Aspirin is a weak acid with a Ka of 3.0x10^-5. Find the pH of a solution by dissolving .65g of aspirin in water and diluting it to 50mL. You can use RCOOH to represent aspirin. Molecular weight of aspirin is 180g/mol
moles aspirin = grams/molar mass = 0.65/180 = approximately 0.004 mole.
.........RCOOH ==> H^+ + RCOO^-
initial....0.004....0......0
change.....-x......x........x
equil...0.004-x.....x........x
Ka = (H^+)(RCOO^-)/(RCOOH)
Ka = (x)(x)/(0.004-x)
What is x. Solve for x = (H^+), then convert to H.
pH = -log (H^+).
The answer that was in my answer key was 2.83 but I got 3.48. Is there something wrong in the calculations?
2.83 is right (and I obtained 3.46 using 0.004, also). Apparently you took my "apparoximately 0.004) and assumed that was gospel. I had to stick something there and didn't want to calculate it so I guessed at it and wrote approximately. I assumed you would substitute the correct value although I didn't say you should confirm my approximate number (but you should have). The molarity of the aspirin is 0.0722. (NOTE: 0.65g x (1 mol/180g) x (1000 mL/50 mL)
For the ICE box, you are supposed to use the molarity of the aspirin. I did not realize that. Thank you so much for your help!
as we know that pH=-log(H^+)
we firstly need to know the concentration of H^+ after dissociating the aspirin.
We should therefore calculate the number of moles of aspirin. Then we should use the ICE table to determine the concentrations of H^+ and the associated conjugate base. The volume in litres may be helpful if we want to get the concentration of aspirin by molarity... remember that we need this in the ICE table.
To find the pH of a solution, we need to determine the concentration of the hydrogen ions (H+ ions) in the solution. The concentration of the H+ ions can be calculated by first determining the concentration of the aspirin (RCOOH) in the solution, and then using the dissociation constant (Ka) to find the concentration of H+ ions.
Step 1: Calculate the number of moles of aspirin (RCOOH):
We are given that the mass of aspirin is 0.65g and the molar mass is 180g/mol. We can use the following formula to calculate the number of moles:
Moles of RCOOH = Mass of RCOOH / Molar mass of RCOOH
= 0.65g / 180g/mol
≈ 0.0036 mol
Step 2: Calculate the concentration of RCOOH in the solution:
The volume of the solution is given as 50mL, so we need to convert it to liters:
Volume of solution in liters = 50mL / 1000 mL/L
= 0.05 L
Concentration of RCOOH = Moles of RCOOH / Volume of solution in liters
= 0.0036 mol / 0.05 L
= 0.072 mol/L
Step 3: Calculate the concentration of H+ ions using Ka:
The dissociation of aspirin can be represented as follows:
RCOOH ⇌ RCOO- + H+
The equilibrium constant (Ka) expression for this reaction is:
Ka = [RCOO-][H+] / [RCOOH]
Since we are assuming that the dissociation of aspirin is small, we can assume that the concentration of RCOOH is approximately equal to the initial concentration. Therefore, [RCOOH] = 0.072 mol/L.
Now, we can rearrange the equation to solve for [H+]:
[H+] = (Ka x [RCOOH]) / [RCOO-]
= (3.0x10^-5) x (0.072 mol/L) / 0.072 mol/L
= 3.0x10^-5 mol/L
Step 4: Calculate the pH:
The pH is a measure of the concentration of H+ ions in a solution. It is defined as the negative logarithm (base 10) of the H+ concentration:
pH = -log[H+]
= -log(3.0x10^-5)
≈ 4.52
Therefore, the pH of the solution is approximately 4.52.