Ranking Salt Solutions by pH

6-2: Ranking Salt Solutions by pH

In this assignment you will be asked to rank aqueous solutions of acids, bases, and salts in order of increasing pH. This is most easily done by first identifying the strong acids that have the lowest pH, the strong bases that have the highest pH, and the neutral solutions that have a pH near 7. The weak acids will have a pH between 1 and 6 and the weak bases between 8 and 14. The exact order of weak acids and weak bases is determined by comparing the ionization constants (Ka for the weak acids and Kb for the weak bases). After ranking the pH of these solutions, you will then test your predictions in the laboratory.

1. Arrange the following 0.1 M solutions in order of increasing pH and state why you placed each solution in that position: NaCH3COO, HCl, HCN, NaOH, NH3, NaCN, KNO3, H2SO4, NH4Cl, H2SO3, NaHCO3, Na3PO4 and CH3COOH.

In order of increasing pH:

1.HCl most powerful acid.
2.H2SO4 a strong acid
3.H2SO3 weak but stronger than 4.HCN from experiments
5.CH3COOH weak acid
6.NH4Cl is an acidic salt
7.KNO3 is Neutral salt
8.NaCH3COO NaCN NaHCO3 Na3PO4 are basic salts.
9.NH3 weak Base
10.NaOH strong Base

After doing the experiment this is the order that I got:

NH4Cl
H2SO4
HCl
H2SO3
KNO3
NaHCO3
CH3COOH
NH3
HCN
NaCH3COO
NaCN
Na3PO4
NaOH

Once you have predicted the nature of each salt solution, you will use Virtual ChemLab to confirm your prediction. Each solution must be approximately 0.1 M for your comparisons to be valid. Most of the solutions in the Stockroom are approximately 0.1 M already. Two solutions will need to be diluted and three solutions will need to be prepared from solid salts. One of these salt solutions is already prepared and on the lab bench ready for you to measure the pH.

2. Start Virtual ChemLab , select Acid-Base Chemistry, and select Ranking Salt Solutions by pH from the list of assignments. The lab will open in the Titrations laboratory.

3. On the stir plate, there will be a beaker of 0.10 M ammonium chloride (NH4Cl) that has already been prepared. The pH meter has been calibrated and is in the beaker. Record the pH of the NH4Cl solution in the data table on the following page. When finished, drag the beaker to the red disposal bucket, and drag the bottle of NH4Cl to the stockroom counter.

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Virtual ChemLab: General Chemistry, Student Lab Manual/Workbook, V. 4.5, by Brian F. Woodfield, Matthew C. Asplund, and Steven Haderlie. Published by Prentice Hall. Copyright © 2013 by Pearson Education, Inc. Acid-Base Chemistry

4. Click in the Stockroom to enter. Double-click on the NH4Cl bottle to return it to the shelf and then double-click on the NaHCO3 and KNO3 bottles to move them to the Stockroom counter. Return to the laboratory

5. Open the beaker drawer (click on it) and drag a beaker to the spotlight next to the Balance. Click and drag the bottle of NaHCO3 and place on the spot light near the balance. Click in the Balance area to zoom in. Place a weigh paper on the balance and tare the balance. Open the bottle by clicking on the lid (Remove Lid). Pick up the Scoop and scoop up some salt by dragging the Scoop to the bottle and then down the face of the bottle. Each scoop position on the face of the bottle represents a different size scoop. Pull the scoop down from the top to the second position (approximately 0.20 g) and drag it to the weigh paper in the balance until it snaps into place. Releasing the scoop places the sample on the weigh paper. Now drag the weigh paper from the balance to the beaker until it snaps into place and then empty the salt into the beaker. Return to the laboratory and drag the beaker to the stir plate.

6. Drag the 25 mL graduated cylinder to the sink under the tap until it fills. When filled, it will return to the lab bench and will indicate that it is full when you place the cursor over the cylinder. Drag the 25 mL cylinder to the beaker on the stir plate and empty it into the beaker. Place the pH probe in the beaker and record the pH in the data table. Drag the beaker to the red disposal bucket. Double-click the bottle of NaHCO3 to move it to the Stockroom counter. Repeat steps 5 and 6 for KNO3.

7. Click in the Stockroom. Double-click on the bottles of NH3 and H2SO4 to move them from the shelf to the counter and return to the laboratory. Drag the bottle of NH3 to one of the three spotlights on the lab bench. Place a beaker from the drawer on the stir plate. Drag the bottle of NH3 to the 5 mL graduated cylinder (the smallest one) by the sink and fill the cylinder by dropping the bottle on the cylinder. Now drag the 5 mL graduated cylinder to the beaker on the stir plate and add the 5 mL of NH3. Add 20 mL water to the beaker by filling and emptying the 10 mL cylinder into the beaker twice. Place the pH probe in the beaker and record the pH in the data table. Drag the beaker to the red disposal bucket. Double-click on the NH3 bottle to move it back to the counter.

8. Repeat step 7 with H2SO4, except that you should use a 10 mL graduated cylinder of H2SO4 and adding 15 mL water.

9. Each of the other solutions is already approximately 0.1 M. With these solutions you can pour a small amount into the beaker that you have placed on the stir plate and place the pH probe in the solution to measure the pH. Record the pH of each solution in the data table. Drag each beaker to the red disposal bucket when you have finished. You must determine the pH for HCl, H2SO3, CH3COOH (HAc), HCN, NaOH, NaCN, Na3PO4, and NaCH3COO (NaAc). You may take two bottles at a time from the stockroom. Data Table
|solution |pH |solution |pH |
|NH4Cl | |CH3COOH (HAc) | |
|NaHCO3 | |HCN | |
|KNO3 | |NaOH | |
|NH3 | |NaCN | |
|H2SO4 | |NaCH3COO (NaAc) | |
|HCl | |Na3PO4 | |
|H2SO3 | | | |

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Virtual ChemLab: General Chemistry, Student Lab Manual/Workbook, V. 4.5, by Brian F. Woodfield, Matthew C. Asplund, and Steven Haderlie. Published by Prentice Hall. Copyright © 2013 by Pearson Education, Inc.
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ISBN 0-558-58862-X

ISBN 0-558-58862-X