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Austin Peay State University Department of Chemistry CHEM 1011 Titration of Hydrochloric Acid with Sodium Hydroxide  Cautions:  Hydrochloric acid solution is a strong acid.  Sodium hydroxide solution is a strong  base.  Both are harmful to skin and eyes.  Affected areas should be washed thoroughly with  copious amounts of water.  Purpose:    The purpose of this lab is to determine the concentration of a hydrochloric acid  so
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   Austin Peay State University Department of Chemistry CHEM 1011 Titration   of    Hydrochloric   Acid   with   Sodium   Hydroxide   Revision SP12 RBR Page 1 of 7   Cautions:   Hydrochloric   acid   solution   is   a   strong   acid.   Sodium   hydroxide   solution   is   a   strong   base.   Both   are   harmful   to   skin   and   eyes.   Affected   areas   should   be   washed   thoroughly   with   copious   amounts   of    water.   Purpose:   The   purpose   of    this   lab   is   to   determine   the   concentration   of    a   hydrochloric   acid   solution   using   acid ‐ base   titration.   Background:   Titration   is   a   technique   that   chemists   use   to   determine   the   unknown   concentration   of    a   known   solution   (we   know   what   chemical   is   dissolved,   but   not   how   much   in   a   solution).   Because   we   know   what   the   chemical   is,   we   know   how   it   will   react   with   other   chemicals   and   we   can   use   that   reaction   to   determine   the   concentration   of    the   solution   by   measuring   the   formation   of    product(s).   In   the   case   of    an   unknown   concentration   of    acid,   we   can   use   a   known   concentration   of    hydroxide   base.   This   type   of    reaction   is   a   neutralization   reaction,   where   salt   and   water   are   products   of    the   reaction:   Acid   +   Base      Salt   +   H 2 O   We   can   use   a   pH   indicator,   a   chemical   that   changes   color   depending   on   the   pH,   to   show   us   when   the   reaction   has   completely   neutralized.   This   point,   where   all   acid   was   consumed   and   there   is   no   excess   of    base,   is   called   the   equivalence   point.   We   can   use   this   equivalence   point   to   determine   the   initial   concentration   of    acid   using   a   series   of    calculations.   The   goal   of    the   titration   is   to   get   as   close   as   possible   to   the   equivalence   point   by   careful   addition   of    the   base;   this   will   ensure   the   calculated   acid   concentration   is   as   close   to   the   true   value   as   possible.   You   will   do   three   titrations   and   average   the   trials.   The   terms   below   will   help   you   understand   the   terminology   used   throughout   the   experiment:      Titrant—the   solution   of    known   concentration   is   also   called   the   standardized   solution.   In   this   lab,   the   titrant   is   sodium   hydroxide   solution.      Buret—a   long,   cylindrical   piece   of    glass   that   can   be   used   to   determine   small,   accurate   quantities   of    a   solution.   A   buret   is   controlled   by   a   stopcock,   a   white   Teflon   piece   that   can   be   turned   to   deliver   the   solution.   The   markings   on   the   buret   are   such   that   you   must   subtract   the   initial   reading   (where   the   titrant   level   is   initially)   from   the   final   reading   to   determine   the   volume   of    base   delivered.   The   buret   measures   2   digits   after   the   decimal   point   accurately.      Volumetric   pipette/pipette   bulb—a   thin   glass   tube   with   only   one   marking   used   to   measure   a   very   specific   volume   of    liquid.   You   will   use   a   pipette   bulb   to   draw   the   liquid   into   the   pipette.     Austin Peay State University Department of Chemistry CHEM 1011 Titration   of    Hydrochloric   Acid   with   Sodium   Hydroxide   Revision SP12 RBR Page 2 of 7      Phenolphthalein—a   pH   indicator.   In   acidic   and   neutral   solutions,   the   indicator   is   colorless,   but   in   a   basic   solution,   the   color   is   a   vibrant   pink.   The   higher   the   pH   is,   the   stronger   the   pink   color   is.   The   equivalence   point   will   be   when   the   color   is   a   very   faint   pink   color.   Keep   your   flask   with   acid   and   indicator   over   a   white   piece   of    paper   to   ensure   you   can   see   the   color   change.   Materials:      50 ‐ mL   Buret   with   clamp      Phenolphthalein   indicator      125   mL   or   250 ‐ mL   Erlenmeyer   flasks      Buret   funnel      250 ‐ mL   beaker      25 ‐ mL   volumetric   pipette      Pipette   bulb   Also   of    importance   in   titrations   are   the   calculations   you   need   to   determine   the   unknown   concentration   of    the   acid.   These   calculations   are   outlined   below.   You   may   want   to   refer   to   your   notes   from   lecture   for   additional   examples.      Determination   of    moles   of    base   delivered:   After   each   titration,   you   will   need   to   determine   the   number   of    moles   of    sodium   hydroxide   used.   First,   you   will   need   to   know   the   molarity   of    the   solution   (the   solution   has   been   previously   standardized  ,   meaning   it   has   a   very   accurate   molarity   that   has   been   experimentally   determined).   