Compounds, formulae, and equations & Amount of substance

• Balancing chemical equations


  The fact that we have to balance chemical equations comes from the law of conservation of mass
• Balancing chemical equations


  • Visualising the atoms can help you to balance
  • At a-level, fractions can be used as well as whole numbers, as this conveys the ratio of moles needed
• Writing chemical equations from supplied formula

  These sorts of questions test your knowledge of chemical formulae, ability to balance equations, and understanding of state symbols
• State symbols


  • Solids (s)
  • Liquids (l)
  • Gas (g)
  • Aqueous (aq)
• Ionic equations


  • When aqueous, elements are in their separate ion forms
  • As solids, liquids, and gases, they are compounds, as in these forms they are insoluble
  • Ions that appear on both sides of the equation are spectator ions and so can be removed, as they have not changed
  • If an equation shows only the non-spectator ions (only the species or particles actually involved in the reaction, and so have changed), it is called an ionic equation
• Substances that exist in the aqueous form

  • Soluble ionic compounds (e.g NaCl, KNO3 ..etc)
  • Strong acids and bases (e.g: HCl, H2SO4, NaOH ..etc)
• Substances that don't exist as aqueous ions

  • Insoluble ionic compounds (e.g: CaCO3)
  • Transition metal hydroxide precipitates
  • Covalent compounds such as CO2
• The mole


  • The amount of any substance that contains the same number of particles as there are carbon atoms in 12g of carbon-12
  • We use moles because the masses for atoms are very very big numbers, so using the mole makes calculations less complicated as it scales the numbers up to much shorter, easier numbers
• Avogadro's number (NA)


  Number of atoms in a mole (6.02 x 10^23)
• The number we see on the periodic table are the mass of 6.02 x 10^23 atoms of each element
• Moles


  • Moles= Mass (g)/Relative mass
  • To work out the number of particles/atoms in a substance, times the number of moles by avogadro's number
  • To know avogadro's number of a compound simply times the result by the sum of the smallest ratio of atoms in the compound (i.e: for water, this number would be three)
• Empirical formula


  The simplest whole number ratio for atoms of each element in a compound
• Molecular formula


  • The actual number of atoms of each element in a compound
  • The ratios refer to moles (i.e: CH2 consists of one mole of carbon to 2 moles of hydrogen)
• For an empirical formula calculation

  1. Put the elements into a columned table

  2. Divide the masses by their Mr (giving you the number of moles present) or percentages by 100

  3. Divide all by the smallest answer

  4. Round the answers you get where necessary, thus telling you the ratio of the elements

  5. Write the compound based on the ratio
• To go from the empirical to the molecular


  Divide the correct Mr of the compound by the mass you have empirically, then times all elements in the empirical compound by the number produced

  
• With empirical formula, until the ratio point, moles should be kept at least as long as 3sf
• Molar gas volume


  One mole of any gas at 100 KPa of pressure and 25 degrees celsius occupies a volume of 24 dm3 (24 dm3 = 24000 cm3)
• When working out the volume of gas produced

  1. Identify the known (value given in question) and unknown (what they want you to find out) in the balanced equation

  2. Calculate the moles of the known using the mole equation

  3. Deduce the moles of the unknown from the mole ratio (answer for the known adjusted for the amount of moles in the unknown)

  4. Calculate the volume of the unknown using the molar gas volume equation

  5. Remember to adjust units for equations and what is asked for in the question (i.e: mass should always be in grams)
• For reacting amount calculations- Masses

  1. We identify the known and unknown substances in the balanced equation

  2. We calculate the moles of the known (using the mole equation)

  3. We deduce the moles of the unknown from the mole ratio

  4. We calculate the mass of the unknown (using the rotated mole equation)
• Hydrated salts


  • When salt crystals form, you sometimes get loosely bonded water molecules attached to them
  • Salts in this condition are known as hydrated salts
  • Because these water molecules are formed when the salt crystals form, they are known as water of crystallization
• Hydrated


  A compound that contains water molecules
• Anhydrous


  A substance that contains no water molecules
• Waters of crystallisation


  Water molecules that form an essential part of the crystalline structure of a compound
• The empirical formula of the compound is separated from the waters of crystallisation by a dot

  
• At a level, the number in front of the water molecules is always a whole number (if you get a decimal for them in practicals, round up or down)
• To work out the amount of waters of crystallization based on mass data

  Work out the difference in mass between the reactant and the product (thus giving you the mass of the water)then treat like empirical formula

  
• To work out the amount of waters of crystallisation based on percentage composition data

  Treat it like an empirical formula question

  
• To identify an unknown metal in a hydrated salt

  1. Look at the charge of the non-metal part of the salt in order to work out which group the metal is in

  2. Work out the difference in mass between the reactant and the product you are interested in (this gives you the mass of the water)

  3. Then work out the moles of the other product

  4. Use your answer (adjust for mole difference between products) to work out the moles of the salt containing the metal you are looking for

  5. Use the mass and moles for the metal salt to work out the Mr

  6. Then remove the Mr of the non-metal bit to get the Mr of the metal
• To work out the number of water of crystallisation experimentally

  1. Heat a hydrated salt to remove the water

  2. Record mass of hydrated salt

  3. Record mass of anhydrous salt

  4. Record the difference in mass (water)
• Ideal gas equation


  • The ideal gas equation is used over the other as it is more accurate
  • The equation is: pV= nRT
• Factors in the ideal gas equation

  • p= pressure in pascals (kPa= 1000 Pa, 1 atmosphere= 101 kPa, or in Nm^-2 (Newton metres), with Nm^-2 = Pa)
  • v= volume in m^3 (a decimetre is 1/1000th of a cubic metre, and a centimetre is one millionth of a cubic decimetre)
  • n= the number of moles
  • R= gas constant (8.314 Jk^-1 mol^-1)
  • T= temperature in kelvin (basically celsius plus 273)
• An ideal gas


  • Has a random motion of particles
  • Has completely elastic collisions between particles
  • Has particles of a negligible size
  • Has no intermolecular forces between particles
• You can tell if a question is an ideal gas question if two or more of the factors in the gas equation from one side are present
• % Composition by mass

  % by mass= (mass of element in 1 mole/ (Mr/Rfm)) x 100