2010 a Level CH P2 Ans

GCE “A” Level H2 Chemistry Nov 2010 Paper 2 Suggested Solutios 1 !a" C2O4 2- 2CO2 + 2e- x 3 FeO4 2- + 8H + + 3e- Fe 3+ + 4H2O x 2 2FeO4 2- + 16H + + 3C2O4 2- 6CO2 + 2Fe 3+ + 8H2O Note: Question says FA1 is acidified, thus must include H + . !#" Making the assumption that no. of dops is die!t"# popotiona" to $o"ume of so"utions used %that means the $o"ume of ea!h dop is &eing !onsistent'( suppose )e pipette 2*. !m 3 of ,a2C2O4 %.1 mo" dm -3 '( the estimated tite $o"ume %FeO4 2- ' )ou"d &e + . 2* 32 1+ × .8 !m 3 . /his tite $o"ume is too sma"" an d "e ad s to p o& "e ms of a!!u a!# in measu emen t an d he n!e the !on!enta tio n of ,a2C2O4 pe paed is not sui ta& "e and shou"d &e "age to o&tain a "age and moe suita&"e tite $o"ume %shou"d &e aound 2*. !m 3 '. !$" 1. 0eigh a!!uate"# a&out 1. g of so"id anh#dous ,a 2C2O4 in a !"ean d# )eighing &ott"e. e!od the eadings a!!oding"#. Mass of )eighing &ott"e + ,a2C2O4 g m1 Mass of empt# )eighing &ott"e g Mass of )eighing &ott"e + esidua" ,a 2C2O4 g m2 Mass of ,a2C2O4 tansfeed g m1 % m2 Note: Reweighing the bottle after transferring the solid is important to mention so that the mass of solid that actually dissolves in the beaker is known accurately. 2. isso"$e the so"id )eighed in the &eake and tansfe it and the )ashings into a 2* !m 3 $o"umeti! f"ask and make up to the mak )ith disti""ed )ate. hake to ensue a homogene ous so"ution o&tained. 5a&e" this so"ution &A 2. Note: annot prepare a standard solution using beakers, conical flasks or measuring cylinders . !tate the volume of flask to be used. ustifi!ation fo !on!entation of &A2 in!e 2FeO4 2- ≡ 3C2O4 2- ( deno ting !on!e ntation of &A 1 as C1( its $o"ume used as 71( &A 2 as C2( its $o"ume as C2( then 2 2 1 1 " " 3 2 an estimate of !on!entation of &A 1 1+ 3 32 1++ . + 2 × × × .213 mo" dm -3 /aking that fo a!tua" titation( 2*. !m 3 of &A 2 is pipetted and 2*. !m 3 of &A1 is euied fo !omp"ete ea!tion( an estimated suita&"e !on!entation of &A 2 2 3 213 . + × .32 mo" dm -3 %highe than .1 mo" dm -3 oigina""#' /his means that a suita&"e amount of ,a 2C2O4 to )eigh .32 134 1+++ 2*+ × × 1. g. Note: #he $ustification for concentration of FA2 is re%uired.

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2010 a Level CH P2 Ans