BRADY PROBLEM 2.83 Tutorial on the procedure of solving for empirical and molecular formulas from a combustion reaction.

Problem Citric Acid, the substance that makes lemon juice sour, is composed of only carbon, hydrogen, and oxygen. When a g sample of citric acid was burned, it produced g CO 2 and g H 2 O. The molecular mass of the compound is 192. What are the empirical and molecular formulas for citric acid? (C + H + O) g CO g + H 2 O g + O2O2

(C + H + O) g CO g + H 2 O g + O2O2 All the hydrogen has gone to the product of H 2 O All the carbon has gone to the product of CO 2 From these gram quantities, convert to moles of compound, then to moles of elements… ( you may wish to subtotal as you will use the number of moles later )… then to grams of elements. Starting with CO 2 ….

Convert grams of CO 2 to moles of CO 2 to moles of C then to grams of carbon g CO 2 1 mol CO 2 1 mol C= mol C 44.0 g1 mol CO 2 = mol C g= grams C 1 mol C

Convert grams of H 2 O to moles of H 2 O to moles of H then to grams of hydrogen g H 2 O1 mol H 2 O2 mol H= mol H 18.0 g1 mol H 2 O = mol H1.008 g= grams H 1 mol H

Find the number of grams of oxygen in the original sample g original sample size subtract grams hydrogen subtract grams carbon g remaining grams are the grams of oxygen in the original sample g O1 mol O= mol O g Convert these grams to moles of oxygen

Find the lowest mole ratio for the empirical formula mol C/ = 1x 6 = mol H/ = 1.333x 6 = mol O/ = 1.166x 6 =7 C6H8O7C6H8O7

Find the Molecular Formula from the given Molecular Weight Given molecular weight = 192 g/mol Empirical Formula determined: C 6 H 7 O 7 Calculate the mass of the empirical formula: C = X 6 = H = x 8 = O = x 7 = g/mol Divide the mass of the empirical into the molecular weight: 192/ = = 1 therefore the empirical formula is the same as the molecular formula