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Buffers. This experiment lets us to know more about the buffer solutions and how the pH changes by adding acid and base to the buffers.

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Introduction

Investigation 20 Buffers Aim: This experiment lets us to know more about the buffer solutions and how the pH changes by adding acid and base to the buffers. Background: 1. Buffer solution is a kind of mixture solution that can resist a change of pH when a small amount of a strong acid or a strong base is added to them, are known as buffer solutions. Usually it is made up of a weak acid and its salt or of a weak base and its salt.[1] 2. Acid, base and salt are electrolytes that can dissociate in water to form ions. Usually a strong acid and base are strong electrolytes that can fully dissociate in water such as HCl, hydrochloric acid and NaOH, sodium hydroxide. Weak base and acid are weak electrolyte such as acetic acid and ammonium hydroxide. But all the salt can fully dissociate in water. 3. Strong acid have a pH about 1 or 2 and strong base have a pH about 13 or 14. Weak acid have a pH about 3 or 4 and weak base has pH about 11 or 12. Water is neutral at 298K that its pH is 7. Method: To examine the pH change of the buffer solutions when adding acid and base, the pH changes of the buffer solutions and that of other solutions are needed to be compared. ...read more.

Middle

Put the buffer solution equally into beaker 2 and 4. 9. Get 10ml of NaHCO3, NaHPO4 and water separately into beaker 5, 6 and 7. 10. Use the pH paper to measure the approximate pH value of beaker 1, 3, 5, 6 and 7. 11. Use the pH meter to measure the exact pH value of beaker 1, 3, 5, 6 and 7. 12. Add one drop of HCl into beaker 1, 3, 5, 6 and 7. 13. Measure the pH value again by pH meter and pH paper. 14. Add two drops of NaOH into the five beakers just measured. 15. Measure the pH one more time by pH meter and pH paper. 16. Put the pH meter into beaker 2 and 4. 17. Add HCl to the two solutions drop by drop until the pH reaches 2 18. Record the number of drops needed. Raw data: Intial pH of beaker 1, 3, 5, 6 and 7 Beaker pH by pH paper pH by pH meter 1 9 9.90 3 7 6.80 5 8 8.43 6 8 9.15 7 6 7.85 pH of beaker 1, 3, 5, 6 and 7 after adding HCl Beaker pH by pH paper pH by pH meter 1 10 9.83 3 7 6.70 5 8 7.65 6 8 7.82 7 2 2.35 pH of ...read more.

Conclusion

Conclusion: 1. Buffer solutions, the mixture solutions, can resist a great pH change when adding small amount of acid and base into that. 2. Solution 1, the mixture of Na2CO3 and NaHCO3 is a better buffer solution than solution 2, the mixture of Na2HPO4 and NaH2PO4. 3. Water cannot resist a pH change at all. Evaluation and improvement: 1. The pH value of water is much greater than the literature value. The beaker must contain some basic solutions that affect the pH of water. Next time the beakers must be fully cleaned and dried so there will be no impurities. 2. When adding HCl to beaker 2 and 4, the final pH is far less than 2. This is because the acid was added so fast and the whole solution was not stirred very well. Next time the solution should be added slowly, drop by drop. After adding one drop, the whole solution should be stirred for a long time. 3. The pH meter has been put into different solutions. It may affect the result. Next time after each trial, take the detector of the pH meter out and clean it. Then put it into another solution. Reference: [1] Buffer solution John Green and Sadru Damji, Chemistry 3rd edition (Victoria: IBID press, 2007), P221 ?? ?? ?? ?? Investigation 20 Name: Yang Yang Student No.: 2007216 Partner's name: Yi Liu Page 1 of 6 ...read more.

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