Common Experiments

A titration (also called volumetric analysis) is a laboratory procedure that usually involves either an acid and base neutralization reaction or a redox reaction. In a titration, two reagents are mixed, one with a known concentration and known volume (or a solid with a known mass) and one with an unknown concentration. The purpose of a titration is to find the concentration of the unknown solution. There must be some way to indicate when the two reagents have reacted essentially completely, and at the end of the titration the unknown solution’s concentration can be calculated since the volume of the solution required to complete the reaction has been accurately measured.

The titrant is the solution of known concentration and is usually placed in the burette. The burette must be rinsed with the solution to be placed in it before filling.

The solution from the burette is added to a flask that contains either a measured volume of a solution or a weighed quantity of solid that has been dissolved. An indicator that changes color at or near the equivalence point is usually added to the solution to be analyzed before titration. The solution of known concentration is then added to the flask from the burette until the color changes. The equivalence point is the point in the reaction where enough titrant has been added to completely neutralize the solution being analyzed. The end point is the point during the titration where the indicator changes color. It is important to choose an indicator that has an end point that is at the same pH as your expected equivalence point. The burette has graduations that are used to read the volume of titrant that’s added to the flask.

The data required for titrations include the mass of the dry substance to be analyzed or an accurately measured volume of the substance to be analyzed, the initial volume and final volume of titrant required to reach the end point, and the molarity of the titrant. At the equivalence point, the moles of the titrant will be equal to the moles of the substance analyzed. To obtain the moles of the unknown substance, multiply the molarity of the titrant by the volume (in liters) of the titrant. Once moles are known, just divide moles by volume and you have the molarity of the unknown substance

M= mole of solute/liter solution

If the substance to be analyzed is a solid, you will be trying to calculate the molecular weight of the unknown solid. Remember that molecular weight is grams per mole. The mass in grams will be known from the beginning of the experiment, when the solid sample was massed. You can find the moles of the unknown substance by multiplying the molarity of the titrant by the volume (in liters) of the titrant. Divide grams by moles to get molecular weight

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If you are doing a titration of a strong base with a strong acid, the equivalence point occurs at a pH of 7.00. The dilution formula can be used to calculate the moles of acid, which will equal the moles of base at the equivalence point:

M₁V₁ = M₂V₂

or moles of acid = moles of base. Don’t try to use this formula if either the acid or base is weak!

Example

Data:

Volume of unknown acid sample:	10.0 mL
Initial volume of titrant (base):	0.0 mL
Final volume of titrant (base):	20.0 mL
Molarity of titrant (base) (must be given):	1.0 M

Find the molarity of the unknown acid solution.

Explanation

Don’t forget to change mL to L.

0.020 L (titrant) × 1.0 mole/liter (titrant molarity) = 0.020 mole of base titrant At the equivalence point: moles acid = moles base = 0.020 mole of acid

Calorimetry is used to determine the amount of heat released or absorbed during a chemical reaction. In the lab we can experiment with finding the energy of a particular system by using a coffee-cup calorimeter. The coffee-cup calorimeter (shown below) can be used to determine the heat of a reaction at constant (atmospheric) pressure or to calculate the specific heat of a metal. The coffee-cup calorimeter is a double plastic foam cup with a lid; the lid has a hole in it where the thermometer pokes through.

The data to be collected include the volumes of the solutions to be mixed, the initial temperatures of each solution, and the highest temperature obtained after mixing. Accurate results depend on measuring precisely and starting with a dry calorimeter. The total volume recorded must be changed into grams (use the density and multiply density × volume = grams). The change in temperature must be calculated by subtracting the final and initial temperatures. To find the heat of reaction, multiply the specific heat capacity, the mass, and the change in temperature: q = mC_pDT.

Example

Data (for a specific heat of a metal):

Mass of the solid metal:	24.00 g
Initial temperature of the metal:	100.0ºC
Mass of water in plastic foam cup:	100.00 g
Initial temperature of the water:	25.0ºC
Highest temperature of the water:	30.0ºC

Find the specific heat of the metal.

q = mC_pDT(C_p = 4.18 J/g ºC)

Explanation

• Temperature change for the metal = 100.0 - 30.0 = 70.0ºC
• Temperature change for the water = 30.0 - 25.0 = 5.0ºC
• We can assume that the heat lost by the metal should equal the heat gained by the water.
• Calculate the heat gained by the water:
• Heat gained by water = (sp. heat water)(mass of water)(DT water)= (4.18 J/g ºC)(100.0 g)(5.0ºC) = 2090 joules gained
• Then find the specific heat of metal:

(Small values for metals are very typical!)

Many experiments require the use of stoichiometry to find an unknown. One typical experiment is the neutralization of an acid with a base to produce a salt and water. If a known volume and concentration of an acid and a base are reacted, the amount of salt produced can be predicted. Typical data would require the accurate recording of molarities and volumes of the acid and the base. The mass of the reacting vessel must be recorded, and then the two solutions are mixed. The mixed solution is then evaporated over a low flame (to avoid spattering and loss of mass) to dryness. The vessel is allowed to cool, and the final weight is then recorded. The vessel should be heated to a constant mass in this type of experiment. This requires heating, cooling, and weighing until two consecutive measurements are within an acceptable range of each other. The mass of the solid salt obtained is calculated by subtracting the mass of the vessel from the total mass of the vessel with the salt. This mass is known as the actual (or experimental) yield. To find the theoretical yield (what should have been produced), a balanced chemical equation must be written. The moles of each substance reacted must be determined (multiply molarity by volume) and also the moles of the limiting reagent (using coefficients). From the moles of limiting reagent, multiply by the mole ratio and then convert to grams using the formula weight of the salt from the periodic table. This gives the theoretical yield of salt. It is typical to calculate the percent yield. The closer to 100%, the better your results were!