# AS level chemistry Isaac Physics help please

Use the ideal gas equation of state to answer the following questions. In SI units, the equation is pV=nRT, where R=8.31JK−1 mol−1.
A barometer shows a reading of 29.2inches of mercury. Use g=9.81 ms−2
for this question.

A) Given that 1inch is approximately 2.54cm, and the density of mercury is 13,590 k gm−3, calculate the atmospheric pressure in pascals. I found the answer Atmosphere pressure is 98900 Pa.

B) A gas man uses a manometer to check the pressure of gas delivered to a home. The manometer contains water with a density of 1.0 gcm−3. The diagram below shows the manometer reading. The scale is marked in decimetres. Give the reading from the manometer shown in Figure*1. I also found this answer too, Gas pressure is 3.2 dm.

C) Calculate the difference between the gas pressure and atmospheric pressure.

D) Calculate the absolute (total) pressure of the gas. (Give your answer to 3 significant figures).

E) Calculate the number of molecules supplied per cubic metre of gas at a temperature of 291K and under the pressure calculated in the previous part. (Give your answer to 2 significant figures).

Original post by qnedrah
Use the ideal gas equation of state to answer the following questions. In SI units, the equation is pV=nRT, where R=8.31JK−1 mol−1.
A barometer shows a reading of 29.2inches of mercury. Use g=9.81 ms−2
for this question.

A) Given that 1inch is approximately 2.54cm, and the density of mercury is 13,590 k gm−3, calculate the atmospheric pressure in pascals. I found the answer Atmosphere pressure is 98900 Pa.

B) A gas man uses a manometer to check the pressure of gas delivered to a home. The manometer contains water with a density of 1.0 gcm−3. The diagram below shows the manometer reading. The scale is marked in decimetres. Give the reading from the manometer shown in Figure*1. I also found this answer too, Gas pressure is 3.2 dm.

C) Calculate the difference between the gas pressure and atmospheric pressure.

D) Calculate the absolute (total) pressure of the gas. (Give your answer to 3 significant figures).

E) Calculate the number of molecules supplied per cubic metre of gas at a temperature of 291K and under the pressure calculated in the previous part. (Give your answer to 2 significant figures).

What specifically are you stuck on, and what have you tried so far?

Spoiler

(edited 8 months ago)
Original post by bl0bf1sh
What specifically are you stuck on, and what have you tried so far?

Spoiler

I’m specifically stuck on part c, because it links to d and e😭 but I managed to work out part an and b.
Original post by qnedrah
I’m specifically stuck on part c, because it links to d and e😭 but I managed to work out part an and b.

It is asking for the difference between the gas pressure and atmospheric pressure, which you find in parts A) and B). Pay attention to units though, you'll probably need to do some converting

Spoiler

(edited 8 months ago)
Original post by bl0bf1sh
It is asking for the difference between the gas pressure and atmospheric pressure, which you find in parts A) and B). Pay attention to units though, you'll probably need to do some converting

Spoiler

So how should I calculate the difference of the pressures, when one of them is in pascals and the other is in dm?? Which unit should I convert both of them too? I can’t attach the screenshots, and thank you for helping me☺️☺️
(edited 8 months ago)
Original post by qnedrah
So how should I calculate the difference of the pressures, when one of them is in pascals and the other is in dm?? Which unit should I convert both of them too? I can’t attach the screenshots, and thank you for helping me☺️☺️

Have a look at parts D) and E) which unit would be more useful to have?
Original post by bl0bf1sh
Have a look at parts D) and E) which unit would be more useful to have?

I think it’s Pascals because when I enter it as an answer it says the unit is correct. So if the atmospheric pressure is already in pascals, how do I convert the gas pressure into pascals when it’s in dm?
Original post by qnedrah
I think it’s Pascals because when I enter it as an answer it says the unit is correct. So if the atmospheric pressure is already in pascals, how do I convert the gas pressure into pascals when it’s in dm?

It's a bit of a funny one, as pressure = force ÷ area, so the units for pressure usually reflect that. dm is just length.

https://www.calculatorsoup.com/calculators/conversions/pressure.php

Found this (ad-filled) webpage which looks like it might be handy: https://engineerexcel.com/manometer-equation/. There are also quite a few other results when you search something along the lines of "manometer pressure decimetre"
Original post by bl0bf1sh
It's a bit of a funny one, as pressure = force ÷ area, so the units for pressure usually reflect that. dm is just length.

https://www.calculatorsoup.com/calculators/conversions/pressure.php

Found this (ad-filled) webpage which looks like it might be handy: https://engineerexcel.com/manometer-equation/. There are also quite a few other results when you search something along the lines of "manometer pressure decimetre"

Ok and thank you, I’ll try that. I will let you know when I get the answer. But could you tell me what I should do for the rest of the questions, like D and E? What method should I use for that? And how do I solve it?
Original post by qnedrah
Ok and thank you, I’ll try that. I will let you know when I get the answer. But could you tell me what I should do for the rest of the questions, like D and E? What method should I use for that? And how do I solve it?

The website I linked had a few equations which looked like they might be helpful for answering D). And I should think that E) requires the ideal gas equation (which you have been given) and then another equation to work out the number of molecules.
Original post by bl0bf1sh
The website I linked had a few equations which looked like they might be helpful for answering D). And I should think that E) requires the ideal gas equation (which you have been given) and then another equation to work out the number of molecules.

Thank you, I think I managed to get the answer as 3100 Pa for part c. The units were just as you said pascals. I only converted the gas pressure into pascals and it gave me the answer!!!! Thank you soooo much, you helped me massively!🥰🥰
Original post by qnedrah
Thank you, I think I managed to get the answer as 3100 Pa for part c. The units were just as you said pascals. I only converted the gas pressure into pascals and it gave me the answer!!!! Thank you soooo much, you helped me massively!🥰🥰

Well done