CHEM 100L: Experiment 8

experiment 1 simplified

-perform a coarse titration -filled a buret with sodium hydroxide -filled a flask with hydrochloric acid, 2 drops phenolphthalein, and water -dispensed buret liquid in 2.0 mL increments until a reaction occurred

experiment 2 simplified

-perform a fine titration -prepared buret and erlenmeyer flask like experiment 1 -added final volume of dispensed buret liquid into flask -dispensed 1 drop of buret liquid until reaction occurred

Assume you titrated 15.00 mL of a 0.100 M NaOH solution to neutralize 25.00 mL of an unknown HCl solution. What is the concentration of HCl? Step 3: Convert to a concentration value using the volume of HCl used

25.00 mL HCl x 1 L HCl / 1000 mL = 0.02500 L HCl Concentration of HCl = 0.00150 moles HCl / 0.02500 L HCl = 0.0600 M HCl

if an acid is strong and the base is strong, the pH at neutralization is =

7

Phenolphthalein changes color between the pH of __ - __

8.2 - 10

HCl(aq) + NaOH(aq) = NaCl(aq) + H2O(l)

Acid Base Salt Water

Simplified equation for when mole ratios are 1:1 --

Balanced equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) M acid x V acid = M base x V base M = concentration and V = volume For this problem we are solving for M acid M acid = M base V base / V acid = (0.100 M)(15.00 mL) / 25.00 mL = 0.0600 M HCl

goal

Finding the concentration (Molarity) of an unknown acid or base using Titration

Experimental Procedures Experiment 2: Perform a Fine Titration

Fine Titration: Identifies the volume of titrant needed to reach the end point Stop the titration at about 2 mL before the coarse titration endpoint Then add titrant dropwise to find endpoint within one drop Remember to record initial and final burette volumes and to find volume dispensed: Volume dispensed = Final burette volume - initial burette volume

Assume you titrated 15.00 mL of a 0.100 M NaOH solution to neutralize 25.00 mL of an unknown HCl solution. What is the concentration of HCl? Step 2: Use a balanced equation to convert from moles of NaOH to moles HCl

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) 0.00150 moles NaOH x 1 mol HCl / 1 mol NaOH = 0.00150 moles HCl

write the chemical equation for neutralization when a strong acid and a strong base react

IDK

What do you expect to happen if you forget to add phenolphthalein to the Erlenmeyer flask?

If we forgot to add the phenolphthalein to the Erlenmeyer flask, no chemical reaction or color change would occur. Therefore, there will not be a change in the pH in the solution and you will not know the equivalence point either.

titration formula =

M acid x V acid = M base X V base

Calculation of Molarity of Acid

Molarity Acid = Molarity Base x Volume Base / Volume Acid

Molarity Acid x Volume Acid =

Molarity Base x Volume Base

experimental objective

Perform an acid-base titration using a solution of NaOH of known concentration as the titrant and a solution of HCl of unknown concentration as the analyte. You will then use your titration data to determine the concentration of the HCl solution. Balanced equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

what was the indicator used in the experiment?

Phenolphthalein

Experimental Procedures Experiment 1: Perform a Coarse Titration

Step 1: To a 50 mL burette, add 52.00 mL of 0.100 M NaOH, open stopcock and allow about 2.00 mL to flow out in order to make sure there are no bubbles at bottom of burette Step 2: Double click on burette and record initial burette volume, note that it does NOT have to be at 0.00 mL Step 3: Into a 150-mL Erlenmeyer flask, add the prepared acid solution and add your indicator Step 4: Begin coarse titration, adding base until you see color change and record the volume at which you first started seeing the color change and the final volume where color is completely pink

hypotheses

When the solution is neutralized, the moles of acid are equal to the moles of base. This is called the equivalence point. The indicator (phenolphthalein) changes color only at the equivalence point, when neutralized.

Suppose you had to titrate a solution of NaOH of unknown concentration with a solution of HCl of known concentration using phenolphthalein as an indicator. How will you determine the color change?

