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Q: when 2gm of A is introduced into an evacuated flask kept at 25^oC ,the pressure is found to be 1atm. if 3gm of B is then added to same flask. the total pressure becomaes 0.5 atm the ratio of molecular weight of A $ B.

ANS: 2:3.

is this question wrong?

coolank2

w(A)= 2 g, w(B) = 3 g.

P(A) = 1 atm. P (when A & B are present) = 0.5

We have to find M(A): M(B)

P(A) = P

n(A) n(A)+n(B)

n(A) = w(A) / M(A) and n(B) = w(B) / M(B)

Let M(A) : M(B) = k (say)

Hence the eqn becomes,

P(A) = P

w(A) w(A) + k.w(B)

Hence on solving we get,

P(B).w(A) - P(A).w(A) = k

P(A).w(B)

Here we get k as negative.

This makes the question wrong.

coolank2

Instead of the the big soln I gave above there's a logical one .......

PV = nRT

Thus, P

n

Thus, when B is added to the vessel already containing A, the number of moles will increase the pressure .......

This just contradicts the question itself ........

kmmankad

I think this question means:
there is a flask maintained @ 298K.
Gas A is introduced into it....P=1
Gas A is removed
Gas B is introduced into it....p=0.5
....Quest:Ratio of molecular wts.......

................I have solved it according to my understanding......
......i have the ans of 2:3............

PV=(w/M)RT
V=Volume of Flask
For Gas A,
1xV= (2/M1)x R x 298 -----------------------------1

For Gas B,
1/2 x V=(3/M2)x R x 298 --------------------------------2

From 1,2
On Dividing,
M1:M2::2:3

coolank2

Ya, your answer is absolutely right.

But what I mean that the question is framed in a wrong way .........

Q: When 2gm of A is introduced into an evacuated flask kept at 25^oC ,the pressure is found to be 1atm. If 3gm of B is then added to same flask, the total pressure becomes 0.5 atm the ratio of molecular weight of A & B .

Cheers !

yogesh_italia

2:3 is also wrong according to me the ans is 1:3

1xV = 2/m1 x R X 298 ------------------------------------------ (1)

1/2xV = 3/m2 x R X 298

V = 6/m2 x R x 298 --------------------------------------------(2)

from 1 and 2

6/m2 = 2/m1

3m1 = m2

1:3

tell my mistake if ne

sorry mankad

kmmankad

yes..small arithematic error...(But then does that mean that the question's ans is wrong?..maybe...)......Yogesh is rite.....its 1:3

yogesh_italia

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