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.

0.17 atm

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