You would determine the color change by stopping the titration at the faintest pink, as it is closest to the equivalence point. This is expected because phenolphthalein is pink when the pH of the solution is above a pH of 7. The solution will be transparent if HCl has a pH that is greater than the equivalence point.

acid-base titration

a solution of one substance is volumetrically added with a burette to a solution of a second substance until the equivalence point

what is base?

a substance that releases HO- ions into a solution; pH of more than 7.0

the four different types of titrations

acid-base, redox, precipitation, complexometric

acid-base neutralization reaction

an acid reacts with a base to form a salt and water

what is an acid?

any compound that releases H+ ions in solution, pH less than 7.0

how does an indicator work?

color changes due to changes in pH

strong acids and bases =

completely dissociate in a solution

how are titrations done?

determined by adding to the measured sample and exactly the known quantity of another substance with which the desired constituent reacts in a definite, known proportion

acids and bases can be classified in terms of how they =

dissociate in water

acids and bases are colorless at the =

equivalence point

when titrating, you want to stop titration at the =

faintest pink, as it is closest to the equivalence

what color will an indicator be in a basic solution?

green to purple

what color will an indicator be in an acidic solution?

green to red

fine titration

identifies the volume of titrant needed to reach the end point

Volume Acid =

initial volume of acid added

if an acid is weak and the base is strong, the pH at neutralization is =

less than 7

how to determine moles of a reactant

mass of reactant / molar masses of reactants

Equivalence point is the point in which the =

moles of acid = moles of base

what is true at neutralization in terms of moles of acid and the moles of base

moles of acid are equal to the moles of base

moles of base added =

moles of acid present

at neutralization point (or equivalence point)

moles of base = moles of acid

if an acid is strong and the base is weak, the pH at neutralization is =

more than 7

Assume you titrated 15.00 mL of a 0.100 M NaOH solution to neutralize 25.00 mL of an unknown HCl solution. What is the concentration of HCl? Step 1: Determine moles of Titrant (NaOH)

n(NaOH) = M(NaOH)V(NaOH) M(NaOH) = concentration of NaOH in M1 mol/L V(NaOH) = volume of NaOH titrated from burette in Liters V(NaOH) = 15.00 mL x 1L / 1000 mL = 0.01500 L n(NaOH) = M(NaOH)V(NaOH) = (0.100 mol/L) (0.01500 L) = 0.00150 moles NaOH

weak acids and bases =

only partially dissociate into ions

equivalence point

point in the titration when the number of moles of OH- (base) equals the moles of H+ (acid)

what solution goes in the buret?

sodium hydroxide

titrant =

solution of known concentration

analyte:

solution of unknown concentration

pH indicators =

substances that change color with pH

titration

the analytical technique used to determine the unknown concentration of a solution using the known concentration of another

endpoint

the point during a titration when an indicator shows that the amount of reactant necessary for a complete reaction has been added to a solution

why are titrations done?

to detect the equivalence point

why is a coarse titration done?

to get an approximate equivalence point

to observe when the equivalence point has been reached =

use a pH indicator

Suppose you titrated a sample of acetic acid (monoprotic acid) with a 0.125 M solution of NaOH. Given the data in the table below, what is the concentration of the acetic acid solution? volume of 0.125 M NaOH dispensed (mL) 22.40 volume of acetic acid solution used (mL) 15.00 volume of water added to the HCl solution (mL) 15.00

volume of 0.0125 M NaOH = 22.40 mL volume of acetic acid solution = 15.00 mL M acid x V acid = M base x V base MV = 0.125 M NaOH (22.4 mL) = 2.8 mol of NaOH Concentration of acetic acid solution = 2.8 mol of NaOH / 15.00 mL = 0.1867 M acetic acid

Volume Base =

volume of base added (final - initial volume)

what solution goes into the Erlenmeyer flask?

water, indicator (phenolphthalein), hydrochloric acid

acid-base neutralization: H+ ions from acid and OH- ions from base form = ________ ; and the solution is at a pH of _._ (H+ + HO- -> H2O)

water; 7.0