Which of the following is the
correct definition of the standard molar gas volume? (A) It is the volume occupied by
one mole of any gas divided by temperature and pressure. (B) It is the volume occupied by
one mole of any gas at a given temperature and pressure. (C) It is the volume occupied by
one gram of any gas divided by temperature and pressure. (D) It is the volume occupied by
one gram of any gas at a given temperature and pressure.
In chemistry, there are many laws
and rules that help us understand the complex chemical relationships that are used
every day in the field. One of these laws, known as
Avogadro’s law, is used in chemistry to help us understand the relationship between
the volume of a gas and the number of moles of the same gas.
Avogadro’s law states that at a
constant temperature and pressure, both the volume and number of moles of a gas are
directly proportional. We can express Avogadro’s law by
using the following proportionality statement: 𝑉 is proportional to 𝑛. 𝑉 is the volume of the gas, and 𝑛
is the number of moles of the gas. We can also express Avogadro’s law
as an equation when we want to perform calculations using the law shown as 𝑉 equals
𝑛 multiplied by 𝑉 𝑚, where 𝑛 is the number of moles and 𝑉 𝑚 is a
proportionality constant that represents molar volume. Molar volume is defined as the
volume occupied by one mole of gas at a specific temperature and pressure and is
typically expressed as liters per mole.
Since we cannot see gas, the
concept of Avogadro’s law and molar volume may be difficult to grasp initially. So let’s look at an example using a
chemical equation that describes the formation of water vapor from hydrogen and
As we can see in the equation, two
moles of hydrogen gas react with one mole of oxygen gas to form two moles of water
vapor. And as we know from Avogadro’s law,
since the number of moles of gas is proportional to the volume of gas, we can also
say that the equation indicates the volumes that react as well. For example, using this equation,
we can say that two volumes of hydrogen gas react with one volume of oxygen gas to
produce two volumes of water vapor. We can further express this as a
ratio of two to one to two, hydrogen to oxygen to water vapor, with the
understanding that this ratio will remain constant regardless of whether it is
expressed in moles or units of volume. This is the case as long as the
given temperature and pressure also remain constant.
Now that we have some background
information, we can focus on our question. Our question asks what the best
definition of standard molar gas volume is given a variety of choices.
If we go through our list, we can
immediately eliminate answer choices (C) and (D) as they mention the volume of one
gram of gas. As molar volume deals only with the
volume of moles and not mass, we can easily see that these choices cannot be
correct. Answer choice (A) correctly
mentions the volume of one mole of gas. However, it mentions that this
value is divided by temperature and pressure. This is incorrect because
Avogadro’s law states that temperature and pressure must only be constant, not
manipulated as variables in the equation.
Therefore, only answer choice (B),
it is the volume occupied by one mole of any gas at a given temperature and
pressure, matches with the definition of standard molar gas volume. “It is the volume occupied by one
mole of any gas at a given temperature and pressure” is the correct answer.