[emgra0]

Got this as a practice exam question.

4.31 g of a compound of carbon, hydrogen and nitrogen contains 2.68 g of carbon and 1.04 g of nitrogen. The compound is a base and one mole of it reacts with two moles of hydrochloric acid. In a typical titration, 0.1174 g of the compound was just neutralised by 20.58 mL of 0.0984 M HCl. What is the molecular formula of the compound.

I've been able to get as far as the emperical formula which is C₃H₈N, and the molar mass of the emperical is 58.104g/mol. The answer given is MF= C₆H₁₆N₂ and a mass of 116g/mol, but I am confused as to how I need to get to that answer

[AWK]

neutral organic compound with formula C₃H₈N cannot exists (one N atom, odd number of H atoms).
You have a hint - compound netralizes 2 moles of  - this means you should double the empirical formula (each amine group neutralize one HCl molecule). Moreover, this is an excessive information - even without this hint the only answer is C₆H₁₆N₂ since this is formula for saturated diamines (to saturated hydrocarbon C₆H₁₄ add 2N+2H).

[KungKemi]

Hey,

So first of all the title of your post is quite contradictory because usually in order to completely determine the molecular formula of a given compound, one must know both the empirical formula as well as the molar mass, or, as stated by AWK, a molar ratio. As such, it is assumed that the molar mass of the compound can be determined via. some means using the information provided from this question...

Now, if you look carefully you will notice that there are two parts: a composition part (to find the empirical formula), and a stoichiometry part (to find the molar mass, and hence the molecular formula).

In order to determine the molar mass of a compound, one must know two pieces of information: the amount of moles present for a given mass of the substance. This can be seen as follows for part two: "In a typical titration, 0.1174 g of the compound was just neutralised by 20.58 mL of 0.0984 M HCl." As can be seen, the mass of the compound is given, and, equally, the moles of the substance are also given however this is concealed within the titration part of the question. Remember that molarity equals moles of a substance over the volume (in L) of said substance. Since molarity and volume of HCl is given, one can determine the amount of moles of HCl required to react with said amount of the unknown compound. And since it was previously stated that the unknown compound reacts with the HCl in a ratio of 1:2, one can solve to find the moles of the unknown, and, as such, determine the molar mass of the compound.

Equally, as stated by AWK, one could use tuition in order to determine the molecular formula, and, as such, side step the stoichiometry part of the question, however, both ways are equally correct.

I hope this helps,
KungKemi