Teacher Notes
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Teacher Notes![]() Concentration and MolaritySuper Value Laboratory KitMaterials Included In KitBromcresol green indicator solution, 0.04%, 70 mL Additional Materials Required(for each lab group) Safety PrecautionsDilute acid and base solutions are severely irritating to eyes and skin and slightly toxic by ingestion and inhalation. Phenolphthalein indicator solution is an alcohol-based solution. It is a flammable liquid and is moderately toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information. DisposalPlease consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Flush all neutralized solutions down the drain with an excess of water according to Flinn Suggested Disposal Method #26b. Neutralize and dispose of unwanted acid solutions according to Flinn Suggested Disposal Method #24b. Neutralize and dispose of unwanted base solutions according to Flinn Suggested Disposal Method #10. Lab Hints
Teacher Tips
Answers to Prelab Questions
Sample Data{13759_Data_Table_3}
Answers to QuestionsPart A. Determining the Molarity of an Unknown Hydrochloric Acid Solution
Part B. Determining the Molarity of an Unknown Sodium Hydroxide Solution
Additional Practice Calulations
ReferencesGriswold, N. E.; Neidig, H. A.; Spencer, J. N.; Stanitski, C. Laboratory Handbook for General Chemistry; Chemical Education Resources: Palmyra, PA, 1996; pp 23–24. Recommended Products
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Student Pages
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Student Pages![]() Concentration and MolarityIntroductionHow is the concentration of a solution determined? With acid and base solutions, titrations are commonly used to determine unknown concentrations. In this laboratory activity, the concentration of an unknown hydrochloric acid solution and an unknown sodium hydroxide solution will be determined using acid/base titrations. Concepts
BackgroundPreparing Solutions {13759_Background_Equation_1}
Used in conjunction with the molecular weight, MW, of a solute, Equation 1 is used to determine the number of grams of solute needed to prepare a given volume of a solution with a specific concentration. For example, consider the preparation of 500 mL of a 0.80 M solution of sodium chloride, NaCl. The following steps outline this procedure.Step 1—Determine the number of moles necessary to prepare this solution. To do this, rearrange Equation 1 to solve for moles. moles of solute = Molarity x volume of solution {13759_Background_Equation_2}
moles of NaCl = 0.40 moles Therefore, distilled or deionized water must be added to 0.40 moles of NaCl and the resulting solution diluted to a total volume of 500 mL to prepare a 0.80 M solution.Step 2—Convert the number of moles to grams using the molecular weight. {13759_Background_Equation_3}
Therefore, 23 g of NaCl is required to prepare 500 mL of a 0.80 M sodium chloride solution.Once the calculations have been done to determine how much solute is needed to correctly prepare the solution, precise analytical techniques must be followed when actually making the solution. The steps below outline proper solution preparation procedures. Step 3—Obtain a piece of volumetric glassware calibrated to the volume needed. Volumetric glassware is glassware that has been calibrated (and marked) to hold a specific volume. The most common form of volumetric glassware used for preparing solutions is the volumetric flask, a flask that has a long, narrow neck with a marking on it. For a 100-mL volumetric flask, the mark on the neck indicates that when filled to the mark, the flask will contain exactly 100 mL. Because volumetric flasks are expensive, they may not be available for every student lab group. However, solutions are not commonly stored in volumetric flasks, so only a few volumetric flasks are necessary for an entire class to prepare solutions. One group can prepare a solution, then empty the solution into a labeled storage bottle and pass the volumetric flask on to another group. If no volumetric glassware is available for preparing solutions, the glassware must be calibrated before preparing the solution. To calibrate a piece of glassware, a specified volume is poured into the container and the liquid level marked. Step 4—Precisely weigh out the required number of grams (determined in step 2) of solid on a balance in a weighing dish. Transfer the solid to a clean, dry beaker (with a larger capacity than the necessary volumetric flask). There may be a few grains of solid left on the weighing dish, so use a wash bottle filled with distilled or deionized water to rinse any remaining solid from the weighing dish into the beaker. Rinse the weighing dish several times to make sure that all of the solid was transferred. This process of transferring every bit of the solid is called quantitatively transferring. The rinse water may be enough water to dissolve all of the solid in the beaker. If it is not, add a minimum amount of distilled or deionized water to dissolve any remaining solid. Step 5—Using a funnel, transfer the solution in the beaker to the volumetric flask. Rinse the beaker with a small amount of distilled or deionized water, transferring the rinse water through the funnel into the volumetric flask. Rinse from the beaker through the funnel into the flask several times to thoroughly rinse the beaker and the funnel. Step 6—Fill the volumetric flask with distilled or deionized water. When the flask is about one-half to two-thirds full, cap the flask and invert it several times to make sure the solution is homogeneous. Continue filling the flask