6.11: Standard Temperature and Pressure
- Page ID
- 213228
As stated in Section 6.3, the original Gas Law experiments were limited to the investigation of two of the four measurable properties of gases. The measurements that were associated with two variables were held constant, so that any experimental change in the third quantity could only influence the value of the fourth. The subsequent sections presented and applied the Gas Laws, which relate the quantities that are altered in a particular trial. However, in order for any collected data to be meaningful, the experiment that was performed must be reproducible. Therefore, in order to complete every experimental trial under the same conditions, scientists were required to standardize the values of the two variables that were held constant.
Standard Temperature and Pressure (STP)
Recall that Avogadro's Law directly relates an amount of gas to its volume under isothermal and isobarometric conditions. As described previously, the temperature and pressure of a gas impact the behaviors of its constituent particles, which, in turn, influence the volume of the gas. The application of pressure causes gaseous particles to move closer to one another, which decreases the overall volume of the gas. Additionally, by decreasing the temperature of a gas, its constituent particles move more slowly and, therefore, collide less often with the surfaces of the container in which they are held, which causes the container to shrink. Since the pressure and temperature at which a gas are held directly influence the volumetric results of an Avogadro's Law experiment, scientists who completed trials using different "constant" conditions generated inconsistent data and, therefore, were unable to corroborate the experimental findings of their peers. In order to eliminate these variations, scientists chose to perform all Avogadro's Law experiments at a temperature of 273 Kelvin and under 1 atmosphere of pressure.
Under these conditions, which became known as standard temperature and pressure (STP), scientists discovered that 1 mole of a gas, regardless of its identity, occupied a volume of 22.4 liters. Recall that the value of the constant, k4, that is associated with the Avogadro's Law equation typically varies based on the identity of the gas that is being investigated. Therefore, since the amount of space that is occupied by a gas is independent of the identity of that gas at STP, the volumetric value that is given above is a chemically-significant quantity and, therefore, is defined as a molar standard.
Equality Pattern
Every molar standard has a corresponding equality pattern, which contains one number and two units on both sides of an equal sign. The left side of the STP equality pattern that is shown below contains a numerical value of "1," which is associated with the unit "mol." The right side of this equality pattern, which utilizes the volumetric value, "22.4," as its numerical quantity, has a primary unit of "L." The secondary unit positions, which are indicated as "blanks" in the equality pattern that is shown below, should be occupied by units that are relevant to the identity of the specific chemical that is referenced in a given problem. In particular, the chemical formula of the gas that is being considered should be inserted into both of these positions. A chemical name should not be used in this, or any, equality, and the relative order of the two units on either side of an STP equality should not be interchanged.
Indicator Phrase
Recall that all molar standards have a corresponding indicator word or phrase that identifies which relationship and, therefore, which equality pattern, must be applied to solve the problem at-hand. Because the volumetric value that is associated with this equality is only valid at an experimental temperature of 273 Kelvin and under 1 atmosphere of pressure, the phrase "at STP" must be present to indicate that an STP equality should be used to solve the given problem.
Calculations
For example, calculate how many moles of molecular chlorine gas are present in 75.0 liters of molecular chlorine at STP.
The phrase "at STP" indicates that the chemical formula for the gas that is referenced in the problem, molecular chlorine, Cl2, should be incorporated into both of the secondary unit positions in the STP equality pattern that was developed above. The resultant equality, 1 mol Cl2 = 22.4 L Cl2, must then be applied as a conversion factor to eliminate the given unit, "liters of molecular chlorine." Applying the correct number of significant figures to the calculated quantity results in the final answer that is shown below.
\( {\text {75.0}}\) \({\cancel{\rm{L} \; \rm{Cl_2}}} \times\) \( \dfrac{1 \; \rm{mol} \; \rm{Cl_2}}{22.4 \; \cancel{\rm{L} \; \rm{Cl_2}}}\) = \( {\text {3.348214...}}\) \({\rm{mol} \; \rm{Cl_2}}\) ≈ \( {\text {3.35}}\) \({\rm{mol} \; \rm{Cl_2}}\)