Chemical Equation

Chemical Equation
1. Importance of chemical equation:
The types of reactants; the physical conditions; the quantity of reactants and products and stated in moles.
nA + nB –> pC + pD
2. Reactants are written in the left side of the reaction and products are written in the right side of the reaction.

3. Information obtainable from chemical equations.

• i) mass of reactants
• ii) volume of reacting gas
• iii) mass of products formed
• iv) volume of gas produced

• Example:
  2 cm³ of lead (II) nitrate solution is added to excess of potassium iodide solution.
  How many molecules of potassium nitrate will be formed?
  [Relative atomic mass: N, 14; O, 16; K, 39; I, 127; Pb, 207; Avogadro's constant: 6.02 x 10²³ mol^-1]

Step 1: Write a complete chemical equation.

• Pb(NO₃)₂(aq) + 2KI(aq) –> PbI₂(s) + 2KNO₃(aq)
• From the equation, 1 mole of Pb(NO₃)₂ reacts with 2 moles of KI formed 1 mole PbI₂ of and 2 moles of KNO₃.

Step 2: Convert to moles.

• No. of moles of Pb(NO₃)₂
  = Mass of Pb(NO₃)₂ / Relative molecular mass
  = 2 / [207 + 2(14 + 3 x 16)]
  = 6.04 x 10^-3 mol

Step 3: Ratio of moles.

• Number of moles of KNO₃/ Number of moles of Pb(NO₃)₂
  = 2/1
• Number of moles of KNO₃
  = (2 x 6.04 x 10^-3) / 1
  = 12.08 x 10^-3 mol

Step 4: Convert to the number of molecules of potassium nitrate.

• Number of molecules of KNO₃
  = 12.08 x 10^-3 x 6.02 x 10²³
  = 7.27 x 10²¹

 

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