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Experiment #4 Determina/on of Empirical Formula A"known"weight"of"magnesium"is"reacted"with"hydrochloric"acid"to"produce"hydrogen"gas"and"magnesium" chloride.""The"product"is"isolated"and"its"weight"used"to"calculate"the"amount"of"chloride"that"combined" with"the"magnesium.""Using"the"atomic"weights,"a"ratio"of"magnesium"atoms"to"chloride"atoms"is" calculated,"which"is,"in"its"simplest"form,"the"empirical"formula."

PURPOSE" " "

" "

To"isolate"a"product"in"reasonable"yield"and"purity." To"use"laboratory"analysis"to"determine"an"empirical"formula."

DISCUSSION((( The"Law$of$De)inite$Proportion"(or"Constant"Composition)"states"that"when"two"elements"combine,"they" do"so"in"a"?ixed"ratio"by"weight.""Stated"another"way,"a"given"compound"always"contains"exactly"the"same" proportion"of"elements"by"weight.""For"example,"when"magnesium"is"reacted"with"oxygen,"the"ratio"of"the" weight"of"magnesium"to"the"weight"of"oxygen"that"reacts"is"always"1.5"to"1.""The"magnesium"oxide"always" contains"magnesium"and"oxygen"in"the"ratio"of"1.5"to"1"by"weight."""

The"Atomic"Theory"of"Matter"and"the"periodic"table"can"help"us"interpret"these"results.""From"their" positions"on"the"periodic"table"it"can"be"seen"that"magnesium"will"lose"two"electrons"and"oxygen"will"gain" two"electrons"to"form"the"neutral"compound"MgO"in"a"ratio"of"1"to"1"by"atoms.""This"integer"ratio"of"atoms" is"the"empirical"formula.""The"ratio"of"Mg"to"O"by"weight"will"be"24.305"g"of"magnesium"to"15.9994"g"of" oxygen,"which"is"equal"to"1.5191"to"1"(1.5"to"1)."

The"purpose"of"this"laboratory"is"to"learn"how"an"empirical"formula"can"be"determined"experimentally."" Consider"an"experiment"in"which"a"known"weight"of"an"element"reacts"with"a"second"element"to"produce" a"new"compound.""By"measuring"the"weight"of"the"compound"produced,"the"weight"of"the"second"element" that"reacted"can"be"determined.""Conservation"of"mass"allows"us"to"subtract"the"weight"of"the"?irst"species" from"the"total"weight"to"give"the"weight"of"the"second"species.""By"using"the"known"atomic"weights"of" these"elements,"the"ratio"of"weight"can"be"easily"converted"to"a"ratio"of"numbers"of"atoms"(or"moles).""This" simplest"ratio"is"known"as"the"empirical"formula."

This"experiment"involves"the"reaction"of"magnesium"with" hydrochloric"acid"to"form"magnesium"chloride,"the"goal"being" determining"the"formula"of"MgxCly.""The"aqueous"acid"dissolves" the"metal"by"reacting"with"it"to"form"soluble"magnesium" chloride"and"hydrogen"gas.""Excess"acid,"water"and"gas"will"be" removed"by"heating"on"a"Bunsen"burner,"leaving"the"dry,"solid" salt"as"a"product." $

X""Mg"(s)"

+"

Y""HCl"(aq)""

!"

MgxCly"+"

Y 2

""H2"(g)"

53 Chem 161 Laboratory - Leeward CC - Spring 2021

Example( Aluminum"is"a"shiny"metal"which,"when"heated"in"the"presence"of"oxygen,"forms"aluminum"oxide,"a"dull" gray"powder.""" A"0.1349"g"sample"of"pure"aluminum"heated"in"oxygen"produces"0.2599"g"of"aluminum"oxide.""Additional" heating"does"not"increase"the"weight"of"the"product,"indicating"that"all"the"aluminum"has"combined"with" oxygen"and,"furthermore,"that"no"more"oxygen"will"combine"with"the"aluminum"oxide"formed.""" The"increase"in"mass"of"0.1250"g"(0.2599g"-"0.1349g)"is"due"to"the"addition"of"oxygen.""Therefore,"0.1349" g"of"aluminum"combines"with"the"?ixed"amount"of"0.1250"g"of"oxygen." The"empirical"formula"of"this"oxide"of"aluminum"can"be"determined"by"converting"the"ratio"of"weights" into"a"ratio"of"atoms.""Since"we"are"working"with"a"large"number"of"atoms,"we"will"determine"the"ratio"of" moles,"which"contain"6.02"X"1023"atoms." "

