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Gas Collection by Water Displacement

Procedure used to collect gas pressure for water vapor present in the sample

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Practice Gas Collection by Water Displacement
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I Forgot To Do That

I Forgot To Do That

Credit: National Health Institute
Source: https://commons.wikimedia.org/wiki/File:NIEHS.jpg
License: CC BY-NC 3.0

Research sponsored by the National Institute of Environmental Health Sciences and the National Toxicology Program provides data that helps determine government policies, regulations and laws regarding health and safety. The information needs to be accurate and reliable since the end-result impacts the lives of everyone.

Why It Matters

  • Scientific research is a demanding endeavor. High standards of performance are expected, with attention to detail being very important. Your results will impact other people, so it’s important that your findings are valid. That’s why it is crucial that each experiment be evaluated in terms of what could go wrong and give incorrect results.
  • Credit: NIAID
    Source: http://www.flickr.com/photos/niaid/5613410129/
    License: CC BY-NC 3.0

    What does it mean if your results cannot be reproduced? Did you make a mistake? Was it a fluke? [Figure2]


  • Gas collection by water displacement is a convenient way to generate small amounts of gases for a lab experiment. But this process also has some built-in sources of error (remember those things you were asked about in your last lab write-up?). The pressure of the gas is a combination of the gas and the water vapor in the gas. For many purposes this will not be a major problem, but could throw things off a little. More important is the fact that water is present – the gas is not dry. If water creates problems with a reaction, then the results will not give an accurate picture of the process being studied.
  • Reproducibility is key to a successful experiment. Can you go back into the lab tomorrow, run the same experiment, and get the same results? More importantly, can someone else take your description of the experiment, run it, and see the same thing you did? If your data cannot be reproduced, it cannot be considered reliable.
  • Watch a video about gas collection at the link below:


Show What You Know

Use the links below to answer the questions that follow.

  1. What are the units for mole and mass fractions?
  2. How does knowing the mole fraction of a gas in a mixture allow you to predict the pressure of that gas?
  3. Do some calculations at the kent chemistry web site. Highlight the column on the right to see the answer.
  4. What is the Reproducibility Initiative?

Image Attributions

  1. [1]^ Credit: National Health Institute; Source: https://commons.wikimedia.org/wiki/File:NIEHS.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: NIAID; Source: http://www.flickr.com/photos/niaid/5613410129/; License: CC BY-NC 3.0

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