# Gas law problem (changing volume, temperature and pressure)

• Tasha Clifford
DanYes. Post your working and answer and we'll check them.In summary, a sample of gas collected at 40##^o## C and 75 mmHg occupies a volume of 65 cm³.f

#### Tasha Clifford

Homework Statement
What volume will 65cm3 of a gas occupy, if it is collected at 40°C and 75mmHg at STP?
Relevant Equations
None
I can’t quite work my head around this question, I am having a difficult time analyzing the question, I can’t seem to make out what the initial and final conditions are

would appreciate all the help I could get cheers

Last edited:

Either:
- it's a trick question and the answer is 65cm³ or
- there is missing information and the question is impossible to answer.

Last edited:
• topsquark
Homework Statement:: What volume will 65cm3 of a gas occupy, if it is collected at 40°C and 75mmHg at stp?
Relevant Equations:: None

I can’t quite work my head around this question, I am having a difficult time analyzing the question, I can’t seem to make out what the initial and final conditions are

would appreciate all the help I could get cheers
If the gas was collected at 40##^o## C and 75 mmHg then that is your initial condition. To find out what the volume is at any other condition you need to know what that condition is. You don't have enough information.

-Dan

On re-reading the question, I see it mentions 'stp' (which is normally written in capitals: STP).

I guess the question could be written as:
"A sample of (ideal) gas collected at 40°C and 75mmHg, occupies a volume of 65cm³.​
What volume will it occupy at STP?"​

• • Lord Jestocost, berkeman, Tasha Clifford and 1 other person
On re-reading the question, I see it mentions 'stp' (which is normally written in capitals: STP).

I guess the question could be written as:
"A sample of (ideal) gas collected at 40°C and 75mmHg, occupies a volume of 65cm³.​
What volume will it occupy at STP?"​
What are your thoughts on this so far? How many moles of gas are collected?

• topsquark
If the gas was collected at 40##^o## C and 75 mmHg then that is your initial condition. To find out what the volume is at any other condition you need to know what that condition is. You don't have enough information.

-Dan
I just edited the question now, it end with “STP” which i didnt write in capitals, does this mean STP conditions are the final conditions

• topsquark
On re-reading the question, I see it mentions 'stp' (which is normally written in capitals: STP).

I guess the question could be written as:
"A sample of (ideal) gas collected at 40°C and 75mmHg, occupies a volume of 65cm³.​
What volume will it occupy at STP?"​
Brilliant, i think it makes much more sense now

What are your thoughts on this so far? How many moles of gas are collected?
On re-reading the question, I see it mentions 'stp' (which is normally written in capitals: STP).

I guess the question could be written as:
"A sample of (ideal) gas collected at 40°C and 75mmHg, occupies a volume of 65cm³.​
What volume will it occupy at STP?"​
And Does this mean STP conditions of 273K and 750atm are the final conditions?

75mmhg

Last edited:
And Does this mean STP conditions of 273K and 750atm are the final conditions?
750 atm! Ouch!

Before 1982 STP was defined as T = 273.15 K and P = 1 atm = 101.325 kPa.
After 1982 STP is defined as T = 273.15 K and P = 100 kPa.

You will need to check your source for the date. Either way, though, your calculations should be essentially the same.

-Dan

• • Lord Jestocost, Tasha Clifford, berkeman and 1 other person
And Does this mean STP conditions of 273K and 750atm are the final conditions?
First tell us what you think!

And (as already noted by @topsquark ) you don't mean "750atm"!

• Tasha Clifford
And Does this mean STP conditions of 273K and 750atm are the final conditions?
the number of moles is the same at both states.

First tell us what you think!

And (as already noted by @topsquark ) you don't mean "750atm"!
I meant 75mmhg and not 750atm sorry for the mixup Last edited:
the number of moles is the same at both states.
I do not quite understand what you mean here sir

Last edited:
Do i now proceed by substituting the said values in the combined gas equation
(P1V1 / T1 = P2V2 / T2 ) then find the final volume?

I meant 75mmhg and not 750atm sorry for the mixup Maybe some confusion remains! Standard Temperature and Pressure (STP) corresponds to 0ºC and 750mmHg (but see note below).

Make sure you know the two different pressures in the question.

Note: In fact for STP, pressure = 750.06mmHg but 750 is often used as it is accurate enough for most purposes.

Note: In fact for STP, pressure = 750.06mmHg but 750 is often used as it is accurate enough for most purposes.
You're certain?

Do i now proceed by substituting the said values in the combined gas equation
(P1V1 / T1 = P2V2 / T2 ) then find the final volume?

As hinted at by @Chestermiller, you can also solve the problem using the ideal gas equation (PV=nRT) because the number of moles doesn't change. You might find it useful to try both methods to see that the two methods are really the same.

Last edited:
• Tasha Clifford Who IS running the asylum these days?

• Steve4Physics
You're certain?
Maybe you mean that there are various definitions of STP (some using 750.06mmHg and others using 760mmHg)?

I guess I should have pointed this out to the OP and suggested that they check which values (of both temperature and pressure) they are meant to use for STP. If @Tasha Clifford is reading this, please note!

However, the OP has used the figure '750' in the context of STP (rather than 760) suggesting 750mmHg is the appropriate value.

• Tasha Clifford and Bystander
Maybe you mean that there are various definitions of STP (some using 750.06mmHg and others using 760mmHg)?

