6.2 ph AND poh CALCULATIONS

Transcription

1 6.2 ph AND poh CALCULATIONS Practice (Page 239) 1. (a) 10 7 mol/l (b) mol/l (c) 10 2 mol/l (d) 10 4 mol/l (e) mol/l 2. (a) 3 (b) 5 (c) 7 (d) When a solution increases by two ph units (e.g., from 3 to 5), the overall concentration of hydronium ions decreases by 100. For example, when ph = 3, [H 3 O + (aq)] = 10 3 mol/l. When ph = 5, [H 3 O + (aq)] = 10 5 mol/l. Lab Exercise 6.A: The Relationship between ph and Hydronium Ion Concentration (Page 240) Purpose The purpose of this investigation is to create a relationship between the magnitude changes in ph and the changes in hydronium ion concentration of a solution. Problem What is the relationship between ph and hydronium ion concentration? Analysis 1. (a) (b) As the concentration of the hydronium ions decreases, the ph increases exponentially. (c) log [H 3 O + (aq)] ph Unit 3 Solutions Manual Copyright 2007 Thomson Nelson

2 (d) As the logarithm of the concentration of the hydronium ions decreases, the ph increases. This graph is unusual because it is in the second quadrant. (e) -log [H 3 O + (aq)] ph (f) As the negative logarithm of the concentration of the hydronium ions increases, the ph increases. This graph is a more practical relationship because ph and the hydronium ion concentration are easily seen as being directly proportional. Practice (Page 242) 4. The number of digits following the decimal point in the ph value is equal to the number of significant digits in the hydronium ion concentration. 5. (a) ph = log[h 3 O + (aq)] = log( ) = 2.68 (b) ph = log[h 3 O + (aq)] = log( ) = 5.0 Copyright 2007 Thomson Nelson Unit 3 Solutions Manual 175

3 (c) ph = log[h 3 O + (aq)] = log( ) = (d) ph = log[h 3 O + (aq)] = log( ) = (a) [H 3 O + (aq)] = 10 ph = mol/l = mol/l (b) [H 3 O + (aq)] = 10 ph = mol/l = mol/l (c) [H 3 O + (aq)] = 10 ph = mol/l = mol/l (d) [H 3 O + (aq)] = 10 ph = mol/l = mol/l 7. If the concentration of hydronium ions in a lake decreased by one thousand times, then the ph of the lake would increase by three ph units. One ph unit corresponds to a change of ten times in the hydronium ion concentration. 8. Bottles of acids and bases are labelled Class E: Corrosive Materials. The symbol shows a solid bar and a hand with liquids being poured on them from test tubes. Practice (Page 243) 9. poh = log[oh (aq)] = log(4.5) = [OH (aq)] = 10 poh = mol/l = mol/l 11. The ph scale provides all of the information necessary to analyze solutions for consumer, technological, and scientific contexts. Use of both scales is repetitive, unnecessary and confusing. Since the scales are opposites of one another, most people need to know only one value. Use of poh in chemistry courses is not necessary but provides more variety of questions for students to answer. Web Activity: Web Quest Bad Hair Day? (Page 243) [The students PowerPoint presentation should include results of secondary research, such as the following.] Background information about ph, the ph scale, and the ph of various household chemicals for comparison The acidity or basicity (sometimes called alkalinity) of a solution is measured using the ph scale. A ph of 7 is considered to be neutral. The more acidic a solution, the lower the ph; the more basic, the higher the ph. 176 Unit 3 Solutions Manual Copyright 2007 Thomson Nelson

