In this lab, you will collect the gas given off from a sample of Alka Seltzer over water. What
types of information can you collect from this? What questions can you pose to answer during
this lab?
Formulate at least 2 beginning questions that you may be able to answer in this lab.
Essentials for collecting gas:
To collect a gas over water, you will need to prepare the set up. Since, gases are soluble in
water and we want to make sure that all of our CO2 that is produced is collected.

Tips for your set-up:
• Maximize your water in your bin but make sure to leave enough room for water to
displace into it and not overflow it.
• Allow the temperature of the water to stabilize to room temperature.
• Start with ZERO air/gas in your graduated cylinder before starting the reaction (any air
in it before you perform the experiment will create error)
o Ideally, use the water in the bin to fill your graduated cylinder (to do this you
can lay it flat in the bin and purge the air from it. Slowly tip the cylinder up
while keeping the opening under the surface of the water.

• Stopper the flask as quickly as you can after dropping the Alka-Seltzer in to capture as
much gas as possible.
• After the reaction is finished, carefully swirl the flask to help the final amount of carbon
dioxide gas move to the cylinder.
• Be as accurate as possible when reading the amount of gas in the cylinder (significant
figures)

Data Collection and Calculations:
You will need data to calculate both an experimental and a theoretical value for carbon dioxide
so that you can compare.
Here is some information that you will need for your process.
For the theoretical yield, you will need the following:
• Mass of your Alka-Seltzer piece (written on your baggie)
• Alka-Seltzer Tablets (according to the box)
o Each tablet is 3.25 g
o Each tablet contains
1916 mg of sodium bicarbonate
1000 mg of citric acid

For the experimental yield, it may be helpful to review collecting water over gas.
Collecting Gas Review Resource
Since we will be using Dalton’s Law of Partial Pressures and the Ideal Gas Law, you will also
need the barometric pressure when you did the experiment. If you have a barometer at home,
use that value, otherwise you can use the National Weather Service to obtain your barometric
pressure. Type in your zip code in the location box (upper left corner) and it will display all of
the current weather data for the station nearest you. (Watch your units!)
NWS

You are also going to need the vapor pressure of the water so it is time for you to Google
yourself a chart of this. Find a chart that gives you a value that is closest to your actual
temperature reading (ie, if you thermometer reads 18°C and the chart only has 15 or 20°C, it
isn’t close enough....find another chart)

Reporting your Results
Complete your lab notebook as previously through data and calculations.
For your final section, we are going to streamline it so it is clearer on the steps and items that
you need to address in your conclusion section.
We are going to be using CER (Claim, Evidence, Reasoning) which is basically what it was
before but this outline will hopefully help you better express your outcomes.
For each beginning question, you should have a claim for and therefore the follow-up evidence
and reasoning. Even if a question was not able to be answered by the lab, you need to state a
claim that it was either not answerable by the data or it was not investigated and explain why.
If you had multiple questions that were asked/answered in the lab, consider how you should
organize the CER format. Should each question/claim have their own CER separately?
Possibly, think about how it should go together so that it makes sense. The bottom line is that
it should all be in a paragraph format but it should be easy to tell what the claims are, what the
evidence is and how the reasoning is linking them both together. Don’t forget, error analysis is
also a claim and needs to follow the same treatment. It wouldn’t be unusual for an analysis of
an experiment to have several claims (ie, more than 3)
Claim: A statement of a student’s understanding about a phenomenon or
about the results of an investigation
• A one-sentence answer to the question you investigated
• It answers, what can you conclude?
• It should not start with yes or no.
• It should describe the relationship between dependent and independent
variables.
Evidence: Scientific data used to support the claim
Evidence must be:
• Sufficient—Use enough evidence to support the claim.
• Appropriate—Use data that support your claim. Leave out information that
doesn’t support the claim.
• Qualitative, Quantitative, or a combination of both.
Reasoning: Ties together the claim and the evidence
• Shows how or why the data count as evidence to support the claim.
• Provides the justification for why this evidence is important to this claim.
• Includes one or more scientific principles that are important to the claim and
evidence.

Can you explain what's the purpose of this lab?