*Final Review Experiment 3*
what is a primary standard?
- a compound of high stability - one that can be used to establish the concentration of a standard solution (NaOH)
on the titration curve, how are the x and y axis labeled?
- x-axis = mL of titrant added - y-axis = pH
what 2 pieces of equipment are used in the titrations?
1. Erlenmeyer flask 2. buret
what are the 4 main parts to this experiment?
1. dry the KHP 2. use KHP to standardize the NaOH 3. determine the molar mass of the unknown acid by titration with the standardized NaOH (by finding endpoint) 4. titrate a specific amount of unknown acid and generate a plot of pH using a pH meter in order to determine pKa.
what are important features of a primary standard? (5)
1. high purity 2. inexpensive 3. high molecular weight 4. air stability 5. not hygroscopic
what two pieces of information do you need to identify your unknown acid?
1. molecular weight 2. pKa
what are the goals of each titration (1/2/3)?
1. to standardize the NaOH (find the known concentration) 2. to find the molar mass of the unknown acid 3. determine pKa by pH titrations
what are the 2 ways to find out the concentration of the standard solution (titrant)
1. weight it out by mass 2. use a primary standard
For every 1 of KHP, how many base molecules do you need to neutralize it? Why?
1; because KHP is monoprotic and therefore only has one acidic hydrogen
how many drops of phenolphthalein do you add per trial?
2 drops
at what pH is the equivalence point when you do a strong acid + strong base titration?
7
During a coarse titration , you placed 200 mg of unknown acid in 80 mL water, and dispensed 4 mL of NaOH to this acid solution (analyte) to reach the endpoint. What should the mass and volume of the analyte so it takes 16mL of NaOH to reach the endpoint?
800 mg in 80mL water
Select the one correct statement regarding acid-base chemistry and titrations.
A Bronsted-Lowry base is a proton acceptor.
what is the 'titration method'?
A known concentration and volume of titrant reacts with a solution of analyte to determine the concentration of the analyte.
what is the general base dissociation equation?
A- + H20 <--> HA + OH-
- The endpoint would not reasonably reflect the equivalence point - If the pKi of the indicator was greater than the pH of the equivalence point for the acid, more NaOH would have to be added to change the indicator color --> An increased volume of NaOH would lead to an artificially low MW of the acid. - If the pKi of the indicator was lower than the pH at the equivalence point for the acid, less NaOH would have to be added to cause the indicator to change color --> A decreased volume of NaOH would lead to an artificially high MW of the acid.
Assumption: Phenophthalein was an appropriate indicator for the unknown acid. If this assumption were invalid, how would the molar mass or the pKa of the unknown acid be affected?
- The mass of KHP would be artificially high because it would be weighed wet - This would require more NaOH to neutralize the KHP standard analyte, leading to an artificially high concentration of NaOH calculated - A higher concentration of NaOH would lead to an artificially low molar mass of the unknown acid
Assumption: The KHP was sufficiently dry. If this assumption were invalid, how would the molar mass or the pKa of the unknown acid be affected?
- CO2 will acidify the solution - Would have to add more NaOH to neutralize the increasingly acidic analyte - larger volume of NaOH would lead to larger moles of NaOH and an artificially high number of moles of unknown acid - Molar mass would be artificially high for the unknown acid
Assumption: The effects of CO2 can be ignored. If this assumption were invalid, how would the molar mass or the pKa of the unknown acid be affected?
You generate a titration curve with the unknown acid as the analyte and NaOH as the titrant. What info is irrelevant to the determination of the pka of the unknown acid?
Concentration of the analyte Conc. of titrant
What'll lead to artificially high molec weight?
During the calculation , you accidentally use a value for 0.010 M for [NaOH] instead of the actual concentration of 0.100 M.
What is the difference between endpoint and equivalence point in your titrations?
Equivalence point refers to the situation when there is stoichiometric equivalence of acid and base while endpoint refers to the stage when an indicator has changed color.
Why did we add phenolphthalein
Gave us the endpoint, allows us to say how many moles of base and of unknown we have in the beaker
what is the general acid dissociation equation?
HA + H20 <---> A- + H30+
- Record pH changes using a pH meter during the titration - Add only enough NaOH to the analyte to raise the pH by about 0.3 - When the pH begins to change dramatically, add NaOH drop wise - Titrate beyond the endpoint to get the data at the end of the curve
How did you collect data for the titration curve?
- Dissolved a known (and calculated) mass of the unknown acid in water and phenophthalein - Titrate the sample with NaOH to the endpoint - There is a 1:1 ratio between moles of NaOH and moles of unknown acid, so the molar mass can be found by dividing the number of moles of NaOH added to the analyte by the mass of the unknown acid first dissolved
How did you determine the molar mass of the unknown acid?
