pH and Serial Dilution CHM 114 S24 (R)

Arizona State University School of Molecular Sciences 1 pH and Serial Dilution Solution a homogeneous mixture of one or more solutes dissolved in a solvent Dilution the process of adding a solvent to a solution to reduce the concentration of the solute . Solute   a substance that can be dissolved into a solution by a solvent Solvent part of a solution that is present in the greatest amount; generally, the liquid that the solute is dissolved   Molarity unit of concentration expressed as the number of moles of dissolved solute per liter of solution Molality measure of solute concentration in a solution Hydronium Ion the hydrogen ion in aqueous solution, written as H 3 O + pH Scale scale used to rank solutions in terms of acidity or basicity (alkalinity ) BACKGROUND Concentration and Dilution The concentration of any solution expresses the amount of solute present in a given amount of solution or solvent. The amount of solute is often included as a mass or number of moles, and the solution is commonly quantified as mass or volume. Dilution is the process of lowering the concentration of a solute in a solution by adding more solvents to the solution, The resulting solution is thoroughly mixed to confirm that all of the solution’s components are the same. For all experiments, it essential to know the concentration of the solution you are using. The concentration of a solution is determined from the amount of solute present in a certain amount of solvent or solution . There are many ways by which concentration is measured including molarity, molality, percent by mass, and mass/volume percent. Molarity (M) is determined by the number of moles of solute divided by the number of liters of total solution (Equation 1). Molarity (M) = molesof solute ( n ) litersof solution ( L ) Equation 1 Molality (ɱ) is determined by the number of moles of solute divided by the number of kilograms of solvent (Equation 2). Molality (m) = molesof solute ( n ) kg of solvent ( kg ) Equation 2

Arizona State University School of Molecular Sciences 2 To determine the percent by mass , divide the number of grams of the solute by the number of grams of the total solution, and then multiply by 100 (Equation 3). Percent by mass (% by mass) = g of solute ( g ) g of solvent ( g ) ∗ 100 Equation 3 To determine the mass/volume percent , divide the number of grams of the solute by the number of milliliters of the total solution, and then multiply by 100 (Equation 4). mass/volume percent (m/v%) = gof solute ( g ) volume of solution ( mL ) ∗ 100 Equation 4 Another concentration unit that is commonly used for dilute solutions is parts per million (ppm). Part per million (ppm) = gof solute ( g ) g of solution ( g ) ∗ 10 6 Equation 5 One part per million (ppm) is equivalent to one mg/L. Therefore, concentrations of molarity can be converted to ppm using this conversion factor and molar mass. The unit PPM is often used when the percentage of the chemical of interest is present in very small quantities. Writing them as percentages requires decimal points and many leading zeros. Therefore, the unit ppm over percentages  is preferred . In the laboratory there are two common types of dilutions: simple dilution and serial dilution. A simple dilution is  one in which a unit volume of a liquid material of interest is combined with  an appropriate volume of a solvent to achieve the desired concentration. A   serial dilution is   a   series of   stepwise   dilutions , where the   dilution   factor   is   held constant at each step.   Students will use a serial dilution method in this investigation. The diluted material of each previous step is used to make the subsequent dilution and the dilution factor in this example is 3. pH and pH Measurements Figure 1. Example of serial dilution of 60 uL into 120 uL. Ref: https://www.quansysbio.com/support/dilutions- explanations-and-examples/

Arizona State University School of Molecular Sciences 4 either direct or indirect effects of the change in acidity (or H +  concentration) in the marine environment. Fundamental physiological processes such as respiration, calcification (shell/skeleton building), photosynthesis, and reproduction respond to the magnitude of changes in CO 2  concentrations in seawater, along with the resultant changes in pH and carbonate ion concentrations that are expected over the next century. Solution or Dilution? Materials within the group (lab bench) kit    pH Probe + instructions Materials within TA bin • 1L bottle of 1M HCl solution  • Extra bags of serological pipets Common Group Glassware Drawer  Beakers  Graduated Cylinders  Additional Glassware PROPOSAL AND DATA The purpose of this investigation is to develop familiarity with measuring liquid volumes using common laboratory glassware, understanding how to accurately make dilutions from stock solutions, and how to use various methods to determine the pH and concentrations of acid and base solutions. In Part 1 , students will work on determining the molarity of a NaCl solution using an evaporative technique using previously determined data . In Part 2 , students will analyze the pH of household solutions, using pH paper and a pH meter using previously determined data . In Part 3 , students will generate a series of HCl samples using a serial dilution technique and measure the pH using pH meters. Students can then determine the hydronium concentration as a function of HCl concentration.

