Analyte: Solution with the unknown concentration Standard: Solution with the unknown concentration You must have a BALANCED chemical equation before looking at the stoichiometry
pH is a measurement of the H+ concentration, pH = - log (H+) pOH is a measurement of the OH- concentration, pOH = - log (OH-) pH + pOH = 14 If you start with a solution of HCl, is the pH going to be high or low? What happens to that pH as base is added? Before doing any calculations you must start with a balanced chemical equation. To predict the products of an acid-base reaction, ions change partners. Result is salt and water. Balance equation before doing calculations. Just about every exam for CHE 115 has had at least one titration calculation without a 1:1 ratio.
The equivalence point is when the moles of H+ and OH- are equal, NOT when the moles of acid and base are equal. For an acid such as HCl and a base like NaOH, when the moles of H+ and OH- are equal, the moles of HCl and NaOH are equal. That is not the case for polyprotic acids such as H2SO4 or H3PO4 or bases such as Ba(OH)2 Endpoint is where the indicator changes color. This is *usually*, but not always after the equivalence point. It is always best to go past the end point to make sure you have completed the titration. If the indicator was chosen well, the endpoint and equivalence point will be very close to the same volume.
The equivalence point can be determined in several ways. Measuring the pH is the most accurate method, but indicators can also be used. Indicators weak acids or bases that change color as the amount of H+ and OH- change. Typically change over 1-2 pH units and different indicators are used for to determine different pH changes. See Appendix in lab manual. The important thing is to understand why we need the equivalence point – to calculate the concentration of the unknown solution.
Explain set-up.
They can view the graph on their calculator or on the computer, but they should have three good titration curves before they leave the lab. What makes a good titration curve? A steep portion near the equivalence point with several data points in the region. If they have multiple outliers (i.e. pH goes up and then back down on the next drop), they should do another trial.
Before leaving the lab, all pH sensors should be stored upright so that the tip is in the storage bottle, which is on the bottom, and the tip is immersed in the solution.
- Students who have had calculus may be familiar with the term second derivative, others may not. It is okay if they don’t understand what it means, just follow the instructions.
Make a big point that acid-base titration problems are just like other stoichiometry problems. Start with a balanced chemical equation, convert to moles, use mole rations, and convert to the desired units. The main difference is that instead of talking about a product and a reactant, you are looking at two reactants. If you start with a balanced chemical equation, it doesn’t matter what of the equation the substance is on.
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