Calculations for titration.

Help with Titration calculations! Please :-)?

• I have done a Mohr titration in 2nd year chem and struggling with the calculations, can someone help please. Have asked my lecturer but been led astray!! Here are the details: Started with an unknown sample (containing .22-.44g of KCl and .50-1.00g KI) Titrated (using .7135M AgNO3) to two end points; first being KI at 12.75mL, then KCl precipitated at 31.97mL. The initial unknown (KI + KCl) was diluted to 100mLs with water, then 25mL sample used in the titration. Here is what I have done... but something doesnt work, thinking maybe I need to work out percentages. ---------------- Mass of KI in sample: C1 V1 = C2 V2 0.07133 x .025 = C2 x 0.01275 C2 =.139901 .139901 x 165.89 (MW KI) =23.20 (23.20/1000) x 25 mL = 0.58 grams KI Mass of KCl in sample: MW KCl = 74.551 C1 V1 = C2 V2 .7133 x .025 = C2 x .01922 C2 = .92780 0..9278 x 74.551(MW KCl) = 41.58 =1.03959

• Answer:

  There are several mistakes in the calculation of KI. You stated that the AgNO3 has a concentration of 0.7135 M and then used 0.07135 M in the calculation. Second, the sample was diluted to 100 mL, but it looks like you used 1000 mL instead. Another problem is that when you multiply the concentration by the MW, you still have a concentration, not a mass (this is why you should always include your units in the calculation). Using your numbers, the last line of the calculation should be 23.20 * (100 / 25) * original volume.