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

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Introduction

HL Chemistry Design Prac Investigate the effect of one factor on the experimentally determined pKa of a weak acid. _______________________________________________________ Research question: How will changing the concentration of the reaction ethanoic acid affect the experimentally determined pKa value of ethanoic acid with hydroxide? Background Research: In chemistry, the acid dissociation constant, Ka, is known as the equilibrium constant for the reaction between weak acid and conjugate base, both in aqueous state. A + B <=> C + D To calculate the value of Ka in the reaction above, the concentration of C and D over A and B is to be calculated. ...read more.

Middle

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: Independent Concentration of OH- * 0.1M * 0.2M * 0.5M * 1M * 2M * 4M Dependent Concentration of ethanoic acid Controlling Variables: Table 1 - Variables and Method of control Type Variable Method of Control Controlled Amount of ethanoic acid Use 100ml measuring cylinder (�0.5ml) One person measuring the amount of propanoic acid to reduce the error in reading Amount of hydroxide Use 100ml measuring cylinder (�0.5ml) ...read more.

Conclusion

* 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- Trials pH Ka pKa 1 2 3 4 5 Table 3 - 0.2M of OH- Trials pH Ka pKa 1 2 3 4 5 Table 4 - 0. ...read more.

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