Quant. Chem Lab Midterm
0.25 ml of an unknown EDTA solution was needed to reach the end in a blank sample. 11.5 ml of an unknown EDTA is need to reach the endpoint with a standard Calcium sample of 100 ml at 0.1252 M. What is the
1.11 M
When titrating uncola drinks, we should theoretically be able to see how many endpoints?
3
Why use a blank titration?
This is to estimate the titration error. The blank titration tells the analyst how much titratnt is needed to create an abservable color change in a solution with no analyze. you will subtract the volume of titratnt (NaOH solution) required for the blank from the volume of titrant required for the titration of the analyze ( KHP).
polyprotic
an acid that can donate more than one proton per molecule
A student obtains the following concentrations for her sodium hydroxide solution fromthree individual titrations: 0.1000 M, 0.0998 M, 0.1004 M. Calculate the average,standard deviation, and %RSD for these results.
average = 0.1 M S= 0.00025820 M %RSD= 0.2582 %
What makes the indicator Calgamite change color?
loss of metal ions
Variance
standard deviation squared
Top-loading balance has a precision of about _______
± 1.0 mg
The phosphoric acid in a 125.00-mL sample of cola drink was titrated with 0.1323 M NaOH. If the first end point occurred after 13.55 mL of base was added, and the second end point occurred after 29.78 mL of base, calculate the concentrations of H3PO4 and H2PO4− in the cola sample.(Hint: where would the second equivalence point have occurred if only H3PO4 were present?
- 0.0143 M of H3PO4 - 0.0028 M of H2PO4-
what is the purpose of the calibration curve of laboratory equipment? How do you apply this correction?
- we use it to correct the volumes/ amount delivered or dispensed. - true volume= correction factor+ volume delivered looking at the graph, the coordinated corresponding to the volume delivered, will allow you to determine the correction factor. The two values you add to get the true volume delivered.
In the analysis of some powdered milk for melamine, the standard deviation of sampling is ± 4%, and the standard deviation of the measurement is ± 3%. What is the the overall standard deviation? To what value must the sampling standard deviation be reduced if we need an overall standard deviation of ± 4% or less? Assume the standard deviation of the measurement cannot be reduced.
-variance of sampling = S^2sampling = 4^2 = 16 variance of measurement = S^2measurement = 3^2 = 9 total variance = variance due to measuring + variance due to sampling = 16 + 9 = 25 sqrt(25) = ± 5% = overall variance -4^2= 4^2 + variance of sampling 16-9= 7 sqrt(7)= ± 2.6%
What is the Molarity of 0.1g of CaCO3 (dry) (MV 100.0869 g/mol) in 0.5 L?
0.001998
Calculate the volume of a concentrated solution of HCl required to prepare a 1 L HC lsolution of approximately 0.1 M. Concentrated HCl is ~12.1 M. Hint: think aboutsignificant figures.
0.008 L
What is the molarity of 7.5046 g of pure and dry KHP (MW 204.23) in 720 mL of DI water?
0.0510 M
A 0.5112-g sample of 99.99% pure KHP (M.W. 204.23) was dissolved in about 25 mL ofdistilled water, and titrated to the phenolphthalein end point with 26.93 mL of a sodiumhydroxide solution. Calculate the molar concentration of the sodium hydroxide solution.
0.09283 M
Calculate the volume of 50 wt% solution of NaOH required to prepare 1 L of 0.1 MNaOH solution. The density of a 50 wt% NaOH solution is 1.53 g/mL
5 mL
The standard deviation of 3 measurements of absorbance of allura red at 504 nm was determined to be 0.03. What is the variance (S^2) of the measurment?
9.0*10^-4
When an ant bites you, the reason it stings because of an injection of formic acid. when 50.00 ml of formic acid solution of 0.11 M is titrated with 40.27 ml of NaOH, what is the concentration of NaOH? A. 0.14 M B. 0.12 M C. 0.08 M D. 0.09 M
A. 0.14 M
Which of the following is true about the end point in titration? A. End point comes after the equivalence point in the titration B. Titrand and titrant are in stoichiometrically equivalent amount at the end point. C. The color of the indicator changes at equivalence point D. All of the above
A. End point comes after the equivalence point in the titration
Hydrochloric acid is a strong acid. This means that ______. A. HCl dissociates completely to H+ and Cl- when dissolved in water B. HCl produces a gaseous product when it is neutralized C. HCl cannot be neutralized by a weak base D. aqeuous solutions of HCl contain equal concentrations of H+ and OH-
A. HCl dissociates completely to H+ and Cl- when dissolved in water
If you weigh a cold object on the balance, the reading will be _____ than if it had been weighed after it warms up to room temperature. A. Higher B. Lower C. Equal D. Weighing hot object is not possible
A. Higher
A cuvette containing DI water is used as a blank in an ocean optics spectrophotometer. Which of the following is not true regarding a blank solution? A. Solution containing a known amount of analytes is the blank solution B. A solution containing all of the reagents and solvents used in the analysis and not deliberately added analyte C. A blank solution measures the response of the analytical procuedure to impurities or interfereing species in the reagents. D. Blank solution is used to calibrate the instrument
A. Solution containing a known amount of analytes is the blank solution
What is alkaline error
At a pH greater than 10.5, the glass electrode responds to other ions alongside H+ ions, making the pH appear lower than it really is
when does the color indicator change color during the titration?
