Lower 6 Chemistry Mole Concept Work sheet1. Empirical and Molecular FormulaeExample: A compound contains 24.24% C, 4.04% H and 71.72% Cl. The molar mass of the compound is 99 g/mol. Calculate the empirical and molecular formulae of the compound.SOLUTIONCHCl

This is the same for mass in 100g% composition24.244.0471.72

Mass(g)24.244.0471.72

Atomic mass for each element12135.5

To find the no. of moles: Mass/atomic massNo. of moles

To find the mole ratio: divide throughout by smallest no. of moleMole ratio

C=1

H=2

Cl=1

NB: If the mole ratio ends in (a) 0.5 then multiply by 2, (b) 0.3 multiply by 3, (c) 0.25 multiply by 4 and (d) 0.9 round up to nearest whole number.Empirical Formula: Molecular Formula: Molar Mass of

Molecular Formula is two (2) times the empirical formula Molecular Formula: Exercise #11. Calculate the empirical formula of a compound composed of 38.67 % C, 16.22 % H, and 45.11 %N2. Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula?3. Ibuprofen, a common headache remedy, has the empirical formula of and molar mass of 215g/mol. Calculate the molecular formula.2. Calculating Empirical formula and Molecular formula using Combustion reactionsExample: 0.5g of an organic compound X containing H:C:O produces 0.733g of carbon dioxide and 0.3g of water on complete combustion. The RMM of the compound is 60. Calculate the empirical and molecular formulae.SOLUTION

The masses for the compound X (0.5g), carbon dioxide (0.733g) and water (0.3g) are given.Carbon dioxide:- Find the mass of C1. . To find the mass of C, we use carbon dioxide since it is the only C containing compound for the products formed. First, find the moles of carbon dioxide

Second, in order to find the mass of C, the mol of C must be known.Carbon dioxide is made up of one Carbon atom and two oxygen atoms. Therefore mole ratio of C: is 1:1. Hence, the number of moles for Carbon is 0.0167 mol.

Lastly, mass of Carbon

The mass of C is 0.2004g and mole of C is 0.0167mol.Water: - Find the mass of H.2. We use water since it is the only H containing compound for the products formed. First, find the moles of carbon dioxide

Second, in order to find the mass of H, the mol of H must be known.Water is made up of one oxygen atom and two hydrogen atoms. Therefore mole ratio of is 1:2. Hence, the number of moles for H is 0.0333 mol H.

Lastly, mass of Hydrogen

The mass of H is 0.0333g and mole of H is 0.0333 mol.Oxygen 3. Mass of oxygen atom = Mass of compound X (Mass of hydrogen + Mass of carbon) Mole of oxygen

Since we know the number of moles for C, H & O, then we go ahead and find the mole ratio rather than using the masses.CHO

No. of moles0.01670.03330.0166

Mole ratio

Empirical Formula: Molecular formula:

Exercise #21. A compound Z contains C:H:O and RMM is 46. Complete combustion of 1.0g of Z gives 1.91g carbon dioxide and 1.17g Hydrogen. Find the empirical and molecular formulae of compound Z.2. When 3.79 grams of an organic compound containing carbon, hydrogen and oxygen is burned, 6.61 grams of CO2 and 3.59 grams of H2O are produced. What is the empirical formula of this compound?3. Molar concentration and Mass concentrations

Example: 4.90g of pure sulphuric acid was dissolved in water; the resulting total volume was 200 cm3. 20.7 cm3of this solution was found on titration, to completely neutralize 10.0 cm3of a sodium hydroxide solution. a) Write the balanced equation for the titration reaction, include state symbols.b) Calculate the molarity of the sulphuric acid solution.c) Calculate the moles of sulphuric acid neutralized.d) Calculate the moles of sodium hydroxide neutralized.e) Calculate the molar concentration of the sodium hydroxide.SOLUTION1. 2.

3.

4. From the balanced equation in part (a), 1 mole of reacts with 2 moles of NaOH. Hence, mole ratio of Therefore, 0.0052 mol reacts with 0.0104 mol NaOH5. Exercise #3 1. A solution of sodium hydroxide contained 0.250 mol dm-3. Using phenolphthalein indicator, titration of 25.0 cm3of this solution required 22.5 cm3of a hydrochloric acid solution for complete neutralization.a) Write the equation for the titration reaction, include state symbols.b) What apparatus would you use to measure out:(i) The sodium hydroxide solution? (ii) The hydrochloric acid solution?c) What is the indicator color change at the end-point?d) Calculate the moles of sodium hydroxide neutralized.e) Calculate the moles of hydrochloric acid neutralized.f) Calculate the molar concentration and mass concentration of the hydrochloric acid.

2. A solution made from pure barium hydroxide contained 2.74 g in exactly 100 cm3of water. Using phenolphthalein indicator, titration of 20.0 cm3of this solution required 18.7 cm3of a hydrochloric acid solution for complete neutralization. (a) Write the equation for the titration reaction and include state symbols.(b) Calculate the molarity of the barium hydroxide solution.(c) Calculate the moles of barium hydroxide neutralized.(d) Calculate the moles of hydrochloric acid neutralized.(e) Calculate the molarity of the hydrochloric acid.

4. Molecular and Ionic/Net ionic equationsA molecular equation is one that shows the chemical formulae of all reactants and products

An ionic equationis an equation that shows only the ions reacting and omits thespectator ions, which go through the reaction unchanged.To write the complete ionic equation: Start with abalancedmolecular equation Break all soluble strong compounds (compounds with (aq) beside them) into their ions Indicate the correct formula and charge of each ion. Indicate the correct number of each ion. Write (aq) after each ion.DO NOT BREAK COMPOUNDS THAT ARE IN SOLID, GASEOUS AND LIQUID STATESExample: 6 Na+(aq) + 2 PO43-(aq) + 3 Ca2+(aq) + 6 Cl-(aq) 6 Na+(aq) + 6 Cl-(aq) + Ca3(PO4)2(s) Cross out the spectator ions that are present6 Na+(aq) + 2 PO43-(aq) + 3 Ca2+(aq) + 6 Cl-(aq) 6 Na+(aq) + 6 Cl-(aq) + Ca3(PO4)2(s) Write the "leftovers" as the net ionic equation2 PO43-(aq) + 3 Ca2+(aq) Ca3(PO4)2(s)

Exercise #4Complete and write the ionic equations for the following:-1. 2. 3. 4.

5. Avogadros Law Avogadro's Law states that equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties.

Mole 1 mole (mol) is the amount of matter that contains as many objects (atoms, molecules) as the number of atoms in isotopically pure 12C1 mole contains 6.02 x 1023 particles6.022 x 1023 is known as Avogadros number 1 mol 12C atoms = 6.02 x 1023 12C atoms 1 mol H2O molecules = 6.02 x 1023 H2O molecules 1 mole NO3- ions = 6.02 x 1023 NO3- ions

Molar mass The massof one mole of a substance (chemical element or chemical compound) and is measured in g/mol