Preparation of Test Tubes Obtain a test tube rack and 10 test tubes. Using a bur
ID: 1039507 • Letter: P
Question
Preparation of Test Tubes
Obtain a test tube rack and 10 test tubes. Using a burette, add exactly 5.0 mL of water to each of
the 10 test tubes. Mark the level of the water on each test tube with a marker or grease pencil.
Label two test tubes as “45 °C”, two as “35 °C”, two as “25 °C”, two as “15 °C”, and two as “5
°C”. Empty the test tubes and allow the water to drain thoroughly.
Preparation of the Saturated Borax Solution
Warm a mixture of about 30 g of borax and 150 mL of water in a 250-mL beaker to at least 45 °C,
but no higher than 50 °C. It is not necessary to record the exact mass of the borax that is added to
water. It is, however, important that the borax solution be saturated. Therefore, if all of the solid
dissolves, add a little more borax until excess solid is present and the solution is truly saturated.
While stirring the borax mixture, allow it to cool to approximately 45 °C. Stop stirring the
solution, and let the borax settle briefly. Then, as quickly as possible:
• Measure the temperature of the solution.
• Pour 5.0 mL of the solution (with no solid!) into one of the test tubes marked 45 °C.
Because the solution will be hot, you will want to hold the test tube with a wire test tube
holder. (Note: when you pour the hot borax solution into a cold test tube, the borax will
instantly precipitate against the cold glass. Don’t worry about this. You will be
redissolving the precipitate in a later step in the lab. The important thing is to not pour
any solid from the beaker into the test tube.)
• Take another temperature measurement.
• Pour another 5.0 mL sample into the second test tube marked 45 °C.
• Take another temperature measurement.
Allow the solution in the 250-mL beaker to cool to approximately 35 °C and take two samples of
the solution as described above. Repeat the sampling procedure for temperatures close to 25 °C,
15 °C, and 5 °C. You will need to use an ice bath to cool the solutions to temperatures lower than
room temperature.
-Obtain about 75 mL of the standardized HCl (about 0.20 M) in a clean, dry beaker. Record
the exact concentration of the HCl solution.
• Rinse and fill a 50-mL burette with the HCl solution so that there are no air bubbles along
the inside surface of the burette. Record your initial burette reading. Burette readings
should be recorded to two decimal places.
• When the borax in the 45 °C test tube has dissolved, pour the 5-mL sample into a clean
125-mL Erlenmeyer flask. Rinse the test tube with several portions of the warm rinse
water, transferring the rinse solutions to the flask and swirling the mixture. Add enough
water to the flask to keep the borax completely dissolved at room temperature and swirl
the mixture again. Add three drops of bromocresol green indicator to the flask.
(Bromocresol green is blue in basic solution and yellow in acidic solution.)
• Titrate the solution with the standardized HCl. The endpoint is reached when all of the
blue color disappears and the solution is yellow. Record your final burette reading.
• Repeat the above procedure for all of the other borax samples. Each lab group should
titrate one sample at each temperature, for a total of 5 titrations. (You collected two sets of
samples so you have back-ups in case there was a problem with one of your titrations.)
• Dispose of all solutions properly, based on the instructions provided by your TA.
????with this setting, if we got HCl value - 10ml, how do i find M of borax and HCl??
Explanation / Answer
Borax reaction with HCl is given by the eqn:
Na2B4O7.10H2O +2HCl---->4H3BO3+2NaCl+5H2O
Thus .molar ratio of HCl to Borax=mol borax/mol HCl=1/2
Now, mass of borax taken=30g
Mol of borax=mass/molar mass=30g/381.38g/mol=0.0787mol
Mol of HCl=2*mol borax=2*0.0787mol=0.157 mol
Molarity*volume=mol
Thus ,Molarity of borax=mol/mol=0.157mol/10ml=0.157mol/0.010L=0.0157M
M of HCl=0.0157M