Specification:
-The concept of amount of substance in performing calculations involving: volume of gases (including the ideal gas equation pV=nRT), balanced chemical equations, enthalpy change; the technique and procedure used in experiments to measure volumes of gases.
The basic understanding of gas and its units:
Gases move around, they occupy a volume, have a certain temperature and have or exert a pressure.
Temperature → kinetic energy the particle has (Unit in kelvin (K), 273 K is 0०C)
Volume → Space occupied (Unit in m3)
Pressure → the collision between the gas particles and the wall of the container (Unit in pascal (Pa) The atmospheric pressure is 101 kPa → 101x103)
Some of the assumptions we made about gases:
-Randomly moving
-Particles are negligible in size compared to separation
-Distance can be vast so don’t collide much
-Many behave as ideal gases at ‘normal’ temperature/pressure.
Some of the relationship between pressure, temperature and volume:
The pressure build is proportional to the temperature and inversely proportional to the volume.
P∝T/V
This means you have some relationship between pressure and volume at constant (This graph can be called the Boyle’s Law but need to know this only for physics.)
You have a relationship with pressure and temperature. (This graph doesn’t have a specific name.)
And the last relationship is between volume and temperature. (This graph is called Charle’s law, here again you don’t need to know but only for physics.)
Formula Pv=nRT
This formula means you can calculate the pressure, volume, moles, or temperature when you have most of the data to find the information you need.
Pressure✖volume=moles✖temperature✖molar gas constant
The molar gas constant is given in the data book but it’s 8.314 J⋅K−1⋅mol−1.
The pressure needs to be in pascal.
The volume needs to be in m3.
Lastly, the temperature needs to be in kelvin (0०C =273k)
An example:
Calculate the number of mol of gas contained in a 3.0 L vessel at 300 K with a pressure of 151.95 kPa.
-The first step will be to see what information you have and convert anything that needs to be converted.
-We want the moles of gas. We have a volume of 3.0 L so this needs to be divided by 1000 which gives 0.003 m3 of volume.
-Then we have the temperature which is 300k. This is already converted to kelvin so we don’t need to do anything.
-Lastly we have the pressure which is 151.95 kPa. This needs to be converted to Pa by multiplying by 1000 which gives 151950 Pa.
-The second step will be to rearrange the formula to get only moles on one side.
n=Pv/rt
-The third step is to put the information we know in the formula and the value of r which is 8.314.
151950✖0.003/8.314✖300
-This will give you an answer of 0.18 to 2 decimal places.
You could use this type of question to find how much mass you need or the mr of the product you are looking for if you have been asked for this as well.
Calculating the volume or the moles when you have a constant pressure and temperature.
Equal volumes of different gases at the same temperature and pressure contain the same number of moles.
Equal number of moles of different gases at the same temperature and pressure have the same volume.
When you have a constant temperature and pressure it’s 298 and 101kPa.
The formulas for this are:
Moles=Volume/24 This for volume in dm3
Moles=Volume/24000 This for volume in cm3
An example for this calculation:
Calculate the volume in cm3 of 0.10 mol of oxygen at room temperature and pressure.
-The first thing you will do is to rearrange the formula to get the volume.
Moles✖24000
-You can then put the information into the equation:
0.10✖24000=2400 cm3
Here again you can be asked to find the moles of the product to then find the mass of the production you need to this volume or the mr of the product if you know the mass.
Way to collect gases during an experiment:
Using a gas syringe:
If you are using soluble water you need to use the gas syringe method as some gas would dissolve before reaching the burette in the second method below.
Collecting gas with a measuring cylinder or inverse burette:
Both methods need to be gas tight for the system to work and collect the gas correctly.
For the inverted burette and the measuring cylinder the volume is the initial volume minus the final volume of the gas.
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