Determine the pH of 0.18M NH4Cl.
how to find the number of moles of water in the sample
Katelynn--I answered this question at one of your other dozen posts of the same question.
Post that are piggy-backed onto another post often are ignored. I suggest you go the top of the page, click on ask a new question, and post your question that way. And while you are at it, provide all of the information needed. The number of moles in a water sample varies with the amount of water.
To determine the pH of 0.18M NH4Cl, we first need to understand the chemical properties of NH4Cl.
NH4Cl is an ionic compound formed from ammonium ion (NH4+) and chloride ion (Cl-). When NH4Cl dissolves in water, it dissociates into its ions:
NH4Cl (s) → NH4+ (aq) + Cl- (aq)
The ammonium ion (NH4+) can act as a weak acid by donating a proton (H+) to water molecules, forming hydronium ions (H3O+). This makes the solution acidic.
The chemical equation for the ionization of NH4+ as a weak acid is:
NH4+ (aq) + H2O (l) ⇌ NH3 (aq) + H3O+ (aq)
Since NH4Cl completely dissociates into its ions, the concentration of NH4+ in the solution will be equal to the initial concentration of NH4Cl, which is 0.18M.
To find the pH of the solution, we need to find the concentration of H3O+ ions.
To do this, we need to calculate the equilibrium concentration of NH3, which is the conjugate base of the weak acid NH4+. We can use the equilibrium constant expression, known as the Ka expression, for this reaction:
Ka = [NH3] / [NH4+]
Assuming that x is the molar concentration of NH3 formed, the equilibrium concentrations can be expressed as [NH4+] - x and x.
The given concentration of NH4Cl is 0.18M. Since NH4+ is in a 1:1 ratio with NH4Cl, the initial concentration of NH4+ is also 0.18M.
Assuming x is small, we can approximate [NH4+] - x as 0.18M and [NH3] as x.
Using the Ka expression, we have:
Ka = x / (0.18 - x)
The Ka value for NH4+ is approximately 5.6 x 10^-10.
To solve for x (concentration of NH3), we can rearrange the equation:
x = Ka * (0.18 - x)
Simplifying the equation:
x = 5.6 x 10^-10 * 0.18 - 5.6 x 10^-10 * x
Combining like terms:
x + 5.6 x 10^-10 * x = 5.6 x 10^-10 * 0.18
Factoring out the x:
x(1 + 5.6 x 10^-10) = 9.9 x 10^-11
Dividing both sides by (1 + 5.6 x 10^-10):
x = (9.9 x 10^-11) / (1 + 5.6 x 10^-10)
Calculating this with a calculator:
x ≈ 8.76 x 10^-11
The concentration of H3O+ ions is equal to the concentration of NH3 formed, which is approximately 8.76 x 10^-11 M.
To get the pH, we need to calculate the negative logarithm (base 10) of the H3O+ concentration:
pH = -log[H3O+]
pH = -log(8.76 x 10^-11)
Calculating this with a calculator:
pH ≈ 10.06
Therefore, the pH of a 0.18M NH4Cl solution is approximately 10.06.