Before beginning the experiment, I knew that density is the measure of how many particles a substance has for every amount of volume and that substances with less density go above more dense substances. I also knew that density can be calculated by dividing an object's mass by its volume and that mass cannot be created or destroyed. I was also aware that when making measurements and calculating, the rules of significant figures must be applied. That means measurements should always be estimated one digit past what you can read and that any measurement that landed on a marker still needs an estimated digit, 0. When adding or subtracting measurements, your significant figures can only go to as many places as the least accurate measurement. When multiplying or dividing, you can only have as many significant figures as your least number does.
Materials: test tube, support ring, stand, trough, bottle, tubing, 1/2 Alka Seltzer tablet, water, graduated cylinder, beaker, electronic balance, glass plate, clamp, marker
Purpose: The purpose of the lab is to find the volume of the gas formed by measuring how much water it displaced, then to find the mass of the gas by subtracting the mass of the water in the test tube after the reaction from the original mass of the water and Alka Seltzer separately, then to use those numbers and the density formula to calculate the density of the gas.
Procedure:
1) Fill the trough 1/2 full of water.
2) Fill the bottle with water until it's overflowing, cover securely with the glass plate, set upside down in the trough of water with the tubing sticking out from under it, and secure with the support ring.
3) Fill the test tube less than 1/2 full of water.
4) Place the test tube in a beaker along with the Alka Seltzer, not letting the tablet make contact with the water.
5) Find and record the mass of the beaker with the contents in it, being sure to set the electronic balance to zero before doing so.
6) Secure the test tube to the stand using the clamp.
7) Place the Alka Seltzer tablet in the water and cover with the corked tubing immediately. Allow the Alka Seltzer tablet to react with the water until there are no more bubbles traveling through the tubing. You may need to gently shake the test tube if the tablet is not fully dissolved.
8) Remove the tubing from the bottle so the bottle is resting flat on the bottom of the tub and mark the water line on the bottle.
9) Empty the bottle and fill it to the water line with water, so that the water is occupying the place where the gas was. Using the graduated cylinder, measure and record the volume of the water.
10) Place the test tube back in the beaker. Find and record the mass the same way you did the first time.
Data Tables:
| Test One | |
| mass before reaction | 187.07 g |
| mass after reaction | 187.06 g |
| volume of gas | 134.0 mL |
Questions and Computations:
1. Use the mass and volume of the gas to determine the density.
2. Determine your percentage error for the density of CO2 gas if the accepted value at room temperature is 2.0 x 10^-3 g/mL.
3. Draw a particle diagram of the CO2 gas produced in reaction that is consistent with your value of density.
Analysis and Reflection:
My results showed that the carbon dioxide formed was not very dense. There was not much change in mass after the reaction occurred, meaning the mass that was converted to a gas was minimal.
Sources of error that I may have encountered are that the stopper wasn't placed on the test tube soon enough after the tablet was placed in the water, meaning that some of the gas produced was not captured and therefore not included in the volume measured. Next time the stopper could be added even sooner to ensure a closed system. The bottle could have lost some water while being placed in the trough, which would make the mass measurement greater than it was supposed to be. That could be avoided by handling the bottle carefully. The water line may not have been level at the time that I drew the mark on the bottle, which would result in an inaccurate measurement of volume. To fix that you could use a level to make sure the top of the bottle is level.
Another group that I compared my results with got a density of 0.24 g/mL. The difference in our results may be due to the possible errors I listed above.
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