Question

For an experiment I have to add 1.5 mL of 0.06M bromine in acetic acid, to 14 test tubes filled with 1.0mL of 0.1M of one of the following solutions: toluene,acetanilide, acetophenone, aniline, anisole, benzoic acid, benzonitrile, biphenyl, bromobenzene, chlorobenzene, ethylbenzene, methyl benzoate, phenol, or phenyl acetate. And I need to determine the limiting reagent and have no idea how to do it, thanks!

Answer 1

No. of mmol of bromine (present in acetic acid), you have to be added to 14 test tubes = 1.5*0.06, i.e. 0.09 = 9*10^-2

No. of mmol of solution filled in the 14 test tubes = 1*0.1, i.e. 0.1 = 10^-1

You have mentioned that the 14 test tubes filled with 1.0mL of 0.1M of one of the following solutions: toluene, acetanilide, acetophenone, aniline, anisole, benzoic acid, benzonitrile, biphenyl, bromobenzene, chlorobenzene, ethylbenzene, methyl benzoate, phenol, or phenyl acetate.

Note: Here, you have to be clear with one thing, i.e. when you mention something as molar solution, you have to think in what it is molar solution, like bromine in acetic acid.

According to the data you have provided, it's enough to say that bromine in acetic acid is the limiting reagent (which is less likely), as its no. of mmol to be added is less than the other reagents.

I feel you want to do bromination of all the 14 reagents present in the test tubes using bromine in acetic acid. Is it right? If it's the case, then you have to check the density (in g/mL) of each reagent. Since there is only 1.0 mL of the 14 reagents in 14 different test tubes, the density will be the weight of each reagent (in g.); then divide all the weights by corresponding reagent molecular weight to get no. of mmol of each of the 14 reagents. Now, you compare the no. of mmol of each reagent with that of bromine (in acetic acid).; which ever is less value, that will be the limiting reagent in each case. Here, I'm presenting all those calculations for you.

1. toluene: density = 0.867 g/mL, (= 0.867 g.), i.e. 0.867/92 = 9*10^-3 mmol. < n_Br2/AcOH (toluene is limiting reagent)

2. acetanilide: density = 1.22 g/mL, (= 1.22 g.), i.e. 1.22/135 = 9*10^-3 mmol. < n_Br2/AcOH (acetanilide is limiting reagent)

3. acetophenone: density = 1.03 g/mL, (= 1.03 g.), i.e. 1.03/120 = 8.6*10^-3 mmol. < n_Br2/AcOH (acetophenone is limiting reagent)

4. aniline: density = 1.02 g/mL, (= 1.02 g.), i.e. 1.02/93 = 11*10^-3 mmol. < n_Br2/AcOH (aniline is limiting reagent)

5. anisole: density = 0.995 g/mL, (= 0.995 g.), i.e. 0.995/108 = 9*10^-3 mmol. < n_Br2/AcOH (anisole is limiting reagent)

Like wise, you perform all the calculations for remaining reagents - benzoic acid, benzonitrile, biphenyl, bromobenzene, chlorobenzene, ethylbenzene, methyl benzoate, phenol, or phenyl acetate.

In every case, n_reagent < n_Br2/AcOH

Hence the 14 different reagents (filled in 14 test tubes) to brominated are the limiting reagents and you have to take the excess of bromine (in acetic acid) in every case.