Suppose a student started with 128 mg of trans-cinnamic acid and 0.52 mL of a 10% (v/v) bromine solution, and after the reaction and workup, ended up with 0.184 g of brominated product. Calculate the student's theoretical and percent yields.

Question

what are the equations and do i need to convert anything?

Answer 1

Here is a link showing cinnamic acid. Br2 adds across the double bond.
http://en.wikipedia.org/wiki/Cinnamic_acid

This is a limiting reagent problem; I now because values are given for BOTH reactants.
1. Convert 128 mg cinnamic acid to mols. mols = grams/molar mass
2. Convert the bromine to mols.

3a. Using the coefficients in the balanced equation, convert mols cinnamic acid to mols of the poduct.
3b. Do the same for mols Br2.
3c.It is likely that the mols for 3a and 3b will be different which means one of them is wrong.
3d. The correct value in limiting regent problems is ALWAYS the smaller value and the reagent providing that value is the limiting reagent.

4. g product = mols x molar mass. This is the theoretical yield (TY)

5. %yield = (actual yield/TY)*100 = ?

Answer 2

how do i find the limiting reagent without a reaction? is everything 1 mol to 1 mol?

i found 0.0008693 mols for the cinnamic acid
and 0.0023 mols for Br
that means the cinnamic is the limiting reagent

so i converted and got 0.069 g Br as the theoretical but that doesnt make any sense because shouldnt the theoretical be higher than the actual? (the actual is 0.184 g Br)

i dono why im having so much trouble, thanks for the help

Answer 3

The product isn't bromine, it's 2 3-dibromo-3-phenylpropanoic acid. The MM for the product should be 307.97g.