until the liquid level is almost at the mark. Fill to the mark with a pipet or wash bottle containing distilled or deionized water drop-by-drop until the bottom of the meniscus is directly on the mark. Again cap the flask and invert it several times to thoroughly mix the solution. Solutions are not generally stored in volumetric flasks, so transfer the solution to a labeled bottle and cap the bottle to prevent evaporation or contamination. Titrations Several techniques can be used to determine the concentration of a solution. When the solution of unknown concentration is an acid or base, one of the most widely used techniques involves carrying out a titration. In a titration involving an acid solution of unknown concentration, a given volume of the acid is placed in a container with an indicator. Then a base solution of known concentration is added to the acid solution until the endpoint is reached. The endpoint is the point at which the number of moles of hydroxide ions, OH–, is equal to the number of moles of hydrogen ions, H+, in solution. The indicator is chosen so that it changes color at the endpoint. How does it do this? Refer to Figure 1 to follow the progress of a titration between hydrochloric acid, HCl, and sodium hydroxide, NaOH. Phenolphthalein is the indicator used in this titration. {13759_Background_Figure_1_An acid–base titration}
Before the Titration Begins. Before any NaOH is added to the acid, there are many H+ ions in solution. The presence of free H+ ions makes the solution acidic (pH below 7). At this pH, the phenolphthalein indicator is colorless.During the Titration. As NaOH is added, the OH– ions from the NaOH pair up with the H+ ions to make water molecules. As this occurs, the number of free H+ ions in solution decreases and the pH begins to rise. As long as there are more H+ ions in solution than OH– ions, however, the pH is still below 7 and the phenolphthalein indicator is still colorless. At the Endpoint. Recall that the endpoint of the titration is defined as the point when the number of OH– ions added is exactly equal to the number of H+ ions in solution. In reality, because the NaOH is being added in drops, and each drop contains about 1019 ions, a titration cannot determine exactly when the number of ions are equal. But, it can come very close. Therefore, in practice, the endpoint is defined as the point at which the indicator changes color. In this titration, by adding just a single drop, the solution turned from colorless to pink. Therefore, at this point, it is assumed for calculation purposes, that the number of moles of OH– ions is equal to the number of moles of H+ ions. Exactly how can the data collected from the above titration be used to calculate the concentration of the HCl solution? Steps 1–3 outline the basic procedure for such titration calculations. For this titration, the following data was taken. {13759_Background_Table_1}
Step 1—Determine the number of moles of base needed to neutralize the acid. Two factors must be known to calculate the number of moles of base needed to neutralize the acid—the volume of base delivered and the molarity of the base. The volume delivered is determined by counting the number of drops of NaOH added and converting drops to liters. The molarity of the NaOH is known. The number of moles of NaOH added to reach the endpoint (the pink color change) is calculated using Equation 2.
{13759_Background_Equation_4}
{13759_Background_Equation_7}
Step 2—Use the balanced chemical equation to determine the number of moles of acid neutralized. The balanced chemical equation must be considered to determine how many moles of NaOH are needed to neutralize each mole of HCl.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) From the balanced equation, it can be observed that one mole of HCl reacts with one mole of NaOH. Therefore, the ratio of NaOH to HCl is 1:1 (1 mole HCl/1 mole NaOH). Using the number of moles of base calculated in Step 1 and this ratio, the moles of acid can be calculated. It is important to make sure that the ratio is used correctly in this calculation. Set up the calculation so that molesbase cancel leaving only molesacid.{13759_Background_Equation_5}
{13759_Background_Equation_8}
molesHCl = 0.00043 moles Step 3—Calculate the molarity of the acid. To determine the molarity of the HCl solution, Equation 1 is used, substituting in the appropriate conversion factors for drops/L.{13759_Background_Equation_6}
{13759_Background_Equation_9}
A similar procedure is used to calculate the concentration of a base solution using an acid solution of known concentration. It must be pointed out that step 2 contains a critical step. In the example shown here, the moles of acid were equal to the moles of base because the ratio of the coefficients in the balanced chemical equation was 1:1. However, in a reaction involving a diprotic or triprotic acid, such as H2SO4 or H3PO4, the ratio will no longer be 1:1, but will instead be 1:2 or 1:3. This new ratio must be accounted for in step 2.
MaterialsBromcresol green indicator solution, 0.04%, 3 drops Prelab Questions
Safety PrecautionsDilute acid and base solutions are severely irritating to eyes and skin and slightly toxic by ingestion and inhalation. Phenolphthalein indicator solution is an alcohol-based solution. It is a flammable liquid and is moderately toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. ProcedurePart A. Determining the Number of Drops in One Milliliter
Part B. Determining the Molarity of an Unknown Hydrochloric Acid Solution
Part C. Determining the Molarity of an Unknown Sodium Hydroxide Solution
Student Worksheet PDF |