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 DO NOT

let the level of solution in your buret fall below the markings!! 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

it out.

. If you have excess 6 M NaOH, run the tap for 10-20 sec when you dump 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!! available at the back of the lab.

BE CAREFUL!!

Do not touch your face!! Gloves are

Be sure to wash and dry your lab bench Wash your hands with soap and water to remove all traces of any spilled chemicals.

before you leave lab.

NaOH • •

Acid-Base Neutralization Rxns

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

.”

H X(aq) + K OH (aq)



KX (aq) + H 2 O (l)

after eq. pt: OH ,Na + , X , H 2 O @ eq. pt: X ,Na + , H 2 O [H + ] = [OH ] before eq pt: H + , X , Na + , H 2 O 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: M 1 V 1 = M 2 V 2 where… M 1 , V 1 M 2 , V 2 = molarity and volume of first solution = molarity and volume of second solution M 1 V 1 = M 2 V V 1 = ?

M 1 = 6 M 2 V 1 = M 2 V 2 M 1 V 2 = 400 mL M 2 = 0.1 M

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.

KHC H O (aq) + NaOH(aq) 8 4 4  H O(l) + NaKC H O (aq) 2 8 4 4 • The known mass of KHP is converted to moles of NaOH.

? mol NaOH =

x

g KHC 8 H 4 O 4 g of KHP

you

weigh out mol KHC 8 H 4 O 4 g KHC 8 H 4 O 4 MM of KHP 1 mol NaOH 1 mol KHC 8 H 4 O 4 • 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)  H O(l) + NaX(aq) 2 • 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 NaOH 1 mol HX ? mol HX =

x

L NaOH L NaOH 1 mol NaOH vol of NaOH

you

delivered [NaOH] you determined previously • Since you know the volume of you acid sample, you can determine the molarity.

? [HX] = mol HX vol of HX solution

x

L HX soln

you

measured out