100 mL volumetric flask? twepealeuly Hise the lo ml beaker into the 5. A 111. 7

Question

100 mL volumetric flask? twepealeuly Hise the lo ml beaker into the 5. A 111. 7 mg ore sample containing iron is digested with acid, filtered and transferred to a volumetric flask and diluted to exactly 200.00 mL. A 10.00 mL aliquot is transferred to a 100.00 mL volumetric flask containing hydroxylamine hydrochloride, sodium acetate and 1,10-phenanthroline and diluted to the mark. The resulting orange-red solution had an absorbance of 0.513 at 508 nm in a 1 cm cell. If the molar absorptivity of the iron- phenanthroline complex at this wavelength is 1.89 x 103 L/molecm, determine the percentage of iron in the ore sample. Procedure (in pairs) o o o All Zincon-containing solutions can be poured down the drain. The stock copper solution is to be poured in the container provided. All waste from the qualitative analysis section is to be placed in the container provided. Preparation of copper standards and unknown copper salt Obtain one of the kits provided as well as one of the unknown copper salts. All of the copper salts (which may or may not be hydrated) are in the +2 oxidation state and will have one the following anions: Ci, Br, so, and CHCOO (acetate). Also, obtain 150 mL of the pH 9 buffer. 1. 2. Using the 10 mL beaker weigh out 0.0300 to 0.0350 g (+0.0001 g) of your salt. Transfer the salt to a 100.00 mL volumetric flask using a small amount of water. Rinse the beaker 3 to 4 times and transfer the washing to the flask each time. Allow the Cu salt to completely dissolve and then dilute to the mark Transfer 1.00 mL of the solution from step 2 (using the provided pipette) into a 100.00 mL volumetric flask. To the flask add 20.00 mL of the pH 9 buffer (pipette) and, from the 3. urette provided, 2 mL of the Zincon reagent. Dilute to the mark and mix thoroughly. This is the unknown solution you will be measuring the a bsorbance of. Prepare the standards as follows: into individual 100 volumetric flasks add 1.00, 2.00, 3.00 and 4.00 ml of the standard Cu solution from the burettes provided (make sure to also record the concentration of the Cu2 solution). It need not be exactly 1.00, 2.00, etc. mL. However, the actual volume added must be known. Therefore, initial and final burette readings are required (±0.02 mL). To the individual flasks also add 20.00 mL of the pH 9 buffer and 2 mL of the Zincon reagent, as in step 3. Dilute to the mark and mix 4. Using a beaker, prepare the blank by adding 20 mL of the pH 9 buffer and 2 mL of the Zincon reagent and dilute to 100 mL with distilled water. NOTE: A VOLUMETRIC FLASK IS NOT NEEDED FOR THIS STEP, SO DON'T USE ONE!! 31

Explanation / Answer

Q5

m = 111.7 mg of ore

A = 0.513, l = 1 cm, e = 1.89*10^3

A = e*L*C

C = A/(e*L)

C = (0.513)/(1.89*10^3 )(1)= 0.0002714 M

this is in the diluted V = 100 mL

C original --> 100/10*0.0002714 = 0.002714 M

now..

Vtotal = 200 mL

mol = MV = (0.0002714)(0.2) = 0.00005428 mol of Iron

mass = mol*MW = 0.00005428*55.8= 0.003028 g = 3.028 mg

% iron = mass of iron / total mass * 100% = 3.028 /111.7*100 = 2.7108 %

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