Aspirin (acetylsalicylic acid, ) is a weak monoprotic acid. To determine its acid-dissociation constant, a student dissolved 2.00 g of aspirin in 0.600 L of water and measured the pH . What was the Ka value calculated by the student if the of the pH of the solution was 2.62?
I know that Ka =[h3O+][a-]/[ha] but i don't know how to solve this.
so far i have
[h3o+]=-log(2.62)=2.4*10^-3
and
[oh-]=4.17*10-12
but i'm not sure what exactly this question is looking for. if you could point me in the right direction that would be great. thank you.
It tells you it wants you to calculate the Ka for ASA. If we call it HA, then
HA ==> H^+ + A^-
Ka = (H^+)(A^-)/(HA)
You know (H^+) and you have that right.
(A^-) is the same as (H^+). I have used H^+ for H3O^+. For HA, you want to plug in the molarity of the aspirin which is moles/L. Find moles by grams/molar mass and you have it in 0.6 L.
I'm still not getting the correct answer:
i have 2.00g/180.15g/mol =1.11*10^-2
1.11*10^-2/.6=1.85*10^-3
and (2.4_10^-3)^2/1.85*10^-3 = 3.11*10^-9.
what am i doing wrong?
To determine the acid-dissociation constant (Ka) of aspirin, you need to use the given pH value and the concentration of the acid (HA) and its conjugate base (A-) in the solution.
Step 1: Calculate the concentration of H3O+ (hydronium ion) in the solution:
[H3O+] = 10^(-pH) = 10^(-2.62) = 2.4 x 10^(-3) M
Step 2: Since aspirin is a monoprotic acid, the concentration of HA is equal to the initial concentration of aspirin (acetylsalicylic acid). To find the concentration, you need to convert the given mass of aspirin to moles and then divide by the volume of the solution in liters:
molar mass of aspirin (C9H8O4) = 180.16 g/mol
moles of aspirin = mass / molar mass = 2.00 g / 180.16 g/mol = 0.0111 mol
concentration of HA = moles / volume = 0.0111 mol / 0.600 L = 0.0185 M
Step 3: Since the aspirin (HA) dissociates to form H3O+ and its conjugate base (A-), the concentration of A- is initially zero but will increase as the dissociation occurs.
Step 4: Substitute the values into the acid-dissociation constant equation:
Ka = [H3O+][A-] / [HA] = (2.4 x 10^(-3)) (0.0185) / (0.0185) = 2.4 x 10^(-3)
Therefore, the calculated Ka value by the student is 2.4 x 10^(-3).
To solve this problem, you are on the right track using the equation Ka = [H3O+][A-] / [HA]. Here's how you can proceed:
1. Calculate the concentration of H3O+ in the solution:
Since the pH is given as 2.62, you correctly calculated [H3O+] = 10^(-pH) = 10^(-2.62) = 2.4 x 10^(-3) M.
2. Determine the initial concentration of the acid (aspirin):
Given that 2.00 g of aspirin is dissolved in 0.600 L of water, you need to convert the mass of aspirin to moles. The molar mass of aspirin is 180.16 g/mol. So, the number of moles of aspirin is:
2.00 g / 180.16 g/mol = 0.0111 mol.
Now, divide the moles of aspirin by the volume of the solution to find the concentration:
0.0111 mol / 0.600 L = 0.0185 M.
3. Since aspirin is a monoprotic acid, the concentration of A- (conjugate base) will be the same as the concentration of the acid. So, [A-] = [HA] = 0.0185 M.
4. Plug the values into the equation Ka = [H3O+][A-] / [HA]:
Ka = (2.4 x 10^(-3) M) * (0.0185 M) / (0.0185 M) = 2.4 x 10^(-3).
Therefore, the calculated Ka value by the student is 2.4 x 10^(-3).
Note: The Ka value represents the acid-dissociation constant and indicates the strength of an acid. A small Ka value indicates a weak acid, as is the case with aspirin (acetylsalicylic acid).