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