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The aim of this experiment is to determine the relative acidity or alkalinity of various salt solutions and determine their pH.

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Introduction

Experiment #12 HL November, 15/2002 Salt hydrolysis AIM: The aim of this experiment is to determine the relative acidity or alkalinity of various salt solutions and determine their pH. INTRODUCTION: Many salts dissolve in water to give neutral solutions. Some salts however, react with water to form acidic or alkaline solutions. These reactions are described as salt hydrolysis. A salt is an ionic compound containing positive ions other than H+ and negative ions other than OH-. The ions contained in salts can act as acids or bases in an aqueous solution. Cations act as acids that react with water to produce hydronium ions and anions as bases that react with water to produce hydroxide ions. Salts can be thought as being derived from the neutralization of an acid or a base. A salt formed from a strong acid and a strong base will not react with water. It will just dissociate and its ions will remain uncombined in the solution. The stronger the conjugate acid/base they are derived from, the weaker the acid/base activity of the ion. The cations derived from strong bases have little acid/base activity and the same happens with the anions derived from strong acids. Therefore, such salts form neutral aqueous solutions. If the anion is derived from a weak acid then the anion will act as a weak base so the solution of the salt of a strong base and a weak acid has a pH less than 7. ...read more.

Middle

Weak 5 Fe(NO3)3 No change HNO3 Strong Fe(OH)3 weak Equations for the dissociation of each salt: 1. KI --> K+ + I- 2. Na2CO3 --> 2Na+ + CO3- 3. SrCl2 --> Sr2+ + 2Cl- 4. NH4C2H3O2 --> NH4- + CH3COO- 5. FE(NO3)3 --> Fe3+ + 3NO3- Ionic Equations: 1. K+ + I- + H2O --> K+ + HI + OH- 2. Na2CO3 + H2O --> HCO3-2 + Na+2 + OH- 3. Strong acid + strong base --> cannot take place 4. NH4C2H3O2 + H2O --> NH4OH + C2H3O2 + H2 5. Fe(NO3)3 + 2H2O --> Fe(OH)3 + 3NO3- + 4H+ DATA ANALYSIS: When the salt is hydrolyzed and the ions contained can act as acids or bases in an aqueous solution. Cations act as acids that react with water to produce hydronium ions and anions as bases that react with water to produce hydroxide ions. According to the theory, salts derived from a strong acid and a strong base will just dissociate and remain uncombined giving a neutral solution. Salts which anions are derived from a weak acid, then the solution will be acidic, giving a pH less than 7. If the cation is derived from a weak base, then the solution will be greater than 7. So, salts formed from a strong acid and a weak base hydrolyse to form a solution that is slightly acidic. ...read more.

Conclusion

In each ionic equation we may observe spectator ions. These ions exist in both the reactants and the products, but they do not take part in the actual reaction. The spectator ions in each ionic equation are: 1. K+ + I- + H2O --> K+ + HI + OH- (potassium ion) 2. Na2CO3 + H2O --> HCO3-2 + Na+2 + OH- (sodium ion) 3. Strong acid + strong base --> cannot take place 4. NH4C2H3O2 + H2O --> NH4OH + C2H3O2 + H2 (ethanoic acid) 5. Fe(NO3)3 + 2H2O --> Fe(OH)3 + 3NO3- + 4H+ (nitrate ion) No experimental errors seem to have happened during the experiment. However there was a possible misinterpretation of the results in the case of solution 1. For more accurate results of the pH values we could have used a pH meter instead of the universal indicator. CONCLUSION: The hypothesis that salts formed from a strong acid and a weak base will give a slightly acidic solution, whereas salts formed from a weak acid and a strong base will give a slightly basic solution was confirmed. Examples indicating this are solutions 2 and 5. In solution 2 the salt originates from a strong acid and a weak base and the pH is very acidic with a value of 1. In solution 5, where the salt derives from a weak acid and a strong base and the solution is alkaline, giving a pH value of 12. ...read more.

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