Gravimetric calculations connect a known chemical amount with the mass of a specified precipitate. Convert the starting mass to moles, apply the balanced equation ratio, then multiply product moles by its molar mass. For CaCl2+2AgNO3→2AgCl + Ca(NO3)₂, each mole of CaCl2 gives two moles of AgCl, assuming complete precipitation and excess silver nitrate.
Reverse the chain when a dry precipitate mass is supplied: divide by precipitate molar mass, use the ratio back to analyte amount, then calculate its mass or solution concentration. Percentage purity is inferred analyte mass divided by original sample mass times 100. The stated complete-reaction, dry-solid and no-interference assumptions are part of each calculation.
Example 1
0.117g pure NaCl reacts with excess AgNO3. Use M(NaCl)=58.5 and M(AgCl)=143.5gmol−1. Find dry AgCl mass.
NaCl and AgCl have a 1:1 ratio. n(NaCl)=58.50.117=0.00200 mol; m(AgCl)=0.00200×143.5=0.287g.
Example 2
A 50.0mL CaCl2 sample gives 0.574g dry AgCl. Use M(AgCl)=143.5gmol−1 and complete precipitation. Find c(CaCl2).
n(AgCl)=143.50.574=0.00400 mol. n(CaCl2)=20.00400=0.00200 mol; c=0.05000.00200=0.0400 mol L−1.
Watch out: Coefficients compare moles; convert with each substance’s own molar mass before and after the ratio step.