4 The ph of skin and the ph of various types of shampoo in comparison. The ph of skin and hair is slightly acidic in the range of 5 to 6 and a ph balanced shampoo is in the same range. A product claiming to be ph balanced is actually just matching the slightly acidic ph of hair. Shampoo name ph Pert Plus Shampoo Plus Medium Conditioner Pert Plus Anti Dandruff Shampoo Plus Conditioner MiglioRin 5.5 Cosval Colourcare 6 System 4 Therapeutic Shampoo Suavital Biologicial Emulsion (shampoo substitute) 5.4 How hair is affected by ph, what is the usual ph of shampoos, and why shampoos are created to have this ph When a slightly acidic product is used on the hair, the hair fibres stay smooth and slippery. When the hair dries, it remains relatively shiny and tangle-free. Shampoos are made to be slightly acidic to take advantage of this effect. Soap is slightly basic, so if soap is used on the hair it actually causes the hair shaft to swell and become rough. Essential oils are removed and the hair fibres become rough and tangle easily. A definition of the term ph Balanced and why shampoo manufacturers claim this to be a good thing (with examples) There may be some misconception among consumers that ph-balanced shampoos bring the ph of the hair closer to that of water (i.e., neutral). This is not the case, as water is considerably more basic than is hair. The ph of shampoo is generally balanced to match that of the hair slightly acidic to keep the hair fibres relatively smooth. Moisturizing shampoos tend to have slightly lower phs than do cleansing shampoos. Some products, namely antidandruff shampoos, have a ph range outside the expected range of skin and hair ( ). Plan for Investigation Purpose The purpose of this investigation is to demonstrate the effect of ph on human hair. Problem What is the effect, on human hair, when it is soaked in solutions of various ph values? Copyright 2007 Thomson Nelson Unit 3 Solutions Manual 177

5 Design Small samples of hair are soaked in solutions with ph values of 2, 4, 6, 8, 10, and 12, and in pure water, for 10 min. The samples are then tested for stretchability and strength, and examined under a microscope. Materials approximately 40 hairs of the same type (preferably from the same person) 6 straws (or wooden splints) 7 clean 50 ml beakers adhesive tape solutions at ph 2.0, ph 4.0, ph 6.0, ph 10.0, and ph 12.0 distilled water paper towels microscope Procedure 1. Label each of the 7 beakers as ph 2.0, 4.0, 6.0, 8.0, 10.0, and 12.0, and control. Place approximately 10.0 ml of the correct solution (or water) into its labelled container. 2. Tape 5 hairs to the end of each straw, allowing most of the hair to be free so that it looks like a paintbrush. 3. Place each straw into one of the solutions. Leave the hair in the solution for 10 min. 4. Remove the hair samples and rinse with distilled water. Set them aside on paper towels to air dry. 5. Create a chart similar to the one below in which to record your observations. Solution ph Texture/Strength Sketch of hair Control (water) Take one hair from each straw, hold it firmly at each end, and lightly pull the hair to see how much it stretches. Qualitatively describe the texture and how brittle or stretchy it is. 7. Set up a microscope and use it to observe one hair from each of the test samples and from the control sample. Section 6.2 Questions (Page 244) 1. Pure water conducts electricity very slightly, suggesting that ions are present in solution. The only source of these ions must be the reaction of water molecules with each other. 2. Advantages to using ph rather than [H 3 O + (aq)] include: The ph scale is more convenient to work with than amount concentration because ph is given in simpler, more convenient numbers (0 14) rather than very small concentrations in scientific notation. The ph scale represents a huge range of hydronium ion concentrations, since one ph unit is equal to a change in concentration of ten times. It is easier and more convenient to determine if a solution is acidic, basic, or neutral using the ph scale. 178 Unit 3 Solutions Manual Copyright 2007 Thomson Nelson