Al"

0.1349"g"

""

1"mole"Al""

" " "

" " O"

" " " " 0.1250"g"

" " ""

26.981"g"Al"

"

"

"

"

15.999"g"O"

The"ratio"in"moles" " " " " " " " " "

"

1"mole"O""

0.007813"mole"O" 0.00500"mole"Al" " " "

="""

0.005000"mole"Al"

="""

0.007813"mole"O"

="

1.562"mole"O"/"1"mole"Al"

="

1.5""mole"O"/"1"mole"Al"

This"leads"to"the"formula"Al1O1.5,"which"cannot"be"true,"since"a"molecule"cannot"contain"a"fraction"of"an" atom;"all"subscripts"in"chemical"formulas"must"be"whole"numbers.""Therefore,"the"least"integer"common" multiple"or"ratio"is"Al2O3,"which"is"obtained"by"multiplying"the"experimentally"determined"ratio"by"two.""" The"other"choice"would"be"to"multiply"by"3,"obtaining"a"ratio"of"3"to"4.686"or"approximately"3"to"5"and"a" formula"of"Al3O5.""Note"that"this"ratio"is"getting"quite"high"and"that"4.686"is"a"long"way"from"5;"this"is"not" the"correct"ratio.""Dalton"emphasized"small"simple"whole"number"ratios.""So"Al2O3"is"the"best"choice.""" The"formula"determined"corresponds"to"that"implied"by"the"periodic"table"and"atomic"theory.""The"stable" aluminum"ion"is"Al+3,"while"that"of"oxygen"is"O-2.""The"formula"of"the"neutral"ionic"compound"formed" from"these"ions"must"be"Al2O3." This"idea"of"small"numbers"in"the"ratios"is"not"true"for"organic"compounds,"which"can"have"high"ratios" such"as"C8H18"(octane)"or"as"C27H46O"(cholesterol).""

54 Chem 161 Laboratory - Leeward CC - Spring 2021

Theoretical(Yield,(%(Error(and(Theoretical(Yield( It"is"unlikely"that"the"experimental"results"obtained"will"be"as"exact"and"precise"as"those"described"above."" It"is"instructive"to"compare"the"experimental"results"with"that"of"theory.""One"way"to"do"so"is"to"compare" the"actual"amount"of"oxygen"combined"to"the"theoretical"amount.""The"difference"between"these"values"is" most"useful"if"expressed"as"a"percentage,"as"percentages"are"independent"of"the"amount"of"sample"used."" All"%"difference"or"%"error"calculations"involve"the"following"equation." " "

" "

%"Error" " "

=""""Experimental"value"-"Theoretical"value"""X"100%" " Theoretical"Value"

In"the"previous"example,"the"theoretical"weight"of"oxygen"combined"is"determined"from"the"amount"of" aluminum"limiting"reagent." "

"

0.1349"g"Al" (

1฀mol฀Al 3฀mol฀O 15.9994฀g฀O )"( )"( )"" 26.981฀g฀Al 2฀mol฀Al 1฀mole฀O

" " " " " " " " " " =""""" " " " Therefore," " " %"Error" =" 0.1250"""-"""0.11999""""X"100"%" =" " " " " " """"""""0.11999" " " " " " " " " " " " " =" 4.2"%" Note"the"2"signi?icant"?igures"from"the"subtraction." "

0.1250"–"0.11999""

"

="

0.11999"g"O"

0.1200""g"O"

4.175"%" " "

"

"

"

="0.00501"(2"s.f.)"

% Errors can be positive or negative. Negative means the answer falls below the true or theoretical value. Positive Error means the measurement falls above the true or theoretical value.

