pH of Salt Solutions
In lab this week we will prepare three salt solutions, calculating and measuring the resulting solution pH in each.
A pH meter is the best way to determine the pH of a solution. pH meters, on the other hand, can be very finicky instruments. A much quicker way to get a measure of a solution pH is a universal indicator. A universal indicator has a color associated with each pH unit over a wide pH range. In this buffer lab we will use a universal indicator useful over the pH 4 to 10. The color : pH code is:
pH 12: blue (grayish)
pH 11: blue (greenish)
pH 10: green (bluish)
pH 9: green
pH 8: green (yellowish)
pH 7: yellow (greenish)
pH 6: yellow (orangish)
pH 5: yellow
pH 4: orange
pH 3: orange (reddish)
pH 2: red
The last page of the lab has the equilibrium constants (Ka and Kb) that you will need in your calculations.
Ammonium nitrate (NH4NO3) solution
There are volumetric flasks of differing sizes at your table. Select a volumetric flask to prepare a 0.18 M solution of NH4NO3.
As you prepare this solution use a plastic weigh boat to measure the NH4NO3 solid. Remember do not add solid to the weigh boat while it is on the balance.
Add the appropriate mass of the solid NH4NO3to the flask and fill to the line with DI water. As you are filling you can add water directly from the tap, but as you get close to the volumetric mark add the water from a squirt bottle.
Place parifilm over the top of the flask and invert the solution (thumb over the top) until the salt dissolves. Is this dissolving endothermic or exothermic?
In your notebook neatly show: 1.) the chemical equations, 2.)I.C.E. table, and 3.) the equilibrium calculations necessary to determine the pH of the solution.
After calculating the pH, measure the pH by pouring approximately 70 mL of the solution into a small beaker. To the solution in the small beaker add three drops of universal indicator. Use the color chart to record the solution pH to one significant digit. (This is of course an estimate.)
After recording the solution pH you can pour the salt mixtures into the waste beaker in the hood.
Calculating and Measuring pH in a Polyprotic System
Sodium hydrogen carbonate (NaHCO3) is an inorganic salt that could behave as an acid or as a base in water. In this salt, sodium (Na+) plays the role of a spectator ion providing a way for the HCO3ion to enter the solution. In your notebook write the two reactions that HCO3 can undergo in water (in one case it acts as an acid and in the other as a base.)
(base) HCO3 + H2O
(acid) HCO3
Examine the Ka and Kb table at the end of this handout and decide which reaction predominately occurs. It turns out that one reaction is 10,000 more prevalent than the other reaction and therefore only one reaction needs to be considered when determining pH.
Calculate the resulting solution pH if the initial salt concentration is [NaHCO3] = 0.3 M. Be clear in showing the calculations, this is primarily what will be graded in this lab work.
Prepare 100 mL of a 0.3 M solution of NaHCO3 using a 100 mL volumetric flask (obtain from your TA). From your analysis above calculate the pH that you expect for this solution. What color will this solution be when universal indicator is added?
Pour some of your solution into your small beaker and add three to five drops of the universal indicator that will allow you to measure the solution pH.
After measuring the solution pH (confirming your pH calculation) you can place the solutions into the waste beaker.
Preparing a solution of predetermined pH
Most common in laboratory work, is preparing a solution of specific pH. The adjustment of pH can be done by determining the initial concentration of a salt needed to produce a desired pH.
From your teaching assistant receive a random pH for you toprepare a 100 mL solutionof that pH. The solution pH will be obtained by dissolving the appropriate amount of the correct salt in 100 mL of H2O.
There are several salts available at the front of the room (NaCH3CO2, NaHSO4, NaH2PO4, and Na2CO3). Some of these salts will prepare solutions that are basic, pH > 7, and some will prepare acidic solutions, pH < 7. You and your partner must decide which salt is appropriate to use to reach your assigned pH.
The ground rules are that you cannot use more than 2.5 grams of the salt. If your calculations show that you need a large amount of the salt (ex. 12.3 g) to make the desired pH you must select a salt with a larger Kb or Ka, so that you do not need as much of the salt.
In your notebook show the calculations you make to arrive at the initial concentration of salt required to prepare the desired pH. From the initial concentration show the calculations for the grams of solid that you will need to add to 100 mL of water to prepare the solution. Note: Some of these salts come as hydrates, so you should check the formula weight on the bottle.
After making your calculations (and showing them to your instructor), prepare this solution in your 100 mL volumetric flask.
Pour enough of the salt solution into your small beaker that you can measure the solution pH with the universal indicator. Record your solutions pH in your notebook.
After completing all of the pH measurements and calculations for this lab, clean and put away the glassware you used leave the volumetric glassware on your lab table. Turn in the duplicate sheets from your laboratory notebook. There is not an abstract for this lab work.