1.2.8 Concentration Calculations

Step by step

• Concentration calculations involve bringing together the skills and knowledge you have acquired previously and applying them to problem solving
• You should be able to easily convert between moles, mass, concentrations and volumes ( of solutions and gases)
• The four steps involved in problem solving are:
  • write the balanced equation for the reaction
  • determine the mass/ moles/ concentration/ volume of the of the substance(s) you know about
  • use the balanced equation to deduce the mole ratios of the substances present
  • calculate the mass/ moles/ concentration/ volume of the of the unknown substance(s)

Answer:

Step 1: Write the balanced equation for the reaction

Na₂CO₃  +  2HCl  →  2NaCl  +  H₂O  +  CO₂

Step 2: Determine the moles of the known substance, in this case sodium carbonate. Don't forget to divide the volume by 1000 to convert cm³ to dm³

moles = volume x concentration

amount (Na₂CO₃) = 0.0250 dm³ x 0.050 mol dm^-3 = 0.00125 mol

Step 3: Use the balanced equation to deduce the mole ratio of sodium carbonate to hydrochloric acid:

1 mol of Na₂CO₃ reacts with 2 mol of HCl, so the mole ratio is 1 : 2

Therefore 0.00125 moles of Na₂CO₃ react with 0.00250 moles of HCl

Step 4: Calculate the concentration of the unknown substance, hydrochloric acid

Answer:

Step 1: Write the balanced equation for the reaction

CaCO₃  +  2HCl  →  CaCl₂  +  H₂O  +  CO₂

Step 2: Determine the moles of the known substance, calcium carbonate

Step 3: Use the balanced equation to deduce the mole ratio of calcium carbonate to hydrochloric acid:

1 mol of CaCO₃ requires 2 mol of HCl

So 0.025 mol of CaCO₃ requires 0.050 mol of HCl

Step 4: Calculate the volume of HCl required