An organic acid containing only C, H, and O has the elemental composition by mass: — Stoichiometry / Organic Chemistry / Colligative Properties Chemistry Question
Problem Context
An organic acid containing only C, H, and O has the elemental composition by mass:
Determine the empirical formula of this acid.
Model Answer
C 40.7 ÷ 12.011 = 3.39 ÷ 3.39 = 1 × 2 = 2 → C2
H 5.1 ÷ 1.008 = 5.06 ÷ 3.39 = 1.5 × 2 = 3 → H3
O 54.2 ÷ 16.00 = 3.39 ÷ 3.39 = 1 × 2 = 2 → O2
Empirical Formula: C2H3O2
When 1.26 g of the acid is dissolved in 20.00 g of H2O, the solution freezes at –1.0 ˚C. Determine the apparent molar mass of the compound to the proper number of significant figures. Suggest an appropriate molecular formula for the compound [Kf = –1.86 ˚C•m–1].
Model Answer
∆T = km –1.93 = –1.86m m = 1.04
Molecular Formula: C4H6O4
When a 4.00 g portion of the solution in b. is titrated with 0.226 M NaOH, 17.8 mL is required to reach a phenolphthalein end point. Calculate the number of moles of NaOH used and the apparent molar mass of the compound.
Model Answer
(4.00g ÷ 21.26) × (1.26) = 0.237g compound
(0.226mol/L) × (0.0178L) = 0.00402 mol NaOH
(0.237g ÷ 0.00402mol) = 58.9/mol (× 2 = 117.8/mol ∴ 2 COOH/mol)
Use the results from b. and c. to draw a possible structural formula for this compound.
Model Answer
Structural Formula: HOOC CH2 CH2 COOH
This compound is refluxed with excess ethanol in the presence of an acid catalyst. Write a structural formula for the organic compound formed.
Model Answer
Structural Formula:
Suppose the reaction in e. does not go to completion. Outline a method to separate the organic product from the unreacted acid.
Model Answer
Aqueous solution of NaOH is added to mixture. Acid is converted to salt. Organic solvent (eg. C6H14 or C2H2Cl2) is added to extract product while salt remains in H2O phase.