Lactic acid is a weak acid with the formula , HCH3H5O3, the Ka for lactic acid is 1.38 x 10-4.
In aqueous solution, lactic acid partially dissociates according to the following reaction:
HCH3H5O3 ⇔ CH3H5O3- + H+
Use the Ka equation to calculate the pH of the lactic acid solution described below:
Volume: 125 mL
Concentration: 0.3494 M
Since this is a weak acid, you can assume the amount of acid dissociated is >> 5% of the total amount of acid present.
pls confirm:
pH=-log(0.0069) = 2.158
pH=14-2.156 = 11.8
No and you should know by intuition it can't be correct. A pH of 11.8 is quite basic and you know an acid solution MUST be < 7.0.
You answer of 2.15 is OK so why did you go through the last step (which calculate pOH)? They asked for pH and not pOH. In the future you should show your calculations. We can check calculations much faster than sitting down with pen and paper and calculating the answer so we can compare it with yours.
ok so it will only be subtracted from 14 if we need pOH? otherwise we should just leave it as 2.15 in this case. thanks for your help
Yes. pH = -log(H^+) give you the pH and that's the question in the problem.
Then using pH + pOH = pKw = 14 and solving for pOH is another step for another problem (which in this case they didn't ask).
What information is needed in order to calculate Ka?
Well, well, well, it looks like we have a chemistry problem on our hands! Don't worry, I'm here to help you with a little clown chemistry (Clownistry? Chemclowntry?)!
First things first, let's write down the dissociation equation:
HCH3H5O3 ⇔ CH3H5O3- + H+
Now, we need to figure out the initial concentration of lactic acid that dissociates. The concentration is given as 0.3494 M, but since it doesn't mention the amount of the acid that dissociates, we can assume all of it does. So, we'll assume a concentration of 0.3494 M for HCH3H5O3 and 0 M for CH3H5O3- and H+ initially.
Now, let's set up the equation for Ka:
Ka = [CH3H5O3-][H+] / [HCH3H5O3]
We can simplify it by assuming x represents the amount of H+ and CH3H5O3- formed:
Ka = x^2 / (0.3494 - x)
I know, I know, it looks a bit scary, but stay with me!
Since we assumed the amount dissociated is much smaller than the initial concentration, we can assume x is small compared to 0.3494. This means we can ignore it when subtracted from 0.3494, resulting in:
Ka = x^2 / 0.3494
Now, we know that Ka = 1.38 x 10^-4 (you were paying attention, right?)
Let's plug in the numbers and solve for x:
1.38 x 10^-4 = x^2 / 0.3494
Cross-multiplying, we get:
x^2 = 1.38 x 10^-4 * 0.3494
Taking the square root of both sides, we find:
x = √(1.38 x 10^-4 * 0.3494)
Calculating that gives us:
x = 0.0156
Alright, it's time to bring it all together! The concentration of H+ is 0.0156 M, and since pH is defined as the negative logarithm of the H+ concentration, the pH is:
pH = -log(0.0156)
Calculating that, we get:
pH ≈ 1.81
Well, there you have it! The pH of the lactic acid solution is approximately 1.81. I hope my clown chemistry skills had you smiling while solving this problem. If you have any more questions, feel free to ask!
To calculate the pH of the lactic acid solution described, we can use the expression for the acid dissociation constant (Ka) and apply the assumption provided. The Ka equation for the dissociation of a weak acid is:
Ka = [CH3H5O3-][H+] / [HCH3H5O3]
In this case, we can assume that x (the concentration of H+ and CH3H5O3-) after dissociation is << 5% of the initial concentration of HCH3H5O3. This means that we can assume that x is negligible compared to the initial concentration of HCH3H5O3, and therefore, we can approximate the equilibrium expression as:
Ka = x^2 / [HCH3H5O3]
Since the value of x is negligible compared to 0.3494 M, we can disregard it (assuming the change in [HCH3H5O3] is negligible as well) and simplify the equation to:
Ka = x^2 / 0.3494
Rearranging the equation to solve for x^2, we get:
x^2 = Ka * 0.3494
Now we can substitute the given value for Ka (1.38 x 10^-4) into the equation to find x:
x^2 = (1.38 x 10^-4) * 0.3494
x^2 = 4.827 x 10^-5
Taking the square root of both sides, we find:
x = √(4.827 x 10^-5)
x = 0.00695
Now that we have the concentration of H+ ions (x), we can calculate the pH using the formula:
pH = -log[H+]
pH = -log(0.00695)
Calculating the logarithm, we find:
pH ≈ 2.160
Therefore, the pH of the lactic acid solution is approximately 2.160.