Question

Using the reaction shown below, calculate the percent yield obtained by a student when reacting 5.0 g of 3-methoxysalicylic acid with 3.9 mL of iodomethane in the presence of 10.3 g of potassium carbonate and obtaining 4.5 g of product, 2,3-dimethoxymethyl benzoate. You should determine which of the three reactants is the limiting reagent and the theoretical yield first.

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Answer 1

We are given, mass of 3-methoxysalicylic acid = 5.0 g , volume of iodomethane = 3.9 mL , potassium carbonate = 10.3 g , Actual mass of product 2,3-dimethoxymethyl benzoate = 4.5 g

Reaction –

3-methoxysalicylic acid + iodomethane + potassium carbonate ------> 2,3 dimethoxymethyl benzoate

Now we need to calculate the moles of each reactant

Mass of iodomethane = density * volume

                                   = 2.28 g/ml * 3.9 mL

                                   = 8.892 g

Moles of 3-methoxysalicylic acid = 5.0 g / 168.15 g.mol-1

                                                      = 0.0297 moles

Moles of iodomethane = 8.892 g / 141.94 g.mol-1

                           = 0.0626 moles

Mole of potassium carbonate = 10.3 g / 138.21 g.mol-1

                                               = 0.0745 moles

Now we need to calculate the moles of 2,3-dimethoxymethyl benzoate from each reactant

From the above-balanced reaction, all have a mole ration of 1:1

So each mole of reactant is equal to the moles of the product.

So the lowest moles of 2,3-dimethoxymethyl benzoate is 0.0297 moles and it is from 3-methoxysalicylic acid since there is mole ratio between the 3-methoxysalicylic acid and 2,3-dimethoxymethyl benzoate is 1:1

So, the limiting reactant is 3-methoxysalicylic acid and

Moles of 2,3-dimethoxymethyl benzoate = 0.0297 moles

Mass of 2,3-dimethoxymethyl benzoate = 0.0297 moles * 196.20 g/mol

                                                                  = 5.83 g

So, the theoretical mass of 2,3-dimethoxymethyl benzoate is 5.83 g

We know, Percent yield = actual mass / theoretical mass * 100 %

                      = 4.5 g / 5.83 g * 100 %

                      = 77.1 %