H2SO4] - [H+] = 0.1 - 0.0794 = 0.0206 M

[SO42-] = [H+] = 0.0794 M

Therefore, the molarities of H+, HSO4-, and SO42- in a 0.1M H2SO4 solution are 0.0794 M, 0.0206 M, and 0.0794 M, respectively.

It is important to note that these calculations are based on the assumption that the first ionization of sulphuric acid is 100% and the K_{a2} of sulphuric acid is 0.10M. If these values are not accurate, the results may be slightly different.
 

What is pH and how is it measured?

pH is a measure of the acidity or basicity of a solution. It is measured on a scale from 0 to 14, with 7 being neutral. A pH of less than 7 indicates acidity, while a pH greater than 7 indicates basicity. pH can be measured using a pH meter or by using indicators that change color in the presence of acids or bases.

What is the relationship between pH and molarity?

pH and molarity are related but not directly proportional. pH is a logarithmic scale, meaning that a change of one unit in pH represents a tenfold change in acidity or basicity. Molarity, on the other hand, is a measure of the concentration of a solution in moles per liter. So while a change in molarity may affect the pH, the relationship between the two is not linear.

How do you calculate pH from molarity?

To calculate pH from molarity, you can use the Henderson-Hasselbalch equation: pH = pKa + log (conjugate base/acid). The pKa is the negative logarithm of the acid dissociation constant, which is a measure of the strength of an acid. The conjugate base and acid refer to the components of the solution that contribute to its pH.

How do you find the molarity of a solution?

The molarity of a solution is calculated by dividing the number of moles of solute by the volume of the solution in liters. This can be determined by measuring the mass or volume of the solute and using its molar mass to convert to moles. The volume of the solution can be measured using a graduated cylinder or other volumetric equipment.

Why is it important to know the pH and molarity of a solution?

Knowing the pH and molarity of a solution is important for a variety of reasons. It can help determine the strength and reactivity of acids and bases, as well as their potential effects on living organisms and the environment. pH and molarity are also important in many industrial and laboratory processes, such as chemical reactions and biological assays.

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