A) an unknown liquid with mass of 0.515 g occupies 223 mL at 99.6 Celsius and 76

Question

A) an unknown liquid with mass of 0.515 g occupies 223 mL at 99.6 Celsius and 765 torr. i) Calculate the number of moles in the gas present ii) Calculate the molar mass of the gas. B) Assume that in A, the atmospheric pressure is mistakenly recorded as 565 torr. i) What would be the calculated molar mass? ii) Calculate the percent error between the molar mass in A(ii) and B(i). iii) In this experiment, how is the mass of a volatile liquid determined? iv) in this experiment, how is the volume of gas determined? What assumption is made? v) In this experiment, how is the temperature of the gas determined? What assumption is made?

Explanation / Answer

Assuming ideal behaviour,

P V = n R T

A.

(i)

(765 / 760) * (223 / 1000) = n * 0.0821 * (99.5 + 273.15)

n = 0.00734 mol

(ii)

Moles = mass / molar mass

Molar mass = 0.515 / 0.00734 = 70.2 g/mol

B.

(i)

(565 / 760) * (223 / 1000) = n * 0.0821 * (99.5 + 273.15)

n = 0.00542 mol

Molar mass = 0.515 / 0.00542 = 95.0 g/mol

(ii)

% error = (70.2 - 95) * 100 / 70.2 = 35.4 %

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