We can also calculate the percent yield. In this case 0.1349"g"sample"of"pure"aluminum"heated"in"oxygen" produces"0.2599"g"of"aluminum"oxide.""But"we"want"to"calculate"how"much"aluminum"oxide"should"have" been"produced"by"the"aluminum"and"compare"that"to"what"we"got. " "

" "

%"Yield" " "

=""""Experimental"value"" """"""Theoretical"Value"

" "

" "

" "

" "

="""""0.1250"g""""""X"100%" """"""""0.11999"g"

"

"

"

"

="""""104.1753"%

"

"

%"yield"

X"100%"

="""""104.2"%

In this case the yield is greater that what we expected, implying there is some extra impurities. This could happen if the aluminum contained other metals that reacted with more oxygen or organic compounds that partially oxidized. In many cases excess yield is due to the sample not being completely dried, the extra weight being water. 55 Chem 161 Laboratory - Leeward CC - Spring 2021

PROCEDURE(

1.""Place"a"small"clean"evaporating"dish"on"a"ring"stand"with"a"clay"triangle"and"heat"over"a"well"adjusted" ?lame"for"a"few"minutes"to"remove"volatiles"such"as"water.""Cool"and"weigh"the"dish"to"the"nearest"mg.""The" ?lame"must"be"well"adjusted"to"avoid"adding"soot"to"the"bottom"of"the"dish,"which"would"alter"its"weight." 2.""Repeat"step"one"and"record"a"second"weight.""The"weights"will"not"be"the"same,"but"they"should"be" close"(a"few"mg).""This"should"give"some"assessment"of"the"accuracy"of"your"procedure"and"technique" while"con?irming"that"the"dish"has"been"?ired"to"a"constant"weight.""If"the"weights"differ"dramatically," repeat"until"a"constant"weight"has"been"obtained." 3.""Obtain"some"magnesium"and"lightly"sand"it"until"it"is"all"shiny.""This"removes"the"magnesium"oxide" coating,"which"will"not"react.""Place"about"0.15"g"of"this"magnesium"in"the"dish"and"weigh"it." 4.""Set"the"dish"in"the"clay"triangle"on"the"ring"stand.""Very"slowly"(one"drop"at"a"time)"add"5"mL"of"dilute" (6M)"HCl"to"the"dish.""Avoid"splattering"and"loss"of"material." 5.""Make"sure"the"reaction"has"subsided"and"is"complete.""If"small"pieces"of"magnesium"have"not"dissolved," a"few"additional"drops"of"HCl"may"be"added." 6.""Gently"warm"the"dish"by"fanning"the"?lame"of"a"Bunsen"burner"under"and"around"the"dish.""Try"to" evaporate"all"the"water"without"splattering"the"salt." 7.""After"it"appears"dry,"the"dish"should"be"heated"for"5"to"10"minutes"to"drive"off"all"moisture.""Allow"the" dish"to"cool"and"weigh"with"the"contents." 8.""Heat"the"dish"again"for"?ive"minutes,"cool"and"reweigh.""Repeat"until"a"constant"weight"(within"the"error" of"the"balance)"is"obtained."

"

56 Chem 161 Laboratory - Leeward CC - Spring 2021

Exp."#4"Composition(of(Magnesium(Chloride-( ( Determination(of(Empirical(Formula"Report(Sheet"

(

(

(

(

(

(

(

Student ID number A

B

C

D

E

F

G

H

For your data set, substitute the number found above for each letter to the below. Data( weight"of"dish"" " " "

st

1 "heating" nd

2 "heating" rd

3 "heating"

"

"

48."3"A"B"g"

"

"

"

48."2"C"D"g"

"

"

"

48."1"E"F"g"

"

"

"

"

"

49."5"A"B"g"

"

49."3"C"D"g" 49."1"E"F"g"

weight"of"dish"and"Mg" " " " " 48."4"G"H"

"

"

"

"

""g"

"

"

"

"

"

""g"

"

"

"

"

"

"

""g"

"

"

"

"

"

"

weight"of"dish"and"product" " " "

st

1 "heating" nd

2 "heating" rd

3 "heating"

You"can"use"this"space"for"some"quick"calculations"to"decide"which"set"of"data"to"use"for"the"?inal"weight"of" dish"and"product.""You"will"present"the"complete"solution"with"the"best"data"on"the"next"page."

