QUESTIONS REVIEW Lab 1 Measuring Devices Which of the following devices is appropriate to measure the stated quantity? 1. 23.4 g NaCl a) analytical balance b) two-pan balance c) top-loading balance 2. 5.4 ml water a) blow-out pipette b) graduated cylinder c) automatic pipetter 3. 0.9 ml 0.005 M HCl a) automatic pipetter b) blow-out pipette c) 10 ml beaker 4. 32 mg NaOH crystals a) top-loading balance b) analytical balance c) two-pan balance True or False: 1 ml of 0.3 M NaCl weighs 1 g Calculating Molarity and Preparing Solutions 45.1 g sucrose is dissolved in water for a final volume of 400 mL sucrose solution. What is the molarity of this solution?
You are trying to prepare a 0.5 M solution of NaCl. Why shouldnt you add a liter of water to 29.22 g NaCl?
You have a stock solution of 2 M CaCl2 and another stock solution of 0.1 M HCl. You want to prepare a solution of 1 M CaCl2 and 0.01 M HCl with a final volume of 500 ml. How would you go about it?
pH and Buffers Why is a neutral solution defined as pH 7?
Statistics For each of the statistics below, determine whether it is a measure of the center of data or a measure of the spread of the data. 1. mean 2. standard deviation 3. mode 4. median 5. 95% confidence interval
Why cant you calculate the population mean length of Paramecia?
Explain the difference between independent and dependent variables.
REVIEW QUESTIONS: KEY Lab 1 Measuring Devices Which of the following devices is appropriate to measure the stated quantity? 5. 23.4 g NaCl a) analytical balance b) two-pan balance c) top-loading balance 6. 5.4 ml water a) blow-out pipette b) graduated cylinder c) automatic pipetter 7. 0.9 ml 0.005 M HCl a) automatic pipetter with a capacity of 1000 ul! b) blow-out pipette c) 10 ml beaker 8. 32 mg NaOH crystals a) top-loading balance b) analytical balance c) two-pan balance True or False: 1 ml of 0.3 M NaCl weighs 1 g False! The relationship 1 ml = 1 g is true for water only. Calculating Molarity and Preparing Solutions 45.1 g sucrose is dissolved in water for a final volume of 400 mL sucrose solution. What is the molarity of this solution? 1) first determine the number of moles of sucrose in 32 g: 45.1 g/ (342.3 g/mol sucrose) = 0.13 mol sucrose 2) then divide by the final volume of the solution to obtain molarity: 0.13 mol/0.4 L = 0.325 M sucrose
You are trying to prepare a 0.5 M solution of NaCl. Why shouldnt you add a liter of water to 29.22 g NaCl? Molarity is defined as moles of solute per liter of solution, not per liter of solvent. Therefore the final volume of the solution, after the addition of solute, is what counts when you are calculating molarity. In this case, the NaCl will take up space in your beaker, so if you were to simply 1 add L of water to the salt, the final volume of the solution would be greater than 1 L and the molarity would be less than 0.5.
You have a stock solution of 2 M CaCl2 and another stock solution of 0.1 M HCl. You want to prepare a solution of 1 M CaCl2 and 0.01 M HCl with a final volume of 500 ml. How would you go about it? 1) Use the formula C1V1 = C2V2 to find the volume of concentrated calcium chloride you will need to add to your final solution (2 M CaCl2)V1 = (1 M CaCl2)(0.5 L) V1 = 0.25 L or 250 ml CaCl2 2) Use the same formula to find the volume of concentrated HCl you will need to add (0.1M HCl)V1 = (0.01 M HCl)(0.5 L) V1 = 0.05 L or 50 ml HCl 3) Now determine how much water you will need to add to bring your solution up to final volume 500 ml final volume (250 ml CaCl2 solution + 50 ml HCl solution) = 200 ml water to add
pH and Buffers Why is a neutral solution defined as pH 7? The pH scale was defined using the ion product constant of water: Kw = [H+][OH-] = 10-14 This equation is telling you that H+ and OH- ions are constantly forming in water from the dissociation of H2O molecules. If you multiply the concentrations of H+ and OH- ions in water at any given moment, the product will always be 10-14. Since pH = - log[H+], pH = 7 when the concentration of H+ ions equals the concentration of OH- ions. [10-7] * [10-7] = 10-14
Statistics For each of the statistics below, determine whether it is a measure of the center of data or a measure of the spread of the data. 1. mean - center 2. standard deviation - spread 3. mode - center 4. median - center 5. 95% confidence interval - spread
Why cant you calculate the population mean length of Paramecia? To calculate the mean length of Paramecia in the population, you would have to measure every individual in the population that is, you would have to measure the length of every paramecium in the world. In biological research, you will always be calculating means, standard deviations, etc. for samples: the subset of the population which you actually measure.
Explain the difference between independent and dependent variables. The change in the independent variable is unaffected by change in the dependent variable. However, the dependent variable must change when the independent variable changes. As an example, imagine you are measuring the weights of caterpillars every day for a month. Your independent variable is time (measured in days) and your dependent variable is caterpillar weight (measured in grams.) How many days have passed does NOT depend on how heavy your caterpillars are. However, your caterpillars likely get heavier as time passes, because each day they eat a little more. Therefore, caterpillar weight is dependent on time.