Question

what is theoretical yield and actual yield of this experiment.

1.104g of cysteine hydrochloride monohydrate added with 2.5 ml of freshly made 10% aqueous sodium hydroxide solution and 10 ml of 3% aqueous hydrogen peroxide solution.

product was 0.336g of Cystine

Answer 1

To determine the theoretical and actual yields of a product from a reaction, first the reactions limiting reagent should be found and that is the reagent with the least no.of moles used. From the molar masses of Cys.HCl as 157.612g/mol, NaOH as 40g/mol and that of H2O2 as 34.0147g/mol it can be determined that the no.of moles of each substance as no.of moles = (mass/molar mass) which gives the no.of moles of the reactants as 7.00454mmoles of Cys.HCl. 10% NaOH means 100g of the compound in 1L of solution implying 0.25g in 2.5mL which gives 6.25mmoles and 3% of peroxide is 0.3g in 10mL which is 8.8127mmoles.

Cystine is the dimer of two molecules of cysteine sans a hydrogen molecule and so in terms of equivalents it can be said that two equivalents of cysteine is required to make one of cystine. This shows that cysteine is the limiting reagent, the final no.of moles of cystine being 7.00454/2 = 3.50227mmoles. Taking the molar mass of Cys which is 240.3g/mol, 0.336g of Cys contains 0.336/121.16 = 2.7732mmoles.

Theoretical yield hence is 3.50227mmoles of Cys which is 0.00350227x240.3 = 0.8416g.

The actual yield or yield % is given as (Experimental yield/Theoretical yield)x100 = 39.924%.