Edexcel A Level Chemistry:复习笔记1.6.1 Empirical & Molecular Formulae

• The molecular formula is the formula that shows the number and type of each atom in a molecule
  • E.g. the molecular formula of ethanoic acid is C2H4O2

   

• The empirical formula is the simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound
  • E.g. the empirical formula of ethanoic acid is CH2O

   

Empirical formula

• Empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
• It is calculated from knowledge of the ratio of masses of each element in the compound
• The empirical formula can be found by determining the mass of each element present in a sample of the compound
• It can also be deduced from data that gives the percentage compositions by mass of the elements in a compound

• The above example shows how to calculate empirical formula from the mass of each element present in the compound
• The example below shows how to calculate the empirical formula from percentage composition

Molecular formula

• The molecular formula gives the exact numbers of atoms of each element present in the formula of the compound
• The molecular formula can be found by dividing the relative formula mass of the molecular formula by the relative formula mass of the empirical formula
• Multiply the number of each element present in the empirical formula by this number to find the molecular formula

Answer

Step 1: Calculate relative mass of the empirical formula

• • Relative empirical mass = (C x 4) + (H x 10) + (S x 1)
  • Relative empirical mass = (12.0 x 4) + (1.0 x 10) + (32.1 x 1)
  • Relative empirical mass = 90.1

Step 2: Divide relative formula mass of X by relative empirical mass

• • Ratio between Mr of X and the Mr of the empirical formula = 180.2 / 90.1
  • Ratio between Mr of X and the Mr of the empirical formula = 2

Step 3: Multiply each number of elements by 2

• • (C4 x 2) + (H10 x 2) + (S x 2)     =    (C8) + (H20) + (S2)
  • Molecular Formula of X is C8H20S2

Ideal Gas Equation

• The ideal gas equation is:

PV = nRT

• • P = pressure in pascals (Pa)
  • V = volume in cubic metres (m3)
  • n = the amount of substance in moles (mol)
  • R = the gas constant, which is given in the Data Booklet as 8.31 J mol-1 K-1
  • T = temperature in Kelvin (K)

• The ideal gas equation can be used to find the amount of moles in a gaseous substance
  • It can also be used for volatile liquids at temperatures above their boiling point
• If the mass of the substance is known, then the molar mass can be calculated
  • This can then be used with empirical formula data to determine the molecular formula of a compound

Answer

Step 1: Calculate the number of moles of carbon, hydrogen and oxygen

Carbon:  moles

Hydrogen:  moles

Oxygen:  moles

Step 2: Divide by the smallest answer to get the ratio

Carbon:

Hydrogen:

Oxygen:

Step 3: State the empirical formula

• • The empirical formula is C4H8O

Step 4: Calculate the amount in moles, using PV = nRT

• • n = moles

Step 5: Calculate the molar mass

• • Molar mass

Step 6: Deduce the molecular formula

• • The empirical formula, C4H8O, has a mass of (4 x 12.0) + (8 x 1.0) + 16.0 = 72.0
  • Therefore, the molecular formula is also C4H8O