Acids, Bases, and p H Buffers SA answers - Drylab PDF

Preview

Summary

Download Acids, Bases, and p H Buffers SA answers - Drylab PDF

Description

Acids, Bases, and pH Buffers Experiment 1: Introduction to pH Indicators Lab Results 1. Fill in the table with the results from your lab notes. light blue Experiment #1: Introduction to pH indicators Solution water acetone 5 M citric acid 5% vinegar 4 M ammonia diluted bleach

Color after addition of bromothymol blue light blue 7.00 yellow yellow dark blue dark blue

pH after addition of bromothymol blue 7.00 7.00 1.22 2.40 11.92 10.67

Data Analysis 1. Based on your results with bromothymol blue, what color would a solution that was pH 13.5 be? blue

Experiment 2: The Phosphate Buffer System Lab Results 1. Fill in the following table with results from your lab notes. Results HCl pH

Tube Name water

7.00

pH after HCl 2.01

NaH2PO4

4.68

3.12

Na2HPO4

9.72

8.16

Change in pH dropped by 4.99 dropped by 1.56 dropped by 1.56 dropped by 0.18

NaOH pH 7.00

pH after NaOH 11.99

4.68

6.25

9.72

11.29

Change in pH increased by 4.99 increased by 1.57 increased by 1.57 increased by 0.17

7.21 7.03 7.21 7.38 NaH2PO4 and Na2HPO4 Data Analysis 1. Which solution was the most sensitive to the addition of acid or base? Tube I-Water 2. Which solution was the least sensitive to the addition of acid or base?

NaH2PO4 and Na2HPO4 3. Which of the four solutions tested is the best buffer against changes in pH caused by the addition of acid or base?

Sodium Dibasic Phosphate and Sodium Monobasic Phosphate

Experiment 3: The Buffering Capacity of a Phosphate Buffer Lab Results 1. In the table below, record the results of the pH of the phosphate buffer solution versus the number of drops of HCl acid and NaOH base added to it. Phosphate Buffer Results Drops Acid pH Drops Base pH 0 0 7.21 7.21 1 1 7.12 7.29 2 2 7.03 7.38 3 3 6.94 7.47 4 4 6.84 7.57 5 5 6.73 7.68 6 6 6.60 7.82 7 7 6.45 7.96 8 8 6.25 8.16 9 9 5.93 8.48 10 10 4.68 9.72 11 11 3.44 10.95 12 12 3.12 11.28 13 13 2.93 11.48 14 14 11.63 15 15 11.75 2. The strong acid and strong base used in this lab are the same concentration. Thus, the magnitude of the pH change caused by addition of a single drop will be the same on either side of the starting pH. Addition of acid lessens the pH, and addition of base raises the pH. Construct a graph of the pH versus the number of drops of acid or base added to the phosphate buffer. To show the relative magnitudes, use a negative value for each drop of acid and a positive value for each drop of base. For example, use -7 to write 7 drops of acid and 5 to show 5 drops of

base.

Data Analysis 1. Suppose you had a buffer containing 0.5 moles of sodium monobasic phosphate and 0.5 moles of sodium dibasic phosphate. How many moles of hydrochloric acid would this phosphate buffer be able to accept before the pH of the solution began to change drastically? At least 5 moles of HCI would be needed in order to change pH level by one point.

Conclusions 1. Which chemical provides the conjugate base in the buffer containing NaH2PO4 and Na2HPO4? Na2HPO4 2. Explain why phosphate buffer needs both Na2HPO4 and NaH2PO4 in order to resist changes in pH.

The equilibrium shifts to the right when additional hydroxide ions are added. However the equilibrium shifts to the left if hydrogen ions are added additionally. 3. Predict what might happen if you made up phosphate buffer with only half as much NaH2PO4 compared to Na2HPO4. The pH level would become 6.30, acidic due to the decrease of the buffering capacity of the solution. 4. Your lab mate attempts to use bromothymol blue to differentiate between two solutions – one that should be pH 7.3, and another that should be pH 6.7. What is your advice to your lab mate? Do you agree with his decision? I do not agree with my lab mate's decision because there will not be change in color. Bromothymol blue only shows change in color when the pH level is 7.6 and above or 6.00 and below....