6 3. (a) Table 2: Acidity of Foods Food [H 3 O + (aq)] (mol/l) ph oranges asparagus olives blackberries (b) On the basis of the generalization that acids taste sour and bases taste bitter, oranges, olives, and blackberries would all taste sour. Oranges would taste the most sour. 4. No. A ph of 0.0 means that the hydronium ion concentration is equal to 1.0 mol/l; [H 3 O + (aq)] = mol/l = 1 mol/l. 5. (a) Table 3: Cleaning Products Cleaning solution [OH (aq)] (mol/l) poh stain remover baking soda bleach drain cleaner (b) baking soda, bleach, stain remover, drain cleaner (c) Safety precautions for handling acids and bases are: Reduce exposure as much as possible. Avoid direct contact with skin. Use in adequately ventilated areas. If spilled, neutralize before cleanup. (Use a large quantity of a weak acid to neutralize small quantities of strong bases, and vice versa.) 6. If the solution is diluted by a factor of 1000, then its hydronium ion concentration has decreased by a factor of 10 3 and the ph has increased by 3 units from 2.00 to A ph change from 4 to 7 is an increase of 3 ph units. If each increase is equivalent to a decrease of ten times the hydronium ion concentration, then the hydronium ion concentration has just decreased by one thousand times. 9. (a) ph = log[h 3 O + (aq)] = log(10 mol/l) = 1.00 (b) ph = log[h 3 O + (aq)] = log( mol/l) = Obtain two samples of vinegar of equal volume: one from a source of normal household vinegar to act as the control and one from a fast food restaurant. Using a ph meter or test strips, measure the ph of each solution. If the ph values of both samples are equal, then the fast food restaurant s vinegar is acceptable. If the ph of the restaurant vinegar is noticeably greater than that of household vinegar, then the restaurant vinegar is diluted. The temperature of both solutions needs to be controlled and the same ph meter should be used. 11. The design is probably valid because toothpaste is diluted in your mouth in any case as you brush, so the ph measured will be approximately correct as long as pure water is used to dilute the toothpaste. An alternative design might be to squeeze toothpaste directly onto ph Copyright 2007 Thomson Nelson Unit 3 Solutions Manual 179

7 paper strips. [Note: ph strips used to test soil samples have a plastic coating on one side so that the colours can be compared without the interference of soil colours.] 12. Consumer products with a high ph include oven cleaner, drain cleaner, and stain remover. Consumer products with a low ph include lime scale remover and some toilet bowl cleaners. Extension 13. Soil ph is important for gardeners because it affects the availability of plant nutrients nutrients become less available at ph extremes. Some plants, mostly broadleaf evergreens (e.g., azalea, rhododendron, holly, and blueberry) prosper in acidic soils. If your soil is too basic, use commercial fertilizer to increase its acidity. Adding sulfur can also lower the ph to a healthy level for your evergreens. Deciduous plants also like acidic soil, but not as acidic as evergreens. Generally, a ph range of 5.2 to 6.2 is ideal. If your soil is too acidic, add lime to raise its ph. 14. Normal arterial blood ph is 7.41 and venous blood ph is A person has acidosis when the ph of arterial blood falls below 7.41, and alkalosis when ph rises above Symptoms of acidosis include osteoporosis, hyperpnea (deep, pauseless breathing), arrhythmia, arteriolar dilation, hypotension, and pulmonary edema. Treatment must involve smoking cessation. Bronchodilator drugs may reverse some airway obstruction. Oxygen may be necessary if the blood oxygen level is low. Mechanical ventilation may be necessary if respiratory acidosis is severe. In certain circumstances, sodium bicarbonate (baking soda) may be given to improve the acidity of the blood. Symptoms of alkalosis include coma, muscle twitching, spasms, nausea, and numbness. Treatment involves taking care of the cause rather than directly treating the ph. Specific treatment depends on the cause. For alkalosis caused by hyperventilation, breathing into a paper bag allows more carbon dioxide to be retained. Oxygen may also be administered. 6.3 ACID BASE INDICATORS Mini Investigation: ph of a Solution (Page 246) [Sample information is shown.] Front of card 1: ph = 6.3 Reverse of card 1: thymol blue is yellow; methyl red is yellow; phenol red is yellow Front of card 2: ph = 10.7 Reverse of card 2: thymolphthalein is blue; indigo carmine is blue; methyl red is yellow Lab Exercise 6.B: Using Indicators to Determine ph (Page 247) Purpose The purpose of this lab exercise is to use the concept of acid base indicators and the reference table of indicator colours to determine the ph of three different solutions. Problem What is the approximate ph of three solutions? Analysis According to the evidence listed below and the table of acid base indicators, the ph of solution A is between 4.4 and Unit 3 Solutions Manual Copyright 2007 Thomson Nelson