Question

At a certain temperature the vapor pressure of pure acetyl bromide (CH_3 COBr) is measured to be 0.75 atm. Suppose a solution is prepared by mixing 51.8 g of acetyl bromide and 123. g of thiophene (C_4H_4S). Calculate the partial pressure of acetyl bromide vapor above this solution. Round your answer to 2 significant digits. Note for advanced students: you may assume the solution is ideal.

Answer 1

The partial pressure of acetyl bromide vapor above the solution is calculated using Raoult's Law and is found to be 0.15 atm.

Step-by-step explanation:

To calculate the partial pressure of acetyl bromide vapor above the solution, we can use Raoult's Law which states that the partial pressure of a component in a solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution. The formula is Pi = XiPio, where Pi is the partial pressure of component i, Xi is the mole fraction of component i, and Pio is the vapor pressure of the pure component i.

Firstly, we need to calculate the molar masses of acetyl bromide (CH3 COBr) and thiophene (C4H4S) to find the number of moles of each component. Molar mass of acetyl bromide =(12.01*2)+(1.008*3)+(79.904)+(35.453) = 136.967 g/mol. Molar mass of thiophene = (12.01*4)+(1.008*4)+(32.065) = 84.143 g/mol.

Next, we calculate the moles of acetyl bromide and thiophene:
moles of acetyl bromide = 51.8 g / 136.967 g/mol = 0.378 moles
moles of thiophene = 123 g / 84.143 g/mol = 1.461 moles

The total number of moles is 0.378 moles + 1.461 moles = 1.839 moles. Now we find the mole fraction of acetyl bromide:
Xacetyl bromide = 0.378 moles / 1.839 moles = 0.205

Finally, we multiply the mole fraction by the vapor pressure of pure acetyl bromide to find the partial pressure:
Partial pressure of acetyl bromide = 0.205 * 0.75 atm = 0.15 atm

Answer 2

0.17 atm

Step-by-step explanation:

For an ideal solution we can use Raoult´s law to solve this question:

Pa= XaPºa (a= acetyl bromide in this case)

where Pa = partial pressure acetyl bromide

Xa = mole fraction

Pºa= vapor pressure of pure acetyl bromide = 0.75 atm

So lets compute the mol fraction of acetyl bromide whice is given by:

Xa = mol of a / ( mol of a + mol of t ) (t stands for thiophene)

We know the masses so find or calculate the molar masses and plug the values:

MW a= 112.95 g/mol ⇒ mol a = 51.8 g/mol x 1 mol/122.95 g

mol a = 0.42

MW t = 84.14 g/mol ⇒ mol t = 123 g x 1mol/84.14 g= 1.46 mol

Xa = 0.42/ (0.42 + 1.46) = 0.22

Pa = 0.22 x 0.75 atm = 0.17 atm