Transcript Chapter 1
Homework Chapter 0 - 0, 1, 2, 4 Chapter 1 – 15, 16, 19, 20, 29, 31, 34 Question: What is the molarity of a 10% (w/v) solution of glucose? Parts per million (PPM) PPM Parts per million is a convenient way to express dilute concentrations. Historically, 1 mg per liter or per 1000 ml is referred to as 1 ppm. However, this is not really the case, as parts per million should be expressed as: c ppm mg solute = 1,000,000m g solution Show that the above equation is equivalent to mg per liter. PPM c ppm mg solute = 1,000,000m g solution For dilute solutions, the density of the solution will be the same as water. Density of solution = Density of water= 1.0 g/ml c ppm mg solute 1000mgsolution 1g solution = 1,000,000m g solution 1ml solution 1g solution c ppm mg solute = 1000 ml solution Question Converting PPM to Molarity The town of Canton prohibits the dumping of copper solutions that have concentrations greater than 0.3969 ppm. When cleaning the quant lab, Dr. Skeels found a bottle labeled “copper standard - 7 mM”, is it permissible to dump this solution down the drain? Volunteers?? Preparation of Stock Solutions Solids Liquids Solution preparation cont’d Describe the Preparation of a 500.0 mL of a solution that contains 8.00 mM Cu2+ using CuSO4.5H2O (MW 149.69). 8.00mmolCu 2 1mol 8.00103 m ol Cu 2 1L 1000 mmol 1L 8.00103 m ol Cu 2 1mol CuSO 4 5 H 2O 8.00103 m olCuSO 4 5H 2O 2 1L 1L 1mol Cu 8.00103 m ol CuSO 4 5H 2O 0.5000 L 4.00103 molCuSO4 5H 2O 1L Solution preparation cont’d Describe the Preparation of a 500.0 mL of a solution that contains 8.00 mM Cu2+ using CuSO4.5H2O (MW 149.69). 4.00103 molCuSO4 5H 2O 249.69g CuSO 4 5H 2O 1 molCuSO 4 5H 2O 0.999g CuSO4 5H 2O Thus … Add ______g CuSO4.5H2O Into a volumetric flask Add about _____ ml of water Swirl to dissolve And fill to the _____ ml mark Question Using the 8 mM Cu2+ solution, prepare 20 mL of a 0.25 mM Cu2+ solution. Dilutions To make dilutions of a solution, the following equation should be employed: M i Vi = M f Vf Question Using the 8 mM Cu2+ solution, prepare 20 mL of a 0.25 mM solution. M i Vi = M f Vf 8mM Vi = 0.25mM 20ml 0.25mM 20ml = 0.625mL Vi = 8mM From a liquid – consider concentrated HCl A more difficult example Prepare a 500.0 mL of 1 M HCl. MW Wt % Density Try it out … Consider it in two steps: (1) Determine concentration of Stock (2) Make dilution (1) Concentration of Stock Must find grams of HCl per liter of solution dHCl=1.19 g/ml %HCl (w/w)=37% MW=36.46 g/mol Mass HCl per Liter Molarity 1.19g solution 1000m l 37g HCl m L solution 1L 100g Solution 4.40102 g HCl 1m ol HCl 36.46g HCl L 12 .1M 4.40102 g HCl L Dilution Determined concentration of stock is ______ M HCl. We want a 500.0 mL solution that is 1M. Mi Vi = Mf Vf 12.1M Vi = 1M 500.0mL 1M 500 .0mL Vi = = 41.322mL 12.1M NOTE Care must be exercised when handling strong acids!! (Always, Always add acid to water) Add about 300 ml of water first Then add acid Dilute to mark Homework Chapter 0 - 0, 1, 2, 4 Chapter 1 – 15, 16, 19, 20, 29, 31, 34 Chapter 3 Experimental Error And propagation of uncertainty Suppose You determine the density of some mineral by measuring its mass 4.635 + 0.002 g And then measured its volume 1.13 + 0.05 ml g m ass( g ) 4.1018 d ml volum e(m l) What is its uncertainty? Significant Figures (cont’d) The last measured digit always has some uncertainty. 3-1 Significant Figures What is meant by significant figures? Significant figures: minimum number of digits required to express a value in scientific notation without loss of accuracy. Examples How many sig. figs in: a. b. c. d. e. 3.0130 meters 6.8 days 0.00104 pounds 350 miles 9 students “Rules” All non-zero digits are significant Zeros: 1. 2. a. b. c. 3. Leading Zeros are not significant Captive Zeros are significant Trailing Zeros are significant Exact numbers have no uncertainty (e.g. counting numbers) Reading a “scale” What is the “value”? When reading the scale of any apparatus, try to estimate to the nearest tenth of a division. 