(b) Write the chemical equation for the dissolution of NaOH in water.

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1 Making a Solution and Measuring ph Prelab Assignment Read the entire lab. Write an objective and any hazards associated with this lab in your laboratory notebook. Answer the following 6 questions in your laboratory notebook before coming to lab. 1. Calculate the molar mass of NaOH. 2. (a) Is NaOH a strong or a weak base and what does this mean? (b) Write the chemical equation for the dissolution of NaOH in water. 3. If you dissolve 20 grams of NaOH in distilled water and bring the total volume of the solution to 0.5 L with distilled water, what will the molar concentration (M, mole/l) of NaOH in your solution be? Show all of your calculations. 4. What is the molar hydroxide concentration, or [OH - ], in the solution described in question 3 above? 5. What is the log[oh - ] for the solution described in question 3 above? 6. What is the ph of the solution you described in question 3 above?

2 Introduction ph ph is commonly used to indicate whether a solution is acidic or basic. ph is the negative log of the molar concentration of the hydrogen cation in solution or, log[h + ], and [H + ] = 10 -ph. poh is the negative log of the molar concentration of the hydroxide anion in solution, or log[oh - ], and [OH - ] = 10 -poh. The dissociation of water, H 2 O(l) H + (aq) + OH - (aq), is a reversible reaction. The equilibrium constant, K, for reversible reactions is the molar concentration of the products raised to the power of their stoichiometric coefficients divided by the molar concentration of the reactants raised to the power of their stoichiometric coefficients at equilibrium. For the dissociation of water the equilibrium constant, K w = ([H + ][OH - ])/[H 2 O]. The concentration of water in an aqueous solution is assigned a value of 1. So the equilibrium constant for the dissociation of water reduces to K w = [H + ][OH - ]. The value of K w has been determined to be So K w = [H + ][OH - ] = Recall that log (XY) = log X + log (Y). So ph + poh = 14, and ph = 14 poh. Freshly distilled water that has not been exposed to air should have a ph of 7 or its [H + ] = [OH - ]. Solutions with a ph <7 have [H+] > [OH-] and are referred to as acidic. Solutions with a ph > 7 have [OH-] > [H+] and are referred to as basic. The composition of air Recall that air is approximately 78% N 2 and 21% O 2. Air also contains approximately % CO 2. Carbon dioxide in air reacts with water exposed to air to form carbonic acid, CO 2 (g) + H 2 O(l) H 2 CO 3 (aq). Therefore, water exposed to air contains carbonic acid and often has a lower ph than anticipated. Cleaning glassware The glassware on the shelves in this laboratory is far from clean. To clean glassware you should use hot soapy water. You will find squeeze bottles near the sinks labeled detergent or soap. This is a detergent solution to be used for cleaning glassware. Once the glassware has been washed with soap and a brush, it should be rinsed thoroughly with tap water and then rinsed 2 to 3 times with small volumes (< 5 ml) of distilled or deionized water. Repeated rinsing with small volumes of water is much more effective than a single rinsing with large volumes of water. Increasing the volume of rinse water does not increase the surface area contacted by the rinse water and most of the water is wasted. Do not

3 dry the glassware with a paper towel. Distilled or deionized water will not contaminate your experiment while a paper towel might introduce contamination. Making a solution To make a solution of a known concentration from a solid 1. First complete all necessary calculations. 2. Weigh out the exact amount of solid needed. 3. Quantitatively transfer the solid into a volumetric flask of the appropriate volume. In other words transfer the solid without spilling any solid. 4. Fill the volumetric flask about ½ full with distilled water. 5. Swirl the solution until the solid has completely dissolved. 6. Add distilled water to the volumetric flask until the level of the distilled water begins to rise in the narrow neck of the flask. 7. Use a squeeze bottle filled with distilled water to carefully add distilled water to the flask until the bottom of the meniscus lies on the line. Make sure you are eye level with the meniscus. If you add too much distilled water you must start over. 8. Cap the flask and mix your solution well by inverting the flask several times. It is best to hold the cap in the neck of the flask during mixing to ensure that the cap does not come off. Measuring ph Conventional glass electrodes are selective for the H + cation and allow H + to diffuse from the solution of interest into the glass surface of the electrode. The electrode measures the electrical potential across the indicator electrode in millivolts (mv) and compares this to a reference electrode which completes the electrical circuit. A calibration curve is used to compare the millivolts measured by the electrode in the solution of interest to the millivolts measured in a solution of known ph. The ph meter contains a small computer chip that performs and stores this calibration. Procedure Making solutions Wash and label two, 500 ml volumetric flasks. Wash and dry a 50 ml beaker. Tare or zero the beaker on a top loading balance and carefully weigh as nearly as possible 1.0 gram of NaOH. Record the exact weight of NaOH measured in your lab notebook. NaOH is highly corrosive. You should not touch the NaOH. Wear gloves and safety glasses when handling the NaOH and solutions made with NaOH. Also do not spill any NaOH on the balance. If you do spill any NaOH clean it up immediately with a brush and paper towel. Quantitatively transfer the NaOH into a clean volumetric flask. Rinse the 50 ml beaker that contained the NaOH with a small volume (<5 ml) of distilled water to remove any NaOH

4 that adhered to the beaker and add this to the flask. Make your solution by adding distilled water as described previously. Wash and dry a 50 ml beaker. Tare or zero the beaker on a top loading balance and carefully weigh as nearly as possible 0.1 grams of NaOH. Record the exact weight of NaOH measured in your lab notebook. NaOH is highly corrosive. You should not touch the NaOH. Wear gloves and safety glasses when handling the NaOH and solutions made with NaOH. Transfer the NaOH into a clean 500 ml volumetric flask and make a solution by adding distilled water as described previously. Calculations Calculate the molar hydroxide concentrations [OH - ], poh and ph for each of the solutions you made. Record your calculations in your laboratory notebook. Measure the ph Measure the ph of each NaOH solution you prepared twice using a ph meter as demonstrated by your instructor. Data Solution 1 Solution 2 Rep 1 Rep 2 Rep 1 Rep 2 Calculated [OH-]: (M) (M) Calculated poh: Calculated ph: Measured ph: Questions: 1. How did your measured ph values compare to your calculated ph values for the NaOH solutions?

5 2. If your measured and calculated ph values differed why do you think this was so? 3. Compare the magnitude of the difference in the molar hydroxide concentration between solutions 1 and 2 with the magnitude (factor of 10) difference in ph between these two solutions.