- Dissolve a known mass of KHP in water and phenolphthalein - Titrate the KHP solution with the NaOH solution until the endpoint (color change from clear to pink) - Calculate the NaOH concentration for each titration (Concentration KHP)(Volume KHP) = (Concentration NaOH)(Volume NaOH)
How did you standardize the NaOH solution?
Choose an indicator with a pKa that is as close as possible to the expected pH of the analyte solution at the equivalence point.
In order for the endpoint to best reflect the equivalence point, how should one choose an indicator?
During lecture we calculated the pH at the equivalence point of a weak acid titrated with a strong base (NaOH) in order to select the correct indicator. What info was not needed to select the appropriate indicator?
Initial pH of the weak acid (before addition of titrant)
what is the primary standard in this experiment?
KHP
Why did we use the acid potassium hydrogen phthalate (KHP) in Exp 3?
KHP is easy to purify, so we used it to standardize the titrant.
How did you standardize your base and why
KHP, it is cheap it is easy
what is the equation for the acid dissociation constant Ka?
Ka = [A-][H3O+]/[HA]
You perform a titration with the unknown acid as the analyte. What info is NEEDED to determine the molecular weight of the unknown acid?
Knowledge of stoichiometry between titrant and analyte Concentration of the titrant Volume of titrant added to analyte to neutralize the analyte
Was the pH at equivalence point valuable to you? Did you need it?
NO, you don't care what the pH at the equivalence point, you only care about the volume because from that volume you use it in the Henderson eq.
what is the standardized solution?
NaOH
what solution is put in the buret for the titrations?
NaOH
greatest
On a titration curve, the equivalence point is the volume at which the slope of the curve is the _________________ (lowest/greatest).
what is different between Titrations 1/2 and Titration 3?
T 1 and T2 use a color indication and T3 uses a pH meter
You generate a titration curve and the first derivative of the titration curve. What is the purpose of the first derivative?
To determine the volume at which the pH changes most rapidly.
Why do we standardize the base?
To find the concentration of the base
How did you get the equivalence volume?
Used pH meter, make a titration curve and identify the largest change in pH per addition of base, you want the biggest slope.
You perform a titration with the unknown acid as the analyte. What info is irrelevant to determine the molecular weight of the unknown acid?
Volume of water added to the analyte prior to starting titration Concentration of the analyte
first derivative
We can find the equivalence point of a titration curve by taking the _________ ___________.
Acid-Base Reactions
What chemistry do you take advantage of in this experiment?
- Molar Mass - pKa
What did you need to determine the identity of your unknown acid?
KCl increases the conductivity of the solution and thereby stabilizes pH measurements.
What is the purpose of adding KCl to the final titration analyte?
To use titration in order to determine the identity of an unknown weak monoprotic acid.
What is the purpose of this experiment?
neutralize
When disposing of liquid waste in this experiment, always __________ the solutions to a pH between 6 and 8.
We performed a first coarse titration in order to calculate the mass of unknown acid that would require between 15 and 20 ml of standardized NaOH solution to reach the endpoint.
Why did you perform a first coarse titration with your unknown acid?
- easy to purify - easy to dry (in the oven) - inexpensive - monoprotic salt (makes calculations easy)
Why did you use KHP to standardize the NaOH solution?
This allows for a 1:1 ratio between the unknown acid and NaOH - makes the calculations easier later on.
Why is it important that the unknown acid is monoprotic?
What situation if any, will definitely lead to an artificially low pka value?
You accidentally record the equivalence point as 8.08ml instead of the actual 8.80 mL.
What'll lead to artificially low molec weight?
You rely on a dark purple solution to indicate that the analyte has been neutralized. The volume of base that corresponds to this purple solution is used in your calculations.
Why did you have to know how much KHP you used?
You took some amount of KHP and you figured out how many moles of base was needed to neutralize
is it safer to add NaOH to water or water to NaOH?
[NaOH] to water
pH Meter
a device used to measure the pH of a solution; the probe used is an ion-selective glass electrode which is sensitive to the concentration of H3O+ in a solution
Indicator
a dye or compound that causes a visible color change of the analyte solution at a specific pH
Buret
a graduated glass tube with a tap at one end, for delivering known volumes of a liquid, especially in titrations.