Write   this   down   when   you   start   the   titration.   Next,   you   must   determine   the   volume   of    the   solution   delivered   to   reach   the   equivalence   point.   Next,   you   will   find   the   moles   of    base   used   in   the   titration:            *Note   that   the   volume   of    base   is   in   L,   not   in   mL      Determine   number    of    moles   of    HCl    in    flask:   If    you   write   the   balanced   reaction   for   the   neutralization   of    sodium   hydroxide   and   hydrochloric   acid,   you   will   see   that   the   reaction   proceeds   in   a   1:1   fashion.   That   is,   for   every   hydroxide   (OH ‐ )   ion   added,   it   can   neutralize   exactly   one   hydronium   (H + )   ion.   This   is   not   always   the   case   for   neutralization   reactions,   and   is   thus   not   always   the   case   for   acid ‐ base   titrations.   The   general   formula   is   below,   where   the   determined   moles   of    base   from   the   equation   above   are   multiplied   by   the   stoichiometric   ratio   found   by   looking   at   the   balanced   equation:    1#  #        Austin Peay State University Department of Chemistry CHEM 1011 Titration   of    Hydrochloric   Acid   with   Sodium   Hydroxide   Revision SP12 RBR Page 3 of 7      Determination   of    acid    concentration:   Now   that   you   know   the   number   of    moles   of    acid   in   the   flask   (at   the   start   of    the   titration,   by   the   end,   there   is   only   water   and   salt),   you   can   determine   its   initial   concentration.   Because   you   know   the   initial   volume   of    acid   used,   you   can   use   the   following   to   determine   the   concentration:          Procedure:   You   will   do   at    least    three   titrations.   If    you   add   too   much   base   and   the   solution   is   too   bright   pink,   you   will   need   to   discard   the   data   and   do   another   run.   Also,   if    your   titrations   are   greater   than   1%   different   from   each   other,   you   will   need   to   conduct   additional   titrations.   (4   columns   of    data   are   provided   for   these   purposes.)   Patience   in   this   lab   will   prevent   you   from   having   to   do   extra   trials!!!   1.   Record   the   molarity   of    the   sodium   hydroxide   solution   on   the   data   sheet   2.   Obtain   about   100   mL   of    the   sodium   hydroxide   solution   in   a   clean   beaker.   This   should   be   enough   for   the   initial   cleaning   of    your   buret   and   for   your   first   3   trials.   3.   Clean   your   buret:   Add   about   5   mL   of    the   base   solution   from   the   beaker   to   the   buret   (use   a   funnel   to   pour).   Move   the   funnel   around   while   adding   to   ensure   the   sides   of    the   buret   are   coated   with   base.   Alternatively,   you   can   remove   the   buret   with   the   5   mL   of    titrant   from   the   buret   stand   and   carefully   tilt   and   rotate   to   coat   all   interior   surfaces   with   the   titrant.   Drain   the   solution   through   the   stopcock   into   a   waste   beaker.   Repeat   this   rinse   with   a   second   5   mL   portion   of    base.   4.   Pour   more   of    the   sodium   hydroxide   solution   into   the   buret   until   it   is   near   the   0.00   mL   mark.   Open   the   stopcock   to   allow   several   drops   to   rinse   through   the   tip   of    the   buret.   This   should   eliminate   any   air   bubbles   in   the   buret   tip.   Record   your   initial   buret   reading   on   the   data   sheet   for   trial   1   (the   volume   does   not   need   to   be   exactly   0.00   mL).   5.   Draw   25.00   mL   of    the   acid   solution   into   the   volumetric   pipette   and   transfer   this   solution   into   an   Erlenmeyer   flask.   Add   2 ‐ 3   drops   of    phenolphthalein   to   the   acid   solution   in   the   flask.   6.   Place   the   flask   under   the   buret   and   start   adding   the   base   solution   to   the   Erlenmeyer   flask.   Have   one   lab   partner   swirl   the   flask   while   the   other   controls   the   stopcock.   When   pink   starts   to   develop,   add   the   solution   more   slowly.   At   this   point   you   should   add   one   drop   at   a   time   followed   by   swirling   until   a   very   light   pink   color   persists   for   at   least   30   seconds.   Remember,   the   lighter   the   pink   the   better!!!   7.   Record   the   final   reading   of    the   buret.   Wash   the   contents   of    the   flask   down   the   drain   with   water.     Austin Peay State University Department of Chemistry CHEM 1011 Titration   of    Hydrochloric   Acid   with   Sodium   Hydroxide   Revision SP12 RBR Page 4 of 7   8.   Refill   the   buret   with   more   sodium   hydroxide   solution   if    necessary.   Record   the   new   volume   under   trial   2   on   the   data   sheet.   Pipette   another   sample   of    acid   and   add   the   phenolphthalein   as   before   and   titrate   as   before.   9.   Conduct   additional   titrations   until   three   of    them   differ   by   no   more   than   1.0%.   10.   Complete   the   data   sheet   and   post ‐ lab   questions.   Show   your   work   for   full   credit!!!  
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