The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non volatile, non electrolyte solid weighing 0.5 g is added to 39.0g of benzene. The vapour pressure of the solution then is 0.845 bar. What is the molecular mass of the solid substance

The molar mass is 169g / mol.

Explanation:

To achieve this result you must consider the formula:

[(pº-p) / pº] = [na / (na + nb)]

This formula should be replaced by the values shown in the question as follows:

pº = 0.850 bar

p = 0.845 bar

To find the values of "na" and "nb", we will consider that the letter "m" represents the molecular mass of benzene that is equal to 78, so we have:

na = 0.5 / m; nb = 39/78 = 0.5

Now we can replace all values in the formula:

0.850-0.845 / 0.850 = (0.5 / m) / (0.5 / m) +0.5 =

0.5 / (0.5 = 0.5m) =

1/1 + m =

m = 0.850 / 0.005 - 1 = 169g / mol

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