Plan for Tues/Wed, 21/22 Oct 08
• Turn in Exp 4 Pre-lab
• Today: Exp 4, Volumetric Analysis
• Purpose:
– To standardize an ~0.1 M aqueous solution of NaOH using a
primary standard (KHP – potassium hydrogen phthalate).
– To use the standardized NaOH solution as a secondary standard
to determine the unknown molarity of an acid solution.
• THIS EXPERIMENT WILL BE WRITTEN UP AS A
FORMAL LAB REPORT.
– The formal report format will be discussed in lecture.
• THIS EXPERIMENT IS DUE AT THE BEGINNING OF
NEXT WEEK’S LAB SESSION.
Tips for success
TECHNIQUE
•
DO NOT let the level of solution in your buret fall below the markings!!
•
DO NOT force the small volume of solution out of the bottom of your pipet tip!!
•
MAKE SURE to have me check your NaOH dilution calculation and your percent-difference calculations
(these must be within 1%!!).
•
MAKE SURE to mix your diluted NaOH solution really really well!!
•
YOU NEED YOUR DILUTE NaOH SOLUTION IN PART B!! Don’t throw it away!!
PROCEDURE CHANGES:
•
•
•
PART A: Weigh out only 0.4-0.5 g of KHP (not 0.9-1.0 g).
PART B: Obtain only 50 mL of unknown acid from stockroom (not 100 mL).
Do not check out a magnetic stir bar…you will swirl your titration flasks with your hand.
WASTE
•
Everything goes in the sink. If you have excess 6 M NaOH, run the tap for 10-20 sec when you dump
it out.
•
Return unused KHP solid to the stockroom.
•
NEVER put a spatula, etc into a reagent bottle. NEVER return unused reagent to the reagent bottle.
Any unused reagent must be placed in the waste container.
•
CLEAN UP THE BALANCE AREA!! THROW AWAY THE WEIGHING PAPERS!!
SAFETY
•
KHP(s) and NaOH(aq) are toxic and corrosive!! BE CAREFUL!! Do not touch your face!! Gloves are
available at the back of the lab.
•
Be sure to wash and dry your lab bench to remove all traces of any spilled chemicals.
•
Wash your hands with soap and water before you leave lab.
Acid-Base Neutralization Rxns
NaOH
•
•
Neutralization: when an acid and a base react to form a molecule
(water or another weak acid) and an aqueous salt.
If the acid and base we use is strong, and we add enough base to
just consume all the available acid, we say the acid has been
“neutralized.”
HX(aq) + KOH(aq)

KX(aq) + H2O(l)
after eq. pt:
OH-,Na+, X-, H2O
@ eq. pt:
X-,Na+, H2O
[H+] = [OH-]
before eq pt:
H+, X-, Na+, H2O
HX
What does an endpoint look like?
TOO DARK!!!
That’s more
like it…
Making your ~0.1 M NaOH Solution
• You have 6 M NaOH, you want to make a solution that is
approximately 0.1 M.
– Using the KHP primary standard, you will determine the actual
concentration of this solution to 4 sig figs.
• Recall the dilution formula:
M1V1 = M2V2
where…
M1, V1 = molarity and volume of first solution
M2, V2 = molarity and volume of second solution
V1 = ?
M1 = 6 M
M1V1 = M2V2
V1 =
V2 = 400 mL
M2 = 0.1 M
M2V2
M1
This volume of NaOH goes in
your 500 mL Erlenmeyer flask,
and you dilute it up to ~400 mL.
Standardizing the KOH Solution
• You will dissolve a known mass of KHP in water, and then titrate it
with your ~0.1 M NaOH.
KHC8H4O4 (aq) + NaOH(aq)  H2O(l) + NaKC8H4O4 (aq)
• The known mass of KHP is converted to moles of NaOH.
? mol NaOH = x g KHC8H4O4
mol KHC8H4O4
1 mol NaOH
g KHC8H4O4
1 mol KHC8H4O4
g of KHP you weigh out
MM of KHP
• Since you know the volume of NaOH(aq) you delivered to reach the
end point, you can determine the molarity of your NaOH solution.
? [NaOH] =
mol NaOH
x L NaOH soln
vol of NaOH solution
you deliver to the
flask from the buret
Determining Unknown Molarity of an Acid
• You will titrate a sample of a monoprotic acid of unknown
concentration using your standardized NaOH solution.
HX(aq) + NaOH(aq)  H2O(l) + NaX(aq)
• Since you know the molarity of your NaOH and the volume needed
to reach the end point, you can determine the moles of acid present
in your sample.
? mol HX = x L NaOH
vol of NaOH you delivered
mol NaOH
1 mol HX
L NaOH
1 mol NaOH
[NaOH] you determined previously
• Since you know the volume of you acid sample, you can determine
the molarity.
? [HX] =
mol HX
x L HX soln
vol of HX solution
you measured out