Titratrions 5 pts ou wish to determine the weight percent of

Titratrions (5 pts) ou wish to determine the weight percent of copper in a copper-containing alloy. After following dissolving a sample of an alloy in nitric acid, an excess of Nal is added, and the following reaction occurs: 2 Cu(NO3 )2 (ag) + 5Nalag) 2Cul (s) Na, (ag) 4 NaNO3(aq) - + + The Nals produced in the above reaction is then titrated with sodium thiosulfate according to the equation 2Na2S2O3 (aq) Nal3 (ag) Na2S406 (aq) 3Nal (aq) + - + If26.32 mL of o.101 M NayS.0, is required for titration to the equivalence point, what is the weight percent of Cu in 0.251 g of the alloy.

Solution

Na2S2O3 moles used = M x V ( in L) = 0.101 x 0.02632 = 0.00266 mol

NaI3 moles reacted = ( 1/2) Na2S2O3 moles ( from reaction coefficients , per every 2Na2S2O3 1NaI3 reacted)

= ( 1/2) ( 0.00266) = 0.00133 mol

Cu(NO3)2 moles = 2 x NaI3 moles ( since from reaction 2 Cu(NO3)2 reacts to give 1 NaI3)

= 2 x 0.00133 = 0.00266 mol

Cu2+ moles = 0.00266 mol ( since 1 Cu(NO3)2 gives us 1Cu2+)

Cu mass = moles of Cu x atomic mass of Cu = 0.00266 mol x 63.546 g/mol = 0.169 g

Cu % in alloy = ( 100 x Cu mass / alloy mass) = ( 100 x 0.169 / 0.251) = 67.3 %