In this experiment we will determine the pH of five salt solutions by using the universal indicator to check for the pH changes and phenolphthalein to detect the alkalinity.
HYPOTHESIS:
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.
MATERIAL AND EQUIPMENT:
Test tubes
Test tube rack
Pipettes
500ml conical flasks
Analytical balance up to 3 digits
Universal indicator (range 0-14)
Phenolphthalein
KI
Na2CO3
SrCl2
NH4C2H3O2
Fe(NO3)3
METHOD:
First we prepare the five salt solutions that we are going to test for their pH. We weight the proper amount of each substance (as shown in the calculations) and dissolve it to 500ml of water. The concentration of each substance should be 0.1M. After producing the five salt solutions, we take 5ml of each and place it in 5 test tubes that we mark A, B, C, D and E. We place the test tubes on the rack. Using a drop pipette, we add two drops of phenolphthalein in each test tube. Each solution is tested for its pH using the universal indicator.
CALCULATIONS:
KI: MM= 166
n = c x v → n = 0.1 x 0.5 = 0.05 moles
g = n x MM = 0.05 x 166 = 8.3g
Na2CO3: MM = 105.98
n = c x v → n = 0.1 x 0.5 = 0.05 moles
g = n x MM = 0.05 x 105.98 = 5.2g
SrCl2: MM = 266.62
n = c x v → n = 0.1 x 0.5 = 0.05 moles
g = n x MM = 0.05 x 266.62 = 13.33g
NH4C2H3O2: MM = 77
n = c x v → n = 0.1 x 0.5 = 0.05 moles
g = n x MM = 0.05 x 77 = 3.85g
Fe(NO3)3: MM = 241.85
n = c x v → n = 0.1 x 0.5 = 0.05 moles
g = n x MM = 0.05 x 241.85 = 12.1g
DATA AND OBSERVATIONS:
Equations for the dissociation of each salt:
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KI → K+ + I-
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Na2CO3 → 2Na+ + CO3-
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SrCl2 → Sr2+ + 2Cl-
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NH4C2H3O2 → NH4- + CH3COO-
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FE(NO3)3 → Fe3+ + 3NO3-
Ionic Equations:
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K+ + I- + H2O → K+ + HI + OH-
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Na2CO3 + H2O → HCO3-2 + Na+2 + OH-
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Strong acid + strong base → cannot take place
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NH4C2H3O2 + H2O → NH4OH + C2H3O2 + H2
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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. Salts formed from a weak acid and a strong base hydrolyse to form a solution that is slightly basic.
The use of phenolphthalein is to test the solutions for their alkalinity, since this indicator can detect pH changes above the value of 8. The only case where a pH change above 8 is detected is in solution 2 that contains Na2CO3. Indeed Na2CO3 derives from the weak acid HCO3 and the strong base NaOH and the pH is 12, that is basic. In all the other solutions the pH change cannot be detected by phenolphthalein, that is their pH is in all cases below 8.
In the first solution containing KI, the anion of the salt comes from the strong base KOH and the cation comes from the weak acid HI. The salt solution has to be basic, but it appears to be slightly acidic with a pH value of 6. That might be due to the lack of precision of the universal indicator. If we had used a pH meter the results would have been mush more accurate. However, the error might also be due a misinterpretation of the results shown by the universal indicator. Between the values of 6 and 8, the color of the universal indicator changes to a different green, we could have possibly seen wrongly the color. Another possible explanation would be that excessive phenolphthalein was added, making the solution more acidic.
In the third solution containing SrCl2, the anion derives from the strong base Sr(OH)2 and the cation from the strong acid HCl. 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, salts form neutral aqueous solutions. That is proved in the case of solution 3, because the pH is indeed found 7, that is neutral.
In the forth solution that contains NH4C2H3O2, the anions are derived from the weak base NH4(OH) and the cations from the weak acid CH3COOH. The solution is found to have a neutral pH of 7. The pH in this case reflects the relative strengths of the acid and the base.
In the case of solution 5 that contains Fe(NO3), the anion is derived from the weak base Fe(OH)3 and the cation from the strong acid HNO3. Salts formed from a strong acid and a weak base hydrolyse to form a solution that is acidic. That is proven in this case, since the pH of the solution is very acidic with a pH value of 1.
From the ionic equations that have resulted, we may see whether a solution where a particular reactions takes place is acidic or alkaline. For instance, in the case of the first and the second reaction we can see the hydroxide ions that are responsible for the alkalinity of the solution (K+ + I- + H2O → K+ + HI + OH-,
Na2CO3 + H2O → HCO3-2 + Na+2 + OH-). In the case of the fifth solution, we are able to detect its acidity from the ionic equation due to the presence of the hydroxide ions (Fe(NO3)3 + 2H2O → Fe(OH)3 + 3NO3- + 4H+).
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:
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K+ + I- + H2O → K+ + HI + OH- (potassium ion)
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Na2CO3 + H2O → HCO3-2 + Na+2 + OH- (sodium ion)
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Strong acid + strong base → cannot take place
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NH4C2H3O2 + H2O → NH4OH + C2H3O2 + H2 (ethanoic acid)
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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.
