chemistry questions

Question

A 12.1912.19 g sample of a compound contains 3.893.89 g of potassium, K,K, 3.533.53 g of chlorine, Cl,Cl, and oxygen, O.O. Calculate the empirical formula. Insert subscripts as needed.

Empirical formula: KClO

A sample of trifluoroacetic acid, C₂HF₃O₂, contains 28.928.9 g of oxygen. Calculate the mass of the trifluoroacetic acid sample.

Answer 1

Find moles of each element:

moles K = 3.89 g K x 1 mol K / 39.10 g =0.0995 mols K

moles Cl = 3.53 g Cl x 1 mol Cl / 35.45 g = 0.0996 mols Cl

moles O = 12.19 g - 3.89 g - 3.53 g = 4.77 g O x 1 mol O / 16 g = 0.298 mols O

Divide all by the lowest value in an attempt to get all whole numbers:

moles K = 0.0995 / 0.0995 = 1.0 mols K

moles Cl = 0.0996 / 0.0995 = 1.0 mols Cl

moles K = 0.298 / 0.0995 = 3.0 mols O

Empirical formula = KClO₃

Find moles of O present:

moles O = 28.9 g O x 1 mol O / 16 g = 1.81 moles O

Find moles and grams of other elements:

moles C = 1.81 mols O x 2 mols C / 2 mols O = 1.81 mols C x 12 g / mol = 21.72 g C

moles H = 1.81 mols O x 1 mol H / 2 mols O = 0.905 mols H x 1 g / mol = 0.905 g H

mols F = 1.81 mols O x 3 mol F / 2 mols O = 2.72 mols F x 19 g / mol = 51.68 g F

Total mass of sample = 28.9 g + 21.7 g + 0.91 g + 51.7 g = 103 g (3 sig. figs.)