Titration Curve
a graph of the pH of a solution as a function of the volume of the added titrant
Titration
a solution of known concentration is used to determine the concentration of another solution
Titrant
a solution of known concentration that is used to titrate a solution of unknown concentration
what is an indicator?
a supplemental substance that produces an OBSERVABLE PHYSICAL CHANGE in the solution at/near the endpoint
what type of reaction occurs between the two components of the titration during the titration?
acid-base reaction
Monoprotic
an acid that can donate (lose) only one proton (H+ ion) per molecule
phenolphthalein exhibits a color change in a acidic/basic pH range?
basic *bc/ the titration is a weak acid w/ a strong base = equivalence point in the basic range*
why do you need to calibrate the pH meter before you use it?
because you need to adjust the meter reading for the correct pH's.
what kind of hazard is NaOH classified as?
corrosive
the dissolution of NaOH is endothermic/exothermic and therefore can cause _____________?
exothermic; burns to the skin
what is the difference between the Titration 1 and Titration 2?
in Titration #2, you are replacing the KHP with the unknown acid.
how do you go from the mass of KHP to the [NaOH]? (1:1 stoichiometry ratio)
mass KHP --> mols KHP --> mols NaOH --> [NaOH]
Volumetric Analysis
method of analysis reliant on measurements of volume; common method used is titration; often used to study acid-base and redox reactions
at what point does the pKa = pH?
midpoint
is KHP a monoprotic or diprotic acid?
monoprotic
does the indicator react with the two components of the titration?
no
is the equivalence point measurable?
no; it is a theoretical or 'ideal' point that can be calculated from titration data.
Henderson-Hasselbalch Equation
pH = pKa + log [A-]/[HA]
At 1/2 equivalence point
pH=pKa
how do you calculate the pKa (from the Ka)?
pKa = -log[Ka]
Henderson-Hasselbalch equation
pKa=pH - log [A-]/[HA]
At 1/3 equivalence point
pKa=pH-log(0.5)=pH+0.30
What is the expression for pKa at the 1/3 equivalence point?
pKa=pH-log(1/2)
At 2/3 equivalence point
pKa=pH-log(2)=pH-0.30
what is used as the indicator?
phenolphthlalein
what is the henderson-hasselbalch equation?
pka = pH - log ([A-]/[HA])
Endpoint
point during a titration at which the indicator added to the analyte changes color
what method is associated with gravimetric analysis?
precipitation method
what does standardize mean?
precisely determine a concentration
what is a BRONSTED LOWRY base?
proton acceptor
what is a BRONSTED LOWRY acid?
proton donor
what is the 'fire diamond' for NaCl (Salt)?
red (flammability) : 0 blue (health): 1 yellow (instability/reactivity): 0 white (special): none
what is the 'fire diamond' for NaCN (sodium cyanide)?
red (flammability) : 0 blue (health): 3 yellow (instability/reactivity): 0 white (special): none
what is the 'fire diamond' for NaOH?
red (flammability) : 0 blue (health): 3 yellow (instability/reactivity): 1 white (special): ALK (alkaline)
what is the 'fire diamond' for ethanol?
red (flammability) : 3 blue (health): 2 yellow (instability/reactivity): 0 white (special): none
Analyte
solution of unknown concentration in a titration
what is another name for the titrant solution?
standard solution
the smaller the pKa value, the stronger/weaker the acid?
stronger
what is the difference between the endpoint and the equivalence point?
the endpoint is a measurable piece of data while the equivalence point is a theoretical piece of data that can be estimated from points in a titration (curve)
what piece of information is found at the end of the second titration?
the molecular weight of the unknown acid. *you can then use this to make a very specific concentration to use for the third titration*
Standardization
titrating a solution of unknown concentration with a primary standard (one that is known with high accuracy and precision)
what method is associated with volumetric analysis?
titration method
what is the purpose of a primary standard?
to find out the concentration of the solution that is going to be the standard solution
what is the point of doing a course titration?
to get a rough estimate where the endpoint is to help save some time.
what is the goal of Experiment 3?
to solve the unknown concentration of the weak organic acid through a series (of 3) titrations.
what is the first titration in this experiment?
to titrate the NaOH again the KHP in order to determine the concentration of the NaOH you will use in the following titration.
Steps
volume of NaOH→moles of NaOH→moles unknown→thru known mass→grams/mole→determine molar mass
what type of analysis are you using in this experiment?
volumetric analysis
the larger the pKa, the stronger/weaker the acid?
weaker
Equivalence Point
when the number of moles of acid and the number of moles of base are equivalent in a titration
what is the equivalence point?
where the amount of added titrant is chemically equivalent to the amount of analyte in the sample * mols of acid = mols of base*
is the endpoint measurable?
yes; it is a point that indicates the end of the titration (color change)
what kind of indicator do you want to use? (include 'endpoint' and 'equivalence point')
you want to use an indicator with a pH with a endpoint that is CLOSE to the estimated equivalence point.
