# Why is it necessary to correct the pressure of a gas that is collected over water?

Basically, what are some mistakes that you can make while using the downward displacement of water method and how to fix them? This is actually a homework question and I'm not too sure what it means either. The original question:

State and explain Dalton’s Law of Partial Pressures. Why is it necessary to correct the pressure of a gas that is collected over water? {Background: Gases formed in a chemical reaction are collected using a technique called the downward displacement of water, feel free to research this technique before you respond to this question}

• Could you please add more context? It's not quite clear how (and why) one would "correct" the pressure in the first place. Posting an example of an experimental setup or citing a textbook where you encountered this info would help to understand your question better. – andselisk Jul 22 '19 at 7:54
• Here's the full question: State and explain Dalton’s Law of Partial Pressures. Why is it necessary to correct the pressure of a gas that is collected over water? {Background: Gases formed in a chemical reaction are collected using a technique called the downward displacement of water, feel free to research this technique before you respond to this question} – Rumaysa Khan Jul 22 '19 at 8:22
• This comment should be an edit to your question. Since it's a homework question, you are expected to demonstrate your effort in solving the problem (literature/internet research also counts). – andselisk Jul 22 '19 at 8:26
• He never actually taught us this so I was hoping to find some help somewhere else. But the question also says that we can research before actually answering. – Rumaysa Khan Jul 22 '19 at 8:29
• – Buck Thorn Jul 22 '19 at 9:37

In this case, the gas you collected by displacing the water will be contaminated by water vapour. Therefore, the pressure of the gas which you measure will be the sum of the partial pressure of the product gas, and partial pressure of water vapour. \begin{align*} Measured\;pressure &= Partial\;pressure\;of\;product\;+\;partial\;pressure\;of\;water\;vapour \\ &=Actual\;pressure + Vapour\;Pressure \end{align*} This means that you must correct the pressure, by subtracting the vapour pressure of water from the measured value to get the actual pressure of the produced gas.