Chemistry 1212 Lab Final Exam Complete Questions and Answers

Chemistry 1212 Lab Final Exam Complete Questions and Answers Introductory Lab - CORRECT ANSWER-Purpose: Review of sig figs, slope formula (Y2- Y1/X2-X1); percent error formula ([Accepted-Experimental (/) Accepted] x 100%); rounding, scientific notation, & excel. Procedure: Measured various objects for circumference (C=2πr) and diameter (2r), then produced a graph comparing the two factors. Results: N/A Significant Figure Rules - CORRECT ANSWER-1. All non-zeros are significant. Ex: 12.12 has 4 sig figs. 2. All zeros between non-zeros are significant. Ex: 1203.03 has 6 sig figs. 3. For #'s less than 1: all zeros to the left of the 1st nonzero are NOT significant. Ex: 0. has 6 sig figs. 4. For every # with a decimal, all zeros expressed to the right of a # are significant. Ex: 0.073500 has 5 sig figs. 73500.00 has 7 sig figs. 5. When a # has no decimal, we must assume the significant numbers end when the trailing zeros begin. Ex: 73500 has 3 sig figs. Rounding Rules - CORRECT ANSWER-A) 5 or greater: round UP B) 4 or below: drop remaining digits C) Exactly 5, with no other nonzero #s to the right of it, round UP if the previous digit is odd, but leave the digit as is if it is even. Lewis Structures, Polarity, and Chromatography - CORRECT ANSWER-Purpose: Apply concepts of shape and electronegativity to determine if a molecule is polar or nonpolar; evaluate effects of intermolecular forces and molecule sizes on retention factors in paper chromatography; determine the identity of an unknown by the comparison of retention factors of known compounds. Procedure: On a 12x25cm piece of paper, spread x's about an inch from the bottom of the paper for each amino acid and the unknown. Roll into tube and place into large beaker. Add 25 ml of chromatography solution (without submerging the x's). Cover beaker with foil. Monitor solvent progress for up to 20 mins, allowing liquid to wick up to 2 cm to the top of the paper. Remove paper and draw a line above where the liquid stops, not focusing on straightness. This is the solvent front. Allow 5 mins for solvent evaporation, then spray paper with 1% ninhydrin solution in ethanol. After 3 mins of drying, heat the oven to 130 degrees for 1-2 mins. Measure the distance from the origin to the center of the darkest part of each sample smudge. The unknown should be two smudges. Results: The higher Rf values came from amino acids that were nonpolar (observed) and that were larger in size. Size is NOT related to polarity. What are the phases of chromatography? - CORRECT ANSWER-Mobile and stationary. In the experiment we performed, the solvent (chrom. solution) was our mobile factor and the stationary medium was the paper tube we created. Freezing Point Depression - CORRECT ANSWER-Purpose: Demonstrate how molar mass can be determined using the freezing point depression formula; determine freezing points of pure solvents and solutions, then use these values to determine freezing point depression; determine molar masses; analyze techniques to recognize experimental error. Procedure: Put 8 grams of lauric acid in a large test tube, then on a hot plate until the solid is completely melted (keep temp near 80 degrees celsius). Leave material in water for an extra 5 mins to ensure temp. is above expected freezing point. Use a wire with a loop to stir the liquid while its cooling to ensure uniformity. Remove test tube from hot water bath and place it into a beaker with room temperature water, immediately recording the first temp. Continue stirring with the wire and record the temp. every 5-10 secs. After the material solidifies, reheat the same sample and repeat the experiment. Next, add 1 gram of benzoic acid to the test tube and place it back in the hot water bath. Stir solute into melted solvent using the wire. After it melts, leave it for an extra 5 min. then transfer the large test tube to beaker of room temp water and record the temp. Reheat the mixture and repeat the experiment. Obtain a fresh 8-gramsample of lauric acid and add 1 gram of the unknown. Follow previous procedures to get 2 values for freezing point using the unknown and average them. Results: N/A Explanation of the freezing point depression trend lines - CORRECT ANSWER-Where the two lines intersect is the freezing point. The 1st is drawn along data points that display where the liquid is cooling or losing heat, and the temp. drops rapidly. The 2nd is drawn along data points that display the phase change that occurs when the liquid begins to solidify and the temp. begins to level out. Absorption VS. Concentration - CORRECT ANSWER-Purpose: Understand the linear relationship between light absorption and concentration for dilute solutions of dyes (Beer's Law); using the best fit line equation to deter