1 of 3 Week 2 Assignment You have been tasked with performing an analysis of chl

Question

1 of 3 Week 2 Assignment You have been tasked with performing an analysis of chloride in a solid sample. To do this you have made use of the precipitation reaction between silver cations and halides, such as chloride, as depicted below: To do the analysis of the solid sample you followed the following procedure to prepare your titrant, standard, and unknown solid. On the subsequent pages are copies of the results you obtained from the analysis. Use the raw data in the results, along with the described procedure, to determine the weight percent (w/w%) of chloride in the unknown solid. Prepare 500 mL of a 0.08 M solution of silver nitrate. On the analytical balance weight the required silver nitrate solid and transfer it to a 50 mL beaker. Dissolve the solid in 20-30 mL of DI water and transfer this to a dark glass bottle using a funnel if necessary. Rinse the beaker and funnel thoroughly into the bottle with DI water. Fill the bottle to approximately 500 mL with DI water. Keep the bottle out of direct light (particularly sunlight) as this can reduce the silver in the solution and weaken the titrant. Roughly weigh 2 g of calcium chloride primary standard solid into a clean, labelled, weighing bottle. Place the solid in the oven for about one hour, then allow it to cool for about 30 minutes in a desiccator before proceeding. Once cooled weigh about 1.5 gram ( 10%), record to 0.1 mg, of calcium chloride into a 50 mL beaker. Dissolve the solid in 20- 30 mL of DI water and quantitatively transfer this solution into a 250 mL volumetric flask. Rinse the beaker (and funnel if used) several times into the volumetric flask to ensure that none of the sample is lost; fill the flask to the mark with DI water. Once diluted to volume and mixed, aliquot 25 mL of this solution, using your pipette, into each of five labelled Erlenmeyer flasks. Transfer the unknown you have been given into a clean, labelled, weighing bottle, and place it in the drying oven for about an hour, allow it to cool for about 30 minutes in a desiccator before weighing the sample. Weigh about 1 gram (± 10%), recorded to 0.1 mg, of the unknown into a 50 mL beaker. Dissolve this salt with about 30 mL of DI water and quantitatively transfer it to a clean 100 mL volumetric flask. Thoroughly rinse the beaker (and funnel if used) with DI water into the flask; fill to the mark with DI water and ensure that the solution is properly mixed. Once diluted to volume and mixed, aliquot 10 mL of this solution, using your pipette, into each of five labelled Erlenmeyer flasks. Determine the weight percent (w/w%) of chloride in the unknown sample.

Explanation / Answer

Preparation of AgNO3 solution:

mass of AgNO3=42.9058g-36.4135g=6.4923g

molarity of AgNO3 prepared=mol of AgNO3/volume of solution

mol of AgNO3=mass of AgNO3/molar mass of AgNO3=6.4923g/169.87 g/mol=0.0382 mol

volume of solution prepared=0.5L(500ml)

molarity of AgNO3=0.0382 mol/0.5L=0.0764M [reference]

Preparation of std CaCl2

mass of CaCl2=44.8916g-43.4078g=1.4838g

mol of CaCl2=1.4838g/110.98g/mol=0.0134 mol

molarity of CaCl2=0.0134mol/0.250L=0.0536 M

preparation of unknown

mass of unknown=40.0479g-39.0041g=1.0749g

volume of unknown=0.1L(100ml)

Volume of titrant (AgNO3) used for std CaCl2:

1) 37.58ml-0.26ml=37.32ml

2)36.71-1.44=35.27ml

3)36.14-0.87=35.27ml

4) 37.8-1.05=36.75ml

5)36.51-1.22=35.29ml

average volume=sum of volumes/5=35.98 ml=Vstd

Using eqn, 2AgNO3 +CaCl2--->2AgCl+Ca(NO3)2

mol AgNO3/mol CaCl2=2/1

or,M(AgNO3)(VAgNO3)/Mstd*Vstd=2/1

2Mstd*Vstd=M(AgNO3)(VAgNO3)

MAgNO3=2*Mstd*Vstd/(VAgNO3)=2*0.0536 M*25.163ml/35.98ml=0.0748 M(standardized molarity of AgNO3 with respect to primary std CaCl2)

volume of titrant used for unknown

1)24.14-1.20=22.94ml

2)23.19-0.94=22.25ml

3)22.93-0.65=22.28ml

4)22.84-0.61=22.23ml

5)22.03-0.84=21.19ml

Average volume=22.18ml

M(AgNO3)(VAgNO3)=M(unknown)*V(unknown)

M(UNK)=M(AgNO3)(VAgNO3)/V(unknown)=0.0748 M*22.18ml/10ml=0.1658 M

Molarity of unknown =0.1658 mol/L So,mol of Cl- in 1L=0.1658mol

mass of Cl- in 1L=0.1658mol*35.5 g/mol=5.886g

The solution prepared was 100 ml with unknown ,so mass in 100ml(0.1L)=5.886g/1L*0.1L=0.5886 g

Actual mass of unknown used for preparing 100ml=1g

So w/w%=(0.5886g of Cl-/1g of sample)*100=58.86%

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