An unknown compound A contains only carbon, oxygen, and chlorine. — Stoichiometry / Gas Laws Chemistry Question
Problem Context
An unknown compound A contains only carbon, oxygen, and chlorine.
A 3.00 g sample of A is completely vaporized in a 1.00 L container at 70.0 ºC and exerts a pressure of 0.854 atm. What is the molar mass of A?
Model Answer
n = PV/RT = (0.854 atm)(1.00 L)/(0.0821 L atm mol-1 K-1)(343.2 K) = 0.0303 mol
3.00 g/0.0303 mol = 99.0 g/mol
A 0.300 g sample of A is added to 100 mL water, which results in the conversion of all the chlorine in A to HCl. After briefly bubbling nitrogen gas through the solution, the hydrochloric acid is titrated with 0.2000 M NaOH solution. The titration requires 30.33 mL added NaOH to reach a phenolphthalein endpoint. What is the mass percentage of chlorine in compound A?
Model Answer
0.03033 L NaOH soln 0.2000 mol L-1 = 0.06066 mol NaOH
There were thus 0.06066 mol Cl 35.45 g mol-1 = 0.2150 g Cl in the sample
(0.2150 g Cl/0.300 g sample) 100% = 71.7% Cl by mass
Propose a molecular formula for A and draw a reasonable Lewis structure for it
Model Answer
In 99.0 g (1 mol) A, there is (0.717 99.0 g)/(35.45 g mol-1 Cl) = 2 mol Cl
The remaining mass in 1 mol A is 99.0 g mol-1 – (2 35.45 g mol-1) = 28.1 g mol-1
The only combination of atomic masses of C and O that is close to 28 g mol-1 is C1O1.
So the molecular formula is COCl2:
Write a balanced equation for the reaction of A with water (as described in part (b)).
Model Answer
COCl2(g) + H2O(l) CO2(g) + 2 HCl(aq)