Learning focus
Use chemical formulae and equations accurately, convert measured quantities into moles, apply balanced-equation ratios and present multi-step calculations with units and checks.
General method
Find moles of the known dissolved reactant using n=cV, apply the balanced-equation ratio, then calculate the requested concentration, volume or mass.
Multiple aqueous reactants
When two solution quantities are provided, limiting-reactant analysis may be required. Convert both to moles and compare relative to coefficients.
Precipitate mass
For precipitation, solution moles lead to precipitate moles through the ionic or molecular equation, then to precipitate mass using molar mass.
Gas from solution
If an aqueous reactant produces gas, find gas moles by ratio and use 24 dm3/mol at r.t.p. for volume.
Worked example
25.0 cm3 of 0.200 mol/dm3 AgNO3 contains 0.00500 mol. With excess NaCl, AgCl forms 1:1, so AgCl=0.00500 mol; mass=0.00500×143.5=0.7175 g.
Practical or data skill
Use solution and precipitation data to predict product mass, then compare with an observed filtered mass.
Examination tip
Do not use concentration values directly in mole ratios; convert to moles first.
Review questions and suggested answers
Question 1
Why convert solution data to moles?
Suggested answer
Equation coefficients compare moles.
Question 2
What mass AgCl forms in the worked example?
Suggested answer
About 0.718 g.
Question 3
When are two solution reactants both considered?
Suggested answer
When neither is stated to be in excess and a limiting reactant may exist.