Arizona State University School of Molecular Sciences 5 In this procedure, students will measure a volume of a sodium chloride solution of unknown concentration. Then, students evaporate the water from the solution to determine the mass of sodium chloride present in the original solution. Use the following questions and tables to collect your data and write the written report. Your discussion should address (in written paragraph form) each question asked. 1. The solute OR solvent is what is being dissolved and the solute OR solvent is what is doing the dissolving. 2. Fill in the following table indicating of the glassware is considered volumetric: Glassware Volumetric - Yes or No Beaker NO Graduated Cylinder YES Serological Pipet YES 3. Why is using volumetric glassware is important for this part of the investigation? Understanding how to accurately make dilutions from stock solutions, and how to use various methods to determine the pH and concentrations of acid and base solutions. 4. What is the molar mass of NaCl (g/mol)? 58.44 g/mol 5. Record your measurements for the evaporation of a NaCl solution – from the previously determined data below (your TA will assign each group #): Data Group # Mass of Evaporating Dish (g) Volume of NaCl solution (mL) Mass of dish and NaCl solution (g) Mass of dish and dry NaCl (g) 1 27.129 10.01 39.14 30.754 2 26.432 9.98 38.41 29.618 3 28.457 11.03 41.69 31.978 4 27.894 10.36 38.25 31.201 Procedure for Part 1 C o n c e n t r a t i o n : Determining the Molarity of a NaCl Solution

Arizona State University School of Molecular Sciences 7 10.Fill in the following table indicating if the samples are acidic, basic, or neutral using the data previously determined from a pH meter. Solution pH Measurement Vinegar 2.44 Ammonia 11.79 Detergent 9.22 Deionized Water 7.32 Mouthwash 4.34 Aspirin 2.96 Solution Expected Color of Universal pH paper Acidic. Basic, or Neutral Vinegar Red Orange Acidic Ammonia Blue Basic Detergent Light blue Basic Pure Water Green Neutral Mouthwash Orange yellow Acidic Aspirin Orange Acidic In this procedure, students will use a serial dilution method to generate a series of four HCl solutions. Then, students will use a pH meter to quantitatively measure the pH of these solutions to understand the relationship between concentration, pH, and hydronium concentration. Dilutions should consist of a 10 mL total volume after dilution using a 1M HCl stock solution to make a total of 4 solutions each decreasing by one order of magnitude. Use Figure 1 to assist in the serial dilution method. After completing the required dilutions, measure the pH of each solution with a pH meter. Be sure to use ONLY volumetric glassware. Procedure for Part 3: S e r i a l Dilutions of HCl and pH Measurements with a pH Meter Figure 5. Serial dilution schematic for the sample

Arizona State University School of Molecular Sciences 8 11.To calculate the dilution factor for each dilution, divide the concentration of the starting solution by the concentration of the diluted solution. What is the dilution factor in your procedure? 12.Complete the following table with your final solution concentrations: Solution Molarity (M) Stock Solution 1 1/10 Stock Solution 1/100 Stock Solution 1/1000 Stock Solution 13.Complete the following table with your pH measurements of the 4 solutions: Solution Molarity (M) pH H+ Concentration Stock Solution 1 1/10 Stock Solution 1/100 Stock Solution 1/1000 Stock Solution 14. Complete the following table to prepare to plot your results – fill in if each variable is the independent or dependent variable: Independent OR Dependent Experimental Variable Solution Molarity (M) pH H+ Concentration Stock Solution 1/10 Stock Solution 1/100 Stock Solution 1/1000 Stock Solution 15. On a scatterplot, the independent OR dependent variable is on the X axis and the independent OR dependent variable is on the Y axis. 16. Complete scatterplots of Concentration and pH, and Concentration and H+ Concentration of the measured data. 17. Describe the two scatterplots above and what they demonstrate. Use technically sound terms such as: linear, non-linear, highly correlated, positive, negative, etc. Do not simply