Help on a chemistry question

So I need help on a chemistry question which is:

Flask Q (volume = 1.00 x 10^3 cm^3) is filled with ammonia (NH3) at 102 kPa and 300 K. The tap is closed and there is a vacuum in flask P

When the tap is opened, the ammonia passes into flask P. The temperature decreases by 5 degrees Celsius. The final pressure in both flasks is 75.0 kPa. Calculate the volume, in cm^3 of flask P.

So I worked it all out completely to get:

1.34x10^-3 m^3 I then convert that into cm^3 and I get 1336cm^3

However the mark scheme says that I need to subtract 1.0 x 10^3 off of 1.34 x 10^-3 to then convert into cm to get 340

I can't for the life of me understand why so can someone help 🤣🤣

Reply 1

username3477548

20

Original post by ksf

So I need help on a chemistry question which is:

Flask Q (volume = 1.00 x 10^3 cm^3) is filled with ammonia (NH3) at 102 kPa and 300 K. The tap is closed and there is a vacuum in flask P

When the tap is opened, the ammonia passes into flask P. The temperature decreases by 5 degrees Celsius. The final pressure in both flasks is 75.0 kPa. Calculate the volume, in cm^3 of flask P.

So I worked it all out completely to get:

1.34x10^-3 m^3 I then convert that into cm^3 and I get 1336cm^3

However the mark scheme says that I need to subtract 1.0 x 10^3 off of 1.34 x 10^-3 to then convert into cm to get 340

I can't for the life of me understand why so can someone help 🤣🤣

lol same i dont get it eitherrr

Reply 2

username3477548

20

Original post by ksf

So I need help on a chemistry question which is:

Flask Q (volume = 1.00 x 10^3 cm^3) is filled with ammonia (NH3) at 102 kPa and 300 K. The tap is closed and there is a vacuum in flask P

When the tap is opened, the ammonia passes into flask P. The temperature decreases by 5 degrees Celsius. The final pressure in both flasks is 75.0 kPa. Calculate the volume, in cm^3 of flask P.

So I worked it all out completely to get:

1.34x10^-3 m^3 I then convert that into cm^3 and I get 1336cm^3

However the mark scheme says that I need to subtract 1.0 x 10^3 off of 1.34 x 10^-3 to then convert into cm to get 340

I can't for the life of me understand why so can someone help 🤣🤣

I understand now lol

basically 1336cm^3 is the volume of NH3 in P and Q. we know the vol of q, so vol in P is TOTAL- vol of Q :smile:

Reply 3

AliHassan1

2

I'm not quite sure about this question too. I was able to get the first part correct but the second part is wrong as the mark scheme has got 340 while I got 99.7 cm^3. I have looked up a worked out answer to the question and how they got 340, but I noticed a mistake straight away. It says the temperature reduced by 5 degrees yet they worked it out as if it was kelvin so they subtracted5 5 (300-5) to get 295. I subtracted 5 in kelvin from the original, so I did 300-(5+273)=32Kelvin

I'm not 100% sure whether I got it right, so if you do something wrong with what I did, please let me know.

(edited 2 years ago)

Reply 4

Nasro_r

1

Original post by AliHassan1

I'm not quite sure about this question too. I was able to get the first part correct but the second part is wrong as the mark scheme has got 340 while I got 99.7 cm^3. I have looked up a worked out answer to the question and how they got 340, but I noticed a mistake straight away. It says the temperature reduced by 5 degrees yet they worked it out as if it was kelvin so they subtracted5 5 (300-5) to get 295. I subtracted 5 in kelvin from the original, so I did 300-(5+273)=32Kelvin

I'm not 100% sure whether I got it right, so if you do something wrong with what I did, please let me know.

I did the same thing as you did, but 300-(5+273) = 22 in kelvin. However, i don't understand why the mark scheme does 300 - 5 as is not even in the form of a kelvin. The mark scheme is really weird :confused:

Reply 5

Anub1s

2

Original post by Nasro_r

I did the same thing as you did, but 300-(5+273) = 22 in kelvin. However, i don't understand why the mark scheme does 300 - 5 as is not even in the form of a kelvin. The mark scheme is really weird :confused:

The scaling for degrees Celsius and Kelvin are the same, its just that with Celsius, readings are offset by -273K, so it doesn't matter if its 5 less degrees. This is the same as the kelvin value, take away 5, if that makes sense.

In other words, when you took away (5+273) you actually solved an equation as though the temperature decreased by 278 degrees, which is definitely not the case!!

I recommend watching some video on YT which explains differences and similarities between Celsius and kelvin, which should help clear things up for you.

Reply 6

k1tzaz

6

A temperature change of X in Kelvin is equal to a temperature change of X in Celsius. A change in K or C is the same, therefore Δx [K] = Δx [C].

Reply 7

DedBread

8

Original post by username3477548

I understand now lol

basically 1336cm^3 is the volume of NH3 in P and Q. we know the vol of q, so vol in P is TOTAL- vol of Q :smile:

I know this is years late, but just in case anyone comes across this thread later: yes, this is the correct explanation of the mark scheme. The total volume of gas in both P and Q is 1.34*10^-3 m^3 (which is 1340cm^3). However, the question asks what the container P's set volume is. Because we know the set volume of container Q (1000cm^3), we can subtract this from the total gas to get 340cm^3.

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