Percent yield/ Theoretical Yield

Question

I was trying to figure out the limiting reagent of a reaction that I did in lab. The reaction is 1-phenyl ethanol that is oxidized to acetophenone using (5% aq. NaOCl (2.2 equiv)) and (HOAc (15M)). ' In the reaction we used 50 mL of Sodium hypochlorite and 2 mL of Acetic acid. Given that i have to convert mL to grams, can you safely assume that 50 mL of sodium hypochlorite is = 50 grams? and 2 mL of acetic acid is = 2 grams? The final product's mass (acetophenone) we achieved is 2.626 grams. Can someone help me find the limiting reagent and the percent yield?

Answer 1

In lab, you should not assume the density of anything is 1 g/mL unless it is distilled water. You would have already listed the physical properties of these compounds in your data table before entering the lab. The density (ρ) should be included. 

 

Please do not just assume mL equals grams of any compound except pure water. No you cannot safely assume the density of either NaOCl or HOAc is 1.0. That is not acceptable when you have the physical data present.

Furthermore, you provide the final mass of the acetophenone you made assuming that is dried to constant mass after satisfactory purification. The problem is you do not list the amount of starting material. It is not sufficient to just include the molar equivalents of reagent unless the grams and therefore moles of 1-phenylethanol is included. There is not enough information to answer the question. The other problem is that all limiting reagent questions require a balanced chemical equation. Please start there first.

Answer 2

The oxidation of the primary alcohol (phenyl ethanol, PE) to acetophenone using NaOCl appears to be on a 1:1 mole ratio of NaOCl : PE.  In this case, you don't need to assume a density of 1 g/ml for the NaOCl, since you are given 2.2 equivalents.  If you know the equivalents of NaOCl that is as good as grams or moles.  You didn't indicate the amount of PE you started with as a reactant, so one cannot tell which reactant is limiting form the information provided.  It will not be the acetic acid as that is only there to provide an acidic environment for the oxidation/reduction reaction.

 

So, in summary, you should compare equivalents of NaOCl to equivalents of PE to determine limiting reactant, and proceed from there to find yield.