You"must"use"the"?inal,"dry"weight"of"the"dish.""You"need"to"choose"the"weight"of"dish"and"product"that" gives"the"best"answer."

I"acknowledge"that"it"is"a"violation"of"the"Student"Conduct"Code"to"use"graded"laboratories"from"previous" semesters"to"assist"me"in"my"work.""I"did"not"and"will"not"use"graded"laboratories"to"assist"me."

"

"

"

"

"

"

"

"

"

"

signed""

"

"

"

"

"

57 Chem 161 Laboratory - Leeward CC - Spring 2021

Calculations$-$show$all$work$$ Weight"of"Magnesium"

"

"

"

"

"

"

"

"

""g"Mg"

Weight"of"chlorine" "

"

"

"

"

"

"

"

"

""g"Cl"

Moles"of"Magnesium""

"

"

"

"

"

"

"

"

""mole"Mg"

Moles"of"Chlorine"

"

"

"

"

"

"

"

"

"

""mole"Cl"

Empirical$Formula$ $

$

$

$

$

$

$

$

$

$

Theoretical"weight"of"chlorine"required"" " to"react"with"the"Mg"sample"

"

"

"

"

"

"g"Cl"theoretical"

%"error"in"mass"of"chlorine" "

"

"

"

"

"

"%"error"

"

"

$

58 Chem 161 Laboratory - Leeward CC - Spring 2021

QUESTIONS( 1.""The"magnesium"chloride"is"dif?icult"to"dry"and"get"to"a"constant"weight.""If"all"the"water"was"not" removed,"how"would"this"affect"the"experimentally"determined"formula"of"the"product"magnesium" chloride?""Will"it"be"more"likely"to"be"MgCl"or"MgCl3?""(Think"about"how"this"mistake"affects"the"data."" What"number"would""be"different"and"what"change"would"that"cause.)"

2."""Heating"the"?inal"product"to"a"constant"weight"can"be"dif?icult.""If"the"product"is"splattered"from"the" dish"or"sublimed"during"drying,"how"would"this"affect"the"experimentally"determined"formula"of"the" product?"

3.""An"acid"was"determined"to"be"40.0%"C","6.7"%"H"and"53.3"%"O"by"weight.""What"is"the"empirical" formula?""Hint:"When"given"experimental"results"in"%’s"by"weight,"assume"100"g,"then"there"is"40.0"g"of"C," 6.7"g"of"H"and"53.3"g"of"O.""With"a"molecular"weight"of"about"60"g/mol"and"using"your"knowledge"of"the" nomenclature"of"acids,"give"the"proper"formula"and"name"this"acid."

59 Chem 161 Laboratory - Leeward CC - Spring 2021

4.""There"are"two"common"oxides"of"chromium;"one"is"76.5"%"chromium"and"the"other"is"68.4"%" chromium"by"weight.""Calculate"the"formula"and"name"each"compound."

5.""A"newly"synthesized"compound"is"known"to"contain"Zn"and"O.""When"a"5.0"g"sample"of"Zn"is"oxidized" (burnt)"it"forms"6.5"grams"of"product,"an"oxide"of"Zn.""What"is"the"formula"and"name"of"the"new" compound?"

6.""What"is"the"percent"error"in"the"weight"of"oxygen"combined"in"the"previous"question?"

60 Chem 161 Laboratory - Leeward CC - Spring 2021

7.""An"acid"is"2.4"%"H,"39.0"%"S"and"58.6"%"O,"by"mass.""What"is"the"empirical"formula?""Using"your" knowledge"of"acids"containing"polyatomic"ions,"give"the"proper"formula"of"the"acid"and"name"it."

8.""An"unknown"compound"is"25.5"%"C,"6.4"%"H"and"68.1"%"O"by"weight.""The"molecular"weight"is"about" 280"g/mol.""What"is"the"molecular"formula?"

61 Chem 161 Laboratory - Leeward CC - Spring 2021...