Coupon Accepted Successfully!


Gases–An Overview

Gases are unique when compared to liquids and solids. Gases can be compressed into smaller volumes, and they can be mixed extensively. They have relatively very low densities. Ideally, a gas exerts pressure on all sides of the container it occupies in a uniform manner. These are some of the properties of gases. Let's explore these ideas in detail.

Standard Temperature and Pressure

It is very important to understand the general aspects of gas pressure, its measurements, and calibrations. Pressure is force exerted over unit area. The unit of pressure is pascal (Pa) which is equivalent to kg/(m.s2). In chemistry, we usually use a mercury-based barometer to measure the pressure. The unit is millimeter of mercury (mmHg). This is the same as the unit torr. Another unit commonly used to denote pressure is atmosphere (atm). You should be able to convert these units back and forth as required.

1 atm = 760 mmHg = 760 torr

1 atm = 1.01 x 105 Pa


Gases can be compressed or expanded by adjusting the temperature and other conditions that prevail. In order to standardize the quantities of gases measured and conveyed, arbitrary reference conditions called standard temperature and pressure (STP) have been chosen and internationally accepted. The specific values of temperature and pressure at STP are 0o C (273 K) and 1 atm (760 mmHg).
Standard Temperature and Pressure

Temperature - 0° C or 273 K


Pressure - 1 atm or 760 mmHg


You should know what STP is, because in many questions and passages, the actual temperature and pressure situations will not be explicit. Rather, they will say, for example, that the reaction was undertaken at standard temperature and pressure. This means, you have to automatically know that they are talking about the temperatures corresponding to STP reference conditions, which you already know.

Molar Volume

Ideally, one mole of gas at STP occupies a volume of 22.4 L. This is known as molar volume. We should bear in mind the fact that this is under standard conditions. Do the example below on your own, before you look at the solution.


Calculate the number of moles of oxygen at STP present in a volume of 78.75 L.



One mole of gas occupies a volume of 22.4 L. To get the number of moles, you have to divide 78.75 L by 22.4 L/mol.



Test Your Skills Now!
Take a Quiz now
Reviewer Name