Continue to add and count the drops of sodium hydroxide until a light pink color remains for at least 30 seconds. (Note: If you add too much sodium hydroxide, add a few more drops of hydrochloric acid until the color disappears.) Record any additional drops of hydrochloric acid that you added. Then, carefully add sodium hydroxide until one drop produces a lasting pink color. Record the total number of drops of sodium hydroxide used.
Use a piece of pH paper to determine the pH of the final solution. Record the pH. If the pH is higher than 7.0, add hydrochloric acid drop by drop, testing the pH with pH paper after each drop, until the pH is equal to 7.0. Record the pH and the total number of drops of HCl you added.
Use the solution in the beaker to fill the watch glass halfway.
Fill the 100-mL beaker about half full of water. Place the beaker on top of the hot plate.
Set the watch glass on top of the beaker.
Turn on the hot plate to a low setting. Adjust the heat as the water in the beaker warms. The water should simmer, but not boil.
CAUTION Do not touch the hot plate or the beaker. Heat until a solid is visible at the edges of the water in the watch glass and the water is nearly evaporated. Turn off the heat.
Allow the remaining water to evaporate. Observe the contents of the watch glass. Record your observations.
When the watch glass has cooled, dispose of the contents as directed by your teacher. Clean up your equipment. Wash your hands with soap and water.
Analyze and Conclude
Comparing and Contrasting What was the total amount of hydrochloric acid used to make the neutral solution? What was the total amount of sodium hydroxide? How do the amounts compare?
Drawing Conclusions What do you conclude about the concentrations of hydrochloric acid and sodium hydroxide in the solutions?
Predicting If the acid had been twice as concentrated as the base, how would your data have changed?
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