Search the World of Chemistry

×

The Equation of Charles's Law

07th Nov 2019 @ 4 min read

Physical Chemistry

The equation of Charles's law is V = kT.

As we can see from the above equation, the law relates the volume of gas to its temperature. The law was discovered by Jacques Charles in the late 1700s. It states the volume of a gas is directly proportional to its temperature unless the pressure and the amount of the gas remain constant.

Let the volume of a gas be V and temperature, T. According to the law at a constant pressure for a fixed amount of gas,

Volume directly proportional to temperature, V ∝ T.

The above expression is the mathematical expression of Charles's law. Removing the proportionality,

Charles's law equation, V=kT

Here, k is a proportional constant. Finally, rearranging the above equation,

Charles's law formula, V⁄T=k

Thus, the ratio of volume to temperature is constant for a constant pressure and a fixed amount of gas

Note: The above expression is valid unless pressure and amount of gas are constant. Also, temperature is in an absolute scale (in the kelvin or rankine).

Atkins Physical Chemistry

The equation of Charles's law in the celsius scale

We can express the law in the celsius scale. The celsius scale uses the freezing point of water (0 °C) and the boiling point of water (100 °C) as reference points. The celsius is related to the kelvin as follows:

T = t + 273.15

Let V0 be the volume of gas at 0 °C i.e. the freezing point of water. And V is the volume at temperature t (in °C). As per Charles's law,

Substituting T = t + 273.15 and T0 = t0 + 273.15,

We also know, t0 = 0 °C.

Rearranging the above expression,

Charles' law equation in the celsius scale

This is the equation of Charles's law in the celsius scale.

Note: When temperature is expressed in the celsius scale, a small t is used. But when temperature is expressed in the kelvin scale, a capital T is used.

Charles's law under different conditions

Let V1 and V2 be volume at temperature T1 and T2 such that pressure and amount of gas is constant. V1, T1 is volume and temperature at condition 1, and V2, T2 is volume and temperature at condition 2.

Condition 1: V1, T1 and condition 2: V2,T2
Condition 1: V1, T1 to condition 2: V2, T2

We can establish the relationship between the two conditions using the law.

Combining both equations,

Using the above formula, we can determine unknown volume or temperature if volume and temperature for a condition are known. Let take an example.

Example 1

Consider an 130 g helium balloon. The balloon is fastened to a taffrail of a ship moving from Miami towards New York City. The volume the balloon at Miami is 1.50 L and the temperature, 27 °C. What will be the volume of the balloon at New York City if the temperature at New York City is 10 °C? Assume the pressure remains 1 atm.

The volume and temperature of the gas balloon from Miami to New York City
V2 is unknown. The volume and temperature of the gas balloon sailing from Miami to New York City

First, we convert the temperatures in the kelvin from the degree celsius.

The initial volume is 1.50 L. The final volume can be calculated from Charles's law.

Thus, the volume of the balloon at New York City is 1.42 L.

The volume and temperature of the gas balloon from Miami to New York City
V2 = 1.42 L

If you have noticed, the volume decreased from 1.5 L to 1.42 L when the temperature of the balloon decreased from 27 °C to 10 °C. This explains Charles's law that volume is directly proportional to temperature.

We can generalise the law to any number of conditions. Consider a fixed amount of gas of pressure P. Let (V1, T1), (V2, T2), (V3, T3) … (Vi, Ti) be volume and temperature at different conditions.

Charles's: A series of i conditions
A series of i conditions

As per the law, the ratio of volume to temperature remains constant.

Charles's law under different conditions

Example 2

Consider the same ship in the above example. The ship after reaching New York City is scheduled to sail towards Saint John, Canada. The temperature at Saint John is 3 °C and pressure, 1 atm. Determine the volume of the balloon at Saint John.

The volume and temperature of the gas balloon sailing from Miami to Saint John via New York City
V3 is unknown. The volume and temperature of the gas balloon sailing from Miami to Saint John via New York City

First, we have to convert the temperature in the kelvin from the degree celsius. 3 °C is 3 + 273.15 = 276.15 K.

As per the law,

Here, V1 is the volume at Miami, V2 is the volume at New York City, which we have already calculated in the previous example, and V3 is the volume at Saint John. Rearranging the above expressions,

We can use any one of two to solve for V3.

The volume at Saint John is 1.38 L.

The volume and temperature of the gas balloon sailing from Miami to Saint John via New York City
V3 = 1.38 L

As observed from the above calculations, the volume decreases even further (1.42 L to 1.38 L) with a decrease in temperature (10 °C to 3 °C). Therefore, the law holds under multiple conditions.

Associated articles

If you appreciate our work, consider supporting us on ❤️ patreon.
Charles Law Gas Law

Copy Article Cite

Thanks for your response!
Write a response


Join the Newsletter

Subscribe to get latest content in your inbox.

newsletter

We won’t send you spam.