Hi A-Level Chemistry students.
Today, we will discuss on the topic of IDEAL GAS LAW, which might be known as THE GASEOUS STATE in your Chemistry syllabus.
Now, in Singapore GCE A-Level H2 Chemistry syllabus, candidates are required to know the following:
1) Basic assumptions of the kinetic theory as applied to an ideal gas
2) Conditions necessary for a gas to approach ideal behavior
3) Limitations of Ideality at very high pressures and very low temperatures
4) General gas equation pV=nRT and the determination of Mr
In the prevIous blog posts, we have discussed on point 1, 2 and 3. You can do a search for those posts by keying in “Ideal Gas Law” on the top RHS field.
Today we will go through the general gas equation which is:
which can be expressed as
pV=(m/Mr)RT since n = m / Mr
It is important to note the units to be used in each term in the equation. Examiners’ reports have always identified wrong use of units to be one of the most common error in A-Level exams.
p = pressure in Pa or N/m2
V = volume in m3
n = no. of moles of gas in mol
R = Gas Constant with a value of 8.31 JK-1mol-1
T = temperature in K
m = mass of gas in g
Let’s take a look at an exam-based question to see how we can apply the equation in examination.
Quick Check 1:
At 27 oC and a pressure of 8 x 104 Pa, 1.00 dm3 of vapour was produced when 8.40 g of a compound was completely vapourised. Determine the relative molecular mass of the compound.
Using the ideal gas equation
we will have the following
and after rearrangement we will have the following
Mr = (mRT)/pV
= 8.40 x 8.31 x (27 + 273) / (8 x 104) x (1.00 x 10-3)
= 262 (to 3 significant figures)
T/K = T/oC + 273
1 dm3 = 1.00 x 10-3 m3
Hope you have learned something today. In the next post we will look at more examples of solving questions using ideal gas equation.
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