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Boyle's law Homework question Help?!

(edited 7 years ago)

Really need help with answering these questions based on the table of results below

The following observations were obtained for a sample of butane at 16.8°C (290.0 K).

pressure P(atm) 0.50 0.70 1.00 1.50 2.00 3.00
volume V(cm3) 30.0 21.5 15.0 9.0 6.0 1.0*

*lncluding 0.1 cm3 of liquid. atm = standard atmospheric pressure.
Explain why Boyle’s law can be applied to calculate the value of V (at 16.8°C) when the pressure is 0.75 atm and also when the pressure is 0.25 atm, but not when the pressure is 1.75 atm nor when the pressure is greater than 3atm.

Give a reason why the value of V (at 16.8 °C) at a pressure of 4 atm is not likely to be much greater than 0.1 cm3.

For Boyle's Law pV must be constant.

Is it? Check out each pair of pV values.
If it isn't constant you can't apply the law.

As for the rest of the question, butane will liquefy if the pressure is large enough.

(edited 7 years ago)

Original post by iamspiderman

3 of the pv values on the table have a constant of 15.0 however the last 3 don't. But a pressure of 0.25 and 0.75 do have a the same constant of 15.0
So this means that the law can be applied to pressures 0.25 and 0.75 ?
I don't understand why the last three pv values on the table are not constant like the rest ?

I drew a graph and plotted the tables pv, and got a curved shape, I know this shows that they are inversely prorational

The values are not constant because the gas does not obey Boyle's law for all values of pressure and volume. There is a range of values around those 1st 3 pairs where it does obey Boyles Law. Butane is not an Ideal Gas. Only Ideal Gases obey Boyle's Law for all values of p and V.
Butane is the stuff used as lighter fuel that you can buy in the shops in pressurised cylinders. Beyond a certain pressure it turns to a liquid. When that happens the volume drastically drops as can be seen on the last pair of values. Look what has happened to the volume when the pressure was increased to 3 Atm.

(edited 7 years ago)

Original post by iamspiderman

Ok thank you so muchh this is really helping me !
So they will only obey boyles law if they are at a low pressure and high temperature.
So are all the p v values inversely proportional to each other ? Even the ones which are not obeying boyle's law?
If I wanted to draw a graph to show this, would it be a straight line ? Or would the pv values which obey the law be in a straight line while the others that do not wont.

You'll only get a straight line while the gas obeys Boyle's Law. That's the 1st 3 pairs of values. In general, real gasses will tend to obey Boyle's Law more and more closely as you reduce the pressure and density. That's why the 1st 3 pairs (low pressure) show agreement with Boyle.
P is only proportional to 1/V (inversely) when Boyle's Law is obeyed. P inversely prop to V is the same thing as saying pV is constant.