A sample of gas (1.9 mol) is in a flask at 21 °C and 697 mmHg. The flask is opened and more gas is added to the flask. The new pressure is 795 mmHg and the temperature is now 26 °C. There are now _ _ mol of gas in the flask.
A) 2.9
B) 0.28
C) 2.1
D) 1.6
E) 3.5

Question

Quizplus · Accepted Answer

We can use the combined gas law to solve for the new number of moles of gas in the flask:

(P1V1)/T1 = (P2V2)/T2

Where P1 = 697 mmHg, V1 is unknown (since we don't know how much additional gas was added), T1 = 21°C + 273.15 (to convert to Kelvin) = 294.15 K, P2 = 795 mmHg, V2 is also unknown, and T2 = 26°C + 273.15 = 299.15 K.

We can rearrange this equation to solve for V2 (the new volume of the flask):

V2 = (P1V1T2)/(P2T1)

Now, since the amount of gas (in moles) in the flask before more gas was added is equal to the amount of gas in the flask after more gas was added, we can set up a proportion to solve for the new number of moles (let n be the amount of additional gas added, in moles):

(1.9 mol)/(V1) = (1.9 + n mol)/(V2)

Substituting in V2 from the previous equation, we get:

(1.9 mol)/(V1) = (1.9 + n mol)(P2T1)/(P1V1T2)

Simplifying, we get:

V1(1.9 + n) = (1.9)(P2T1)

V1(1.9 + n) = (1.9)(795 mmHg)(294.15 K)/(697 mmHg)(299.15 K)

V1(1.9 + n) = 2.0667V1

1.9 + n = 2.0667

n = 0.1667 mol

Therefore, the total number of moles of gas in the flask after more gas is added is:

1.9 mol + 0.1667 mol = 2.0667 mol (which we can round to 2.1 mol)

Therefore, the answer is C) 2.1.

A Sample of Gas (1