Tutorial – Charles’ Law (Std 4c)
Read through this tutorial very carefully. It has almost everything that needs to be done in the most difficult of Charles’ Law problems, and goes through the logic of why each step is required. I hope
With Boyle’s Law pressure and volume were inversely proportional. That means that as one
increases the other decreases.
PV = k k is a constant
With Charles’ Law temperature and volume are directly proportional. That means that as
temperature increase, volume must increase also.
---- = b b is a constant
Using Charles’ Law:
V1 V2 V1 – starting volume
--- = --- V2 – ending volume
T1 T2 T1 – starting temperature in Kelvins only
T2 – ending temperature in Kelvins only
As with Boyle’s law, the unit of measure for Volume must be the same for both V1 and V2. Another important thing to remember is that pressure and amount of gas cannot change for this law to work.
We start with temperature at 25 °C and an end with a temperature of 58 °C. The
volume starts at 75 ml. What is the final volume?
T1 = 25 °C T2 = 58 °C V1 = 75 ml and V2 = unknown
We cannot use Charles’ Law until the temperatures are in Kelvins. Remember that:
°C + 273 = K which also means that K = °C - 273
25 °C + 273 °C = 298 K = T1
58 °C + 273 °C = 331 K = T2
Now we can use these temperatures in Charles’ Law as follows:
75 ml T2
-------- = ---------
298 K 331 K
The unit of measure K crosses out on both sides of the = sign. The unit of measure
left is ml. To finish off the math, multiply 75 ml x 331 and then divide your answer
by 298: Answer 83.3 ml = T2