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Unformatted text preview: choudhury (fac489) – Postlab 9 Aspirin – lyon – (51155) 1 This printout should have 20 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Answer to six significant figures. 10.0 ml of a 0.0680 g/L Cr 3+ (aq) solution is transferred to a 50.0 ml volumetric flask and enough deionized water is added to bring the total volume of solution to the 50.0 ml mark. Then 20.0 ml of the prepared solution is mixed with 60.0 ml of deionized water. Calculate the molarity of Cr 3+ (aq) in the final solution. Correct answer: 6 . 53896 × 10 5 M. Explanation: Start with the initial concentration and for each dilution multiply by the initial volume over the final volume. The first dilution is 10.0 ml to 50.0 ml, and the second dilution is 20.0 ml to 80.0 ml. So the concentration at the end of both dilutions is Conc final = 0 . 0680 g / L × 10 . 0 ml 50 . 0 ml × 20 . 0 ml 80 . 0 ml Conc final = 0 . 0034 M . But the question asks for the concentration in molarity, so you have to divide by the atomic weight of chromium (51.996 g/mole) to get moles/liter. M final = 6 . 53896 × 10 5 M . Your answer is limited to three significant digits by the volumes and concentration given in the problem. 002 10.0 points Answer to six significant figures. Suppose that 1 . 407 g of impure barium hy droxide is dissolved in enough water to pro duce 206 . 2 mL of solution and that 61 . 2 mL of this solution is titrated to the stoichiomet ric point with 32 . 7 mL of 0 . 0662 M HCl(aq). What is the percentage purity of the original sample? Correct answer: 44 . 4101%. Explanation: Ba(OH) 2 + 2 HCl ⇀ ↽ BaCl 2 + 2 H 2 O The mass of pure Ba(OH) 2 is m = (0 . 0327 L) parenleftbigg . 0662 mol HCl 1 L HCl parenrightbigg × parenleftbigg 1 mol Ba(OH) 2 2 mol HCl parenrightbigg × parenleftbigg 171 . 342 g Ba(OH) 2 1 mol Ba(OH) 2 parenrightbiggparenleftbigg 206 . 2 mL 61 . 2 mL parenrightbigg = 0 . 62485 g % purity = . 62485 g 1 . 407 g × 100% = 44 . 4101% 003 10.0 points Answer to the correct number of significant figures. If a 0.0200 M solution exhibits 50.0% trans mittance, what will be the percent transmit tance for a 0.0400 M solution of the same substance measured at the same wavelength? Correct answer: 25 . 0% transmittance. Explanation: Percent transmittance is not linearly re lated to concentration, but ansorbance is, so first you have to convert percent transmit tance to absorbance, then double the ab sorbance (because you’re doubling the con centration), and then convert the new ab sorbance back to percent transmittance. First convert percent transmittance to transmittance. 50.0% transmittance is a transmittance of 0.500....
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 Fall '08
 leytner

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