3-2 Significant Figures in Arithmetic We often need to estimate the uncertainty of a result that has been computed from two or more experimental data, each of which has a known sample uncertainty. Significant figures can provide a marginally good way to express uncertainty! 3-2 Significant Figures in Arithmetic Summations: When performing addition and subtraction report the answer to the same number of decimal places as the term with the fewest decimal places +10.001 + 5.32 + 6.130 21.451 ? ___ decimal places Try this one + 1.632 x 105 4.107 x 103 0.984 x 106 0.1632 x 106 0.004107 x 106 6 0.984 x 10 + 1.151307 1.151307 xx 10 1066 3-2 Significant Figures in Arithmetic Multiplication/Division: When performing multiplication or division report the answer to the same number of sig figs as the least precise term in the operation 16.315 x 0.031 = 0 ? .505765 ___ sig figs ___ sig figs ____ sig figs 3-2 Logarithms and Antilogarithms From math class: log(100) = 2 Or log(102) = 2 But what about significant figures? 3-2 Logarithms and Antilogarithms Let’s consider the following: An operation requires that you take the log of 0.0000339. What is the log of this number? log (3.39 x 10-5) = -4.469800302 Between -5 and -4 ____ sig figs 3-2 Logarithms and Antilogarithms Try the following: 23442 x 104 Antilog 4.37 = 2.3442 ___ sigs “Rules” Logarithms and antilogs 1. In a logarithm, keep as many digits to the right of the decimal point as there are sig figs in the original number. 2. In an anti-log, keep as many digits are there are digits to the right of the decimal point in the original number. 3-4. Types of error Error – difference between your answer and the ‘true’ one. Generally, all errors are of one of three types. Systematic (aka determinate) – problem with the method, all errors are of the same magnitude and direction (affect accuracy) Can be detected and eliminated or lessened Random – (aka indeterminate) causes data to be scattered more or less symmetrically around a mean value. (affect precision) Estimated Gross. – occur only occasionally, and are often large. Treated statistically Absolute and Relative Uncertainty Absolute uncertainty expresses the margin of uncertainty associated with a measurement. Consider a calibrated buret which has an uncertainty + 0.02 ml. Then, we say that the absolute uncertainty is + 0.02 ml Absolute and Relative Uncertainty Relative uncertainty compares the size of the absolute uncertainty with its associated measurement. Consider a calibrated buret which has an uncertainty is + 0.02 ml. Find the relative uncertainty is 12.35 + 0.02, we say that the relative uncertainty is 0.02 absolute uncertainty 0.002 RelativeUncertainty magnitudeof measurement 12.35 3-5. Estimating Random Error (absolute uncertainty) Consider the summation: + 0.50 (+ 0.02) +4.10 (+ 0.03) -1.97 (+ 0.05) 2.63 (+ ?) s y sa2 sb2 sc2 ... Sy = + 0.06 3-5. Estimating Random Error Consider the following operation: 4.10(0.02) 0.0050(0.0001) 0.010406( ?) 1.97(0.04) 2 2 2 2 2 2 sb0.0001 sc 0.04 0s.a02 ... yy 4a.10 b 0.0050 c 1.97 sy 0.02897 s y 0.02897 0.010406 y = y ssyy Try this one 14.3(0.2) 11.6(0.2) 0.050(0.001) 820(10) 1030(5) 42.3(0.4) 3-5. Estimating Random Error For exponents For ya x uncert ainty in a is Sa sa x y a sy 3-5. Estimating Random Error Logarithms antilogs For For y log a y antilog a uncertainty in a is Sa uncertainty in a is Sa sa s y 0.434 a sy y 2.303sa Question y log a Calculate the absolute uncertainty in a is Sa standard deviation for a sa the pH of a solutions s y 0.434 whose hydronium ion a concentration is 0.02 -4 s y 0.434 2.00 (+ 0.02) x 10 2.00 pH = 3.6990 + ? Question y antilog a Calculate the absolute value for the hydronium uncertainty in a is Sa ion concentration for a sy 2.303sa solution that has a pH of y 7.02 (+ 0.02) [H+] = 0.954992 (+ ?) x 10-7 s y 2.303sa y Suppose You determine the density of some mineral by measuring its mass 4.635 + 0.002 g And then measured its volume 1.13 + 0.05 ml g m ass( g ) 4.1018 d ml volum e(m l) What is its uncertainty? The minute paper Please answer each question in 1 or 2 sentences 1) 2) What was the most useful or meaningful thing you learned during this session? What question(s) remain uppermost in your mind as we end this session?