Design - Investigate the effect of one factor on the experimentally determined pKa of a weak acid

Ka could be calculated with pH.

           

The Ka value of propanoic acid with water would be;

CH3COOH + OH- ⬄ CH3COO- + H2O

As the Ka value gets bigger, the stronger the acid is. Ka could be expressed as pKa which is the logarithm of Ka.

As the pKa value gets smaller, the stronger the acid is. However, it is preferable to measure in Ka value instead of pKa value, because adding excess amount of water will affect the pKa value but not Ka value.

Defining Independent and Dependent Variables:

Controlling Variables:

Materials and Equipments List:

• 100ml Measuring Cylinder
• Beaker
• Thermometer
• 1M of ethanoic acid
• Hydroxide (concentration of 0.1, 0.2, 0.5, 1, 2, 4M)
• GLX
• pH meter

Method:

1. Measure 20ml of 1M of ethanoic acid and 20ml of 0.1M of hydroxide and place them into a beaker.
2. Use GLX and pH meter to find the pH of the solution.
3. Calculate the Ka from the pH
4. Calculate the pKa from the Ka by taking negative log of Ka.

5. Repeat step 1 to 4, four more times to minimise the uncertainties
6. Repeat step 1 to 5, with different concentration of OH-        

 

Results:

Table 2 – 0.1M of OH-

Table 3 – 0.2M of OH-

Table 4 – 0.5M of OH-

Table 5 – 1M of OH-

Table 6 – 2M of OH-

Table 7 – 4M of OH-

Bibliography:

Avogadro.co.uk. Acid Dissociation Constant, Ka. 2007. 31 01 2010 <http://www.avogadro.co.uk/definitions/acidka.htm>.

ChemTeam, The. Calculate the Ka of a Weak Acid Given the pH and Molarity. 02 02 2010 <http://www.chemteam.info/AcidBase/Calc-Ka-from-data.html>.

Publishing, Integrated. What is the hydrogen ion concentration [H ] and the hydroxyl concentration + [OH ] in a solution with a pH of 5.5? 31 01 2010 <http://www.tpub.com/content/doe/h1015v1/css/h1015v1_72.htm>.