I guess I should have pointed this out to the OP and suggested that they check which values (of both temperature and pressure) they are meant to use for STP. If @Tasha Clifford is reading this, please note!

However, the OP has used the figure '750' in the context of STP (rather than 760) suggesting 750mmHg is the appropriate value.
Note taken

• Steve4Physics

As hinted at by @Chestermiller, you can also solve the problem using the ideal gas equation (PV=nRT) because the number of moles doesn't change. You might find it useful to try both methods to see that the two methods are really the same.
I don't really understand how i can solve this question using the ideal gas eqaution, i just tried and it’s giving me an absurd value, would appreciate if you can do so , i want to see how you go about it

As hinted at by @Chestermiller, you can also solve the problem using the ideal gas equation (PV=nRT) because the number of moles doesn't change. You might find it useful to try both methods to see that the two methods are really the same.
This is my working using the combined gas equation

#### Attachments

• B2A4CE1B-457C-4C2C-B78A-C11D359B3EE9.jpeg
56.6 KB · Views: 45
I don't really understand how i can solve this question using the ideal gas eqaution, i just tried and it’s giving me an absurd value,
You need to show your working and the value you got.

would appreciate if you can do so , i want to see how you go about it
The rules here require you to show evidence of your own work before we can offer guidance. See https://www.physicsforums.com/threads/homework-help-guidelines-for-students-and-helpers.686781/

Did you manage to solve the problem using P₁V₁/T₁ = P₂V₂/T₂? Post your working - even if you got stuck at some point.

This is my working using the combined gas equation
The original question says that the initial pressure, P₁ = 75mmHg, not 750mmHg.

The final conditions are SPT, so P₂ = 750mmHg. (That means pressure has increased by a factor of 10.)

Also, 1 atm is 760mmHg, not 750mmHg. It's wrong to say 750mmHg is 1 atm.

Try again!

• Tasha Clifford
• Tasha Clifford
The original question says that the initial pressure, P₁ = 75mmHg, not 750mmHg.

The final conditions are SPT, so P₂ = 750mmHg. (That means pressure has increased by a factor of 10.)

Also, 1 atm is 760mmHg, not 750mmHg. It's wrong to say 750mmHg is 1 atm.

Try again!
I am confused it’s actually an mcq qeustion, here is the question The original question says that the initial pressure, P₁ = 75mmHg, not 750mmHg.

The final conditions are SPT, so P₂ = 750mmHg. (That means pressure has increased by a factor of 10.)

Also, 1 atm is 760mmHg, not 750mmHg. It's wrong to say 750mmHg is 1 atm.

Try again!
I just did the necessary corrections and solved it solved it again but my answer still doesn't fall among the provided answers I just did the necessary corrections and solved it solved it again but my answer still doesn't fall among the provided answers View attachment 313765
I agree with your answer. Well done! It looks like the question and/or answer-list have one or more mistakes. It happens sometimes.

If we use the STP definition where pressure is 760mmHg and temperature is 0ºC, then I get the same answer as you.

(I would have rounded the answer to 2 or 3 significant figures, i.e. 5.6cm³ or 5.60cm³.)

If we use the STP definition where pressure is 750mmHg and temperature is 0ºC, then this changes the answer slightly, to 5.67cm³.

Make sure you know which definition of STP you are supposed to use. Using the incorrect definition will change the answer (slightly).

A very minor point: you correctly converted 0ºC to 273.15K. But the number you put into your equation was 273.

• Tasha Clifford and Chestermiller
I agree with your answer. Well done! It looks like the question and/or answer-list have one or more mistakes. It happens sometimes.

If we use the STP definition where pressure is 760mmHg and temperature is 0ºC, then I get the same answer as you.

(I would have rounded the answer to 2 or 3 significant figures, i.e. 5.6cm³ or 5.60cm³.)

If we use the STP definition where pressure is 750mmHg and temperature is 0ºC, then this changes the answer slightly, to 5.67cm³.

Make sure you know which definition of STP you are supposed to use. Using the incorrect definition will change the answer (slightly).

A very minor point: you correctly converted 0ºC to 273.15K. But the number you put into your equation was 273.

Yh i agree that there seems to be a problem with the qeustion because i realized that if you change the 75mmhg to 750mmhg then the answer becomes D) 55.9cm3

I still want to know how i could go about with the problem using the ideal gas equation as you said

Thanks anyway @Steve4Physics and @Chestermiller for all your contributions, I really do appreciate
Until next time

I still want to know how i could go about with the problem using the ideal gas equation as you said
##P_1V_1 = nRT_1##. Solve your initial condition to find the number of moles of the gas. Since this does not change, plug that into ##P_2 V_2 = nRT_2##.

-Dan

• Tasha Clifford
##P_1V_1 = nRT_1##. Solve your initial condition to find the number of moles of the gas. Since this does not change, plug that into ##P_2 V_2 = nRT_2##.

-Dan
Ok ##P_1V_1 = nRT_1##. Solve your initial condition to find the number of moles of the gas. Since this does not change, plug that into ##P_2 V_2 = nRT_2##.

-Dan
Please how did you type the equation , i see it as( ##P_1V_1 = nRT_1##)
but it actually appears in the message itself as the ideal gas equation ??

Last edited:
Please how did you type the equation , i see it as( ##P_1V_1 = nRT_1##)
but it actually appears in the message itself as the ideal gas equation ??
We use a version of LaTeX here. The basic stuff is actually pretty simple. We have an actual Forum but see this link here to get you started.

-Dan