[kladd56]

Acetylsalicylic acid (HC9H7O4) is the active ingredient in aspirin.  Two tablets each containing 500 mg of acetylsalicylic acid are dissolved in 500 mL of water.  What is the pH of this solution, if the Ka of the acid is 3.3 x 10^-4

[Will]

Acetylsalicylic acid (HC9H7O4) is the active ingredient in aspirin.  Two tablets each containing 500 mg of acetylsalicylic acid are dissolved in 500 mL of water.  What is the pH of this solution, if the Ka of the acid is 3.3 x 10^-4

pH = -log[H⁺]
                                                        
[H⁺] = _/K_a x [C₆H₄(COOH)(OAc)]

You just have to work out what the concentration of the acid is (moles is mass/M_r and concentration is moles/volume).

[Dan]

Use the previously posted equations, here is the procedure I would use

1. Determine relative molecular mass of the acid
2. Determine number of moles of the acid in two 500 mg tablets
3. Determine the concentration of the acid in the solution
4. using the pKa, determine the concentration of H⁺ ions in solution, [H⁺]
5. Determine pH

[Borek]

                                                        
[H⁺] = _/K_a x [C₆H₄(COOH)(OAc)]

Wrong. This equation (8.13 in terms of the lecture pointed to below) holds only for dissociation percentage below 5%.

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

[Borek]

Acetylsalicylic acid (HC9H7O4) is the active ingredient in aspirin.  Two tablets each containing 500 mg of acetylsalicylic acid are dissolved in 500 mL of water.  What is the pH of this solution, if the Ka of the acid is 3.3 x 10^-4

Please read forum rules, as Will suggested.

[Will]

                                                        
[H⁺] = _/K_a x [C₆H₄(COOH)(OAc)]

Wrong. This equation (8.13 in terms of the lecture pointed to below) holds only for dissociation percentage below 5%.

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

Woops! That is proof of just how cr*ppy the A-Level syllabus is.

If I did it my way I would've gotten an inaccurate pH of 3.019085045, when in fact the real pH is 3.093596791 (I think!).

[Borek]

Woops! That is proof of just how cr*ppy the A-Level syllabus is.

If I did it my way I would've gotten an inaccurate pH of 3.019085045, when in fact the real pH is 3.093596791 (I think!).

Hmm, either your syllabus is even more crappy or my BATE isn't worth money I am asking for

2.75 here (2.8 to be correct in terms of significant digits).

[Will]

Woops! That is proof of just how cr*ppy the A-Level syllabus is.

If I did it my way I would've gotten an inaccurate pH of 3.019085045, when in fact the real pH is 3.093596791 (I think!).

Hmm, either your syllabus is even more crappy or my BATE isn't worth money I am asking for

2.75 here (2.8 to be correct in terms of significant digits).

It's probably me, my room is boiling hot and I can't think properly!

Edit: my calculator messed me up on the concentration, it showed me 0.002775... instead of 0.0111014...

[Will]

The pH I get now is 2.755446731 or 2.76 to 2dp, probably due to the small difference in concentration (I used 180.15742 as M_r) and I would've gotten a pH of 2.718054109 if I used the inaccurate equation.

[Borek]

Other probable source of difference is Ka value - BATE requires logarithmic input, so it got 3.48 for pKa, instead of 3.3*10^-4. But 2.75 or 2.76 is perfectly acceptable here.

Note that BATE uses accurate value for molar mass, it just displays only two decimal digits.