What effect on the molar mass of the unknown acid would each of the following
have-that is-would it make the calculated molar mass high, low, or would it have no effect?
1. Adding 5 drops of phenolphthalein instead of 1 drop.
2. Using a standard NaOH solution that has been left
unstoppered for several days.
Explain:
3. Failing to remove an air bubble in the tip of the buret
containing the NaOH solution prior to titration.
4. Failing to rinse down the unknown acid solution from the
inside walls of the flask near the end of the titration
5. Placing the unknown acid in a wet container prior to weighing it.
1. Adding 5 drops of phenolphthalein instead of 1 drop:
Adding more drops of phenolphthalein would have no effect on the molar mass of the unknown acid. Phenolphthalein is not involved in the chemical reaction or calculation of molar mass in this context. It is only used as an indicator to determine the endpoint of the titration.
2. Using a standard NaOH solution that has been left unstoppered for several days:
Using a standard NaOH solution that has been left unstoppered for several days would have no effect on the molar mass of the unknown acid. The molar mass of the unknown acid is determined by the chemical reaction with the NaOH solution during the titration, and the concentration and stability of the NaOH solution will not impact the molar mass calculation.
3. Failing to remove an air bubble in the tip of the buret containing the NaOH solution prior to titration:
Failing to remove an air bubble in the tip of the buret containing the NaOH solution prior to titration may affect the accuracy of the volume of NaOH solution delivered. This can result in an incorrect calculation of the molar mass of the unknown acid if the volume of NaOH solution used is not accurate. However, it would have no direct effect on the molar mass itself.
4. Failing to rinse down the unknown acid solution from the inside walls of the flask near the end of the titration:
Failing to rinse down the unknown acid solution from the inside walls of the flask near the end of the titration may result in a higher calculated molar mass. This is because the remaining acid solution on the walls of the flask would not react with the NaOH solution, leading to an underestimation of the amount of NaOH required. Consequently, the molar mass of the unknown acid would appear higher than it actually is.
5. Placing the unknown acid in a wet container prior to weighing it:
Placing the unknown acid in a wet container prior to weighing it may result in a higher calculated molar mass. This is because the presence of water or moisture from the wet container could increase the mass of the unknown acid sample, leading to an overestimation of the molar mass. It is important to ensure that the container is completely dry before weighing the unknown acid to obtain accurate results.
To determine the effect on the molar mass of the unknown acid, we need to consider each scenario separately:
1. Adding 5 drops of phenolphthalein instead of 1 drop:
Phenolphthalein is used as an indicator to determine the endpoint of the titration. Adding more drops may slightly increase the volume of the indicator in the solution, but since it doesn't affect the chemical reaction itself, it will have no significant effect on the molar mass calculation.
2. Using a standard NaOH solution that has been left unstoppered for several days:
When NaOH is exposed to the air, it can absorb moisture from the surroundings and react with atmospheric carbon dioxide, which forms sodium carbonate. This reaction reduces the concentration of the NaOH solution. Using a less concentrated NaOH solution can result in a higher calculated molar mass of the unknown acid because a larger volume of less concentrated NaOH solution is needed to reach the endpoint of the titration.
3. Failing to remove an air bubble in the tip of the buret containing the NaOH solution prior to titration:
If there is an air bubble in the buret, it may cause an inaccurate reading of the volume of NaOH delivered during the titration. This can lead to underestimation or overestimation of the volume of NaOH used, resulting in a lower or higher calculated molar mass of the unknown acid, respectively.
4. Failing to rinse down the unknown acid solution from the inside walls of the flask near the end of the titration:
If some of the unknown acid solution remains on the walls of the flask, it won't react with the NaOH solution during the titration. This will result in a lower volume of NaOH being consumed, leading to a lower calculated molar mass of the unknown acid.
5. Placing the unknown acid in a wet container prior to weighing it:
If the container used to weigh the unknown acid is wet, the moisture may be absorbed by the acid, increasing its mass. This will lead to an inaccurate measurement of the mass of the unknown acid and consequently affect the calculated molar mass, making it appear higher than it actually is.
In summary, scenarios 1 and 2 have no significant effect on the calculated molar mass of the unknown acid. Scenarios 3 and 4 can lead to an inaccurate volume of NaOH used, resulting in a lower or higher calculated molar mass, respectively. Scenario 5 can introduce additional mass due to absorbed moisture, leading to a higher calculated molar mass.