What is the original molarity of a solution of weak acid whose Ka= 3.5 x 10^ -5 and pH is 5.20...
I'm lost, please help!
Call the weak acid HA.
HA ==> H^+ + A^-
Ka = (H^+)(A^-)/(HA)
pH = 5.20 = -log(H^+)
Solve for (H^+), substitute into Ka expression above for H^+ as well as for A^-. For (HA) substitute X-(H^+) and solve for X. I think you can neglect the (H^+) in the X-(H^+) and simply call X - (H^+) = X
To find the original molarity of a solution of weak acid, you can use the formula relating the concentration of the acid to its dissociation constant (Ka) and the concentration of its dissociated species ([H+]).
Step 1: Calculate the concentration of [H+] using the pH value.
pH is defined as the negative logarithm (base 10) of the concentration of [H+], given by the formula pH = -log[H+]. Rearranging the equation, [H+] = 10^(-pH). In this case, the pH value is 5.20, so the concentration of [H+] is 10^(-5.20).
Step 2: Use the dissociation constant (Ka) and the concentration of [H+] to calculate the concentration of the weak acid ([HA]).
The dissociation constant (Ka) is the ratio of the concentration of the dissociated species [A-] to the concentration of the undissociated weak acid [HA].
Ka = [A-] / [HA]
Since the weak acid fully dissociates into [H+] and [A-], [A-] will have the same concentration as [H+]. Therefore, we can substitute [A-] with [H+] in the Ka equation and solve for [HA].
Ka = [H+] / [HA]
[HA] = [H+] / Ka
Using the value of [H+] from Step 1 and the given value of Ka (3.5 x 10^(-5)), plug in these values to calculate [HA]:
[HA] = (10^(-5.20)) / (3.5 x 10^(-5))
Step 3: Calculate the original molarity using the concentration of [HA].
The original molarity, represented as M, is the concentration of the acid [HA]. It is defined as the moles of solute (in this case, the acid) divided by the volume of the solution in liters (L).
M = (moles of [HA]) / (volume of solution in L)
If you are given the volume of the solution, plug in the values of [HA] and volume to calculate the original molarity.
To find the original molarity of a solution of a weak acid with a given Ka value and pH, you can follow these steps:
Step 1: Convert the pH value to the [H+] concentration.
- The pH scale is a logarithmic scale used to represent the concentration of hydrogen ions (H+) in a solution.
- The formula to convert pH to [H+] concentration is [H+] = 10^(-pH).
In this case, the pH is 5.20, so the [H+] concentration is [H+] = 10^(-5.20).
Step 2: Calculate the concentration of the weak acid.
- For a weak acid, the concentration of the acid and its conjugate base are equal at equilibrium.
- Since the Ka value is given, we can assume that the equilibrium expression for the dissociation of the weak acid can be written as follows:
Ka = [H+][A-] / [HA]
- Since the Ka value is small (3.5 x 10^(-5)), we can assume that the dissociation of the weak acid is negligible, and the concentration of the weak acid ([HA]) is equal to the initial molarity of the solution.
Step 3: Solve for the initial molarity.
- Rearrange the equilibrium expression to solve for the initial molarity ([HA]) of the weak acid solution:
[HA] = (Ka * [HA]) / [H+]
- Substitute the values into the equation: [HA] = (3.5 x 10^(-5)) / (10^(-5.20))
Now, you can calculate the original molarity of the solution by evaluating the expression [HA] = (3.5 x 10^(-5)) / (10^(-5.20)).