51 Experiment 9: Buffer Investigation Exp 9: Investigation of Buffers Objectives

Question

51 Experiment 9: Buffer Investigation Exp 9: Investigation of Buffers Objectives Irvestigate how buffers pft changes upon (1) mixing ratios of acid/conjugate base. (2) addition of a strong acid or base, and (3) dilution. Introduction The hydronium ion concentration or pH is an exceedingly important characteristic of aqueous solutions. For example, the speed and direction of many chemical reactions are influenced by the prevailing pH of the solution. This is particularly true of biochemical systems where the correct action of many enzymes is critically dependent upon the pH of the solution. is the name given to the condition that occurs when the blood pH is les than normal. Acidosis is one of the results of uncontrolled diabetes mellitus. For good health, the body must have a mechanism for the control of pH. It does this through the use of chemicals that are known as buffers. The establishment and control of the hydronium ion concentration is an important goal not only in living systems but in many other laboratory experiments as well. In this experiment you will discover some of the theoretical and practical considerations necessary to achieve this objective. Pre-Lab Questions 1. What is the pH of 200 mL of a 0.15 M aqueous solution of the strong acid HNOs at 25°C? At 25°C, one dilutes the solution in problem 1 with 100 mL of water. Do you expect the pH to change, remain the same, or decrease? Justify your answer. Read through the experimental procedure. What data will be important to record for this experiment? Propose a data table for each of the three different investigations (at least three tables) to include in your laboratory notebook. 2. 3. 4. If you wanted to make a 20 mL. solution that was 1/9 solution A/solution B, what volume of A and what volume of B would you add, respectively. The Ka of acetic acid is 1.8 x 10% at 25°c. a) What is the pH of a 0.5 M solution of acetic acid at 25°C? 5. b) What is the pH of a solution made up of 20 mL of 0.5 M solution of acetic acid and 10 mL of 0.5 M sodium acetate? c) If 10 mL of 0.1 M NaOH is added to the solution in part b, what is the final pH? d) What is the pH of a solution containing equal concentrations of acetic acid and sodium acetate?

Explanation / Answer

1)

HNO3 is a very strong acid

so it undergoes 100 % dissociation

HNO3 ---> H+ + NO3-

all of the HNO3 will be converted to H+

so

[H+] = 0.15 M

pH = -log [H+]

pH = -log 0.15

pH = 0.8239

2)

now upon dilution , concentration of H+ will decrease , so pH increases

now

for dilution

C1V1 = C2V2

0.15 M x 200 ml = C2 x 300 ml

C2 = 0.1 M

so

[H+] = 0.1 M

now

pH = -log [H+]

pH = -log 0.1

pH = 1

so

the pH has increased from 0.8239 to 1

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