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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. Number the following 0.1 M solutions in order of increasing pH (1 being the lowest and 13 being the highest pH value) and state why you placed each solution in that position: NaCH3COO, HCl, HCN, NaOH, NH3, NaCN, KNO3, H2SO4, NH4Cl, H2SO3, NaHCO3, Na3PO4 and CH3COOH. Use _ for subscripts.
Order Solution Reasoning
1  
2  
3  
4  
5  
6  
7  
8  
9  
10  
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12  
13  

 

Once you have predicted the nature of each salt solution, you will use Beyond Labz 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.

 

  1. To start this part of the activity, click this link for Ranking Salt Solutions by pH. The lab will open in the Titrations laboratory.
  2. 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 below. When finished, drag the beaker to the red disposal bucket.
  3. Open the Stockroom tab in the tray. Click the trash can to the right of NH4Cl in the Acids menu to remove it from the table and then select the NaHCO3 and KNO3 bottles from the Base and Inert Salt menus to move them to the counter.
  4. 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). Each scoop position on the face of the bottle represents a different size scoop. Click the the third image from the top (approximately 0.20 g) and drag the scoop 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 that will empty the salt into the beaker. Close the balance view and drag the beaker to the stir plate.
  5. 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 hover over it. 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. Move the bottle of NaHCO3 back to the counter. Repeat steps 5 and 6 for KNO3.
  6. In the Stockroom tab delete the NaHCO3 and KNO3. Select the NH3 and H2SO4. 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 holding the bottle over 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. 
  7. Repeat step 7 with H2SO4, except that you should use a 10 mL graduated cylinder of H2SO4 and add 15 mL water.
  8. Each of the other solutions is already approximately 0.1 M. With these solutions you can pour a small amount into a 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). 

Data Table

Solution pH
NH4Cl
NaHCO3
KNO3
NH3
H2SO4
HCl
H2SO3
CH3COOH (HAc)
HCN
NaOH
NaCN
NaCH3COO (NaAc)
Na3PO4