At the end point, the solution changes from colorless to a very faint pink color
Which of the following volumetric glassware do you use to transfer the 25 ml of analytical reagent solution? A. 25 ml volumetric flask B. 25 ml pipet C. 50 ml beaker D. 25 ml erlenmeyer flask
B. 25 ml pipet
Calculate the volume of 50 wt% solution of NaOH required to prepare 735 ml of 0.15 M NaOH solution. The density of 50 wt% NaOH solutino is 1.53 g/ml. A. 0.576 ml B. 5.76 ml C. 22.42 ml D. 2.24 ml
B. 5.76 ml
The sources of variance in any analysis are ______ A. subtractive B. Additive C. Exponential D. constant
B. additive
What is the purpose of boiling the water used to make the NaOH solution? How might yourresults for the concentration of NaOH be affected if you did not boil the water?
Boiling helps to get rid of any dissolved carbon dioxide, and other gas, that may be present in the NaOH solution. The concentration of NaOH will decrease due to it having impurities, if it is not boiled prior to standardization.
The total variance for analysis (S^2 total) is composed of A. variance due to analytical equipment B. variance due to sampling C. Both A and B D. None of the above
C. Both A and B
In "weighing by difference", the term "difference" refers to the difference between.... A. before and after weighing B. before and after adding C. before and after dispensing D. before and after taring
C. before and after dispensing?
Beer's equation is A=εbC. Where ε is the molar absorptivity. Which of the following is true about molar absorptivity? A. ε decreases with increase in concentration of the chemical species B. ε depends on the path length of the cuvette C. ε is the measure of how strongly a chemical species absorbs light at a given wavelength D. ε is a constant, and is therefore independent of chemical species
C. ε is the measure of how strongly a chemical species absorbs light at a given wavelength
In experiment 5, The second titration, calcon indicator and pH of 12, gives the moles of
Ca2+
In experiment 5, The results of the first titration, calmagite indicator and pH of 10, give the total moles of
Ca2+ and Mg2+
What metal ions generally contribute to water hardness?
Ca2+ and Mg2+
A 100.00-mL water sample was adjusted to pH 10 and titrated to the calmagite endpoint with 10.87mL of 0.0125 M EDTA solution. Then the pH of another 100.00-mL water sample was adjusted to pH12 and titrated with EDTA to the calcon endpoint. In this second titration, 2.63 mL of titrant wasrequired. Calculate the concentration of Mg2+ and Ca2+ in the water. Express your answers in mg/L ofCaCO3 and MgCO3, respectively.
CaCO3 mg/L: 32.88 mg/L MgCO3 mg/L: 86.52 mg/L
What is the relationship between wavelength and frequency of a light wave? A. Wavelength and frequency are same at the wavelength of maximum absorbance B. wavelength and frequency are independent to each other C. As the wavelength increases, the frequency also increases D. As the wavelength increases, the frequency decreases.
D. As the wavelength increases, the frequency decreases
Which of the following statements is true about using an analytical balance. A. The weighin bottle is not used for weighing liquid and solid samples B. You can handle dry chemicals with your finger C. You should clean the balance if you fing unknown spilled chemicals on it D. The analytical balance you will use in the lab are accurate to ± 0.1 mg.
D. The analytical balance you will use in the lab are accurate to ± 0.1 mg.
Give a property that is unique to a secondary standard that primary standard does not have: A. hygroscopic B. Known molecular weight C. Accurate and precise mass D. hygroscopic
D. hygroscopic
50 mL of soft drinks was pipetted out instead of the 100 mL called for in the protocol. This was titrated with 0.10 M NaOH to determine the acid concentration. The titrated volume was used to calculate the molarity of the polyprotic acid in soft drinks. How will the change in volume change the results?
No change to the measured acid concentration
What is a metal chelation?
The capture of a metal by a multivalent organic compound
What is the purpose of sampling step in analysis? What factors would you consider in choosing a representative bulk sample of a mixture of particles? How might you reduce the sampling uncertainty in such a mixture?
Purpose of sampling step in analysis is to ensure that the sample is almost a complete reflection of the population it was sampled from. The factors that would need to be considered in choosing a representative bulk sample of a mixture of particles are the use upon which the analytical data will be put on, the accuracy and precision desired, and the costs incurred in obtaining suitable samples. How sample uncertainty will be reduced in a mixture is by homogenizing the pool from which sampling will be done and performing statistical analysis that will account for random and experimental errors.
Describe the difference between the endpoint of a titration and the equivalence point
The difference between endpoint and equivalence point is that equivalence point is where the titrant has completely neutralized the analyte. This means that the number of moles of titrant equals the same moles of analyte. Endpoint on the other hand occurs after the equivalence point. This is the point where the indicator changes color and indicates that there are more moles of titrant than there is analyte.
We need to use the pH electrode to detect the pH value of the solution. Which is correct about the pH electrode:
The difference in H+ concentration inside the electrode and in solution produces a voltage difference (Eglass) across the membrane, which is correlated to pH
Where is the equivalence point on a titration curve?
The equivalence point is the point of maximal slope on the plot of pH as a function of volume.
What are three important characteristics of a primary standard? Why does the validity of ananalytical result depend on knowing the composition of a primary standard?
Three characteristics are pure, not hydroscopic, very stable. The validity of an analytical result depends on knowing the composition of a primary standard because this value will allow you to accurately determine the concentration of the analyte at an equivalence point. Equivalence point is where the moles of primary standard equals the same moles of analyte.
Refer to the figure below. You use calibrated buret to deliver 40.21 ml of DI water. What is the true volume delivered? figure description: (x,y) = (40.21, -0.012) A. 40.21 ml B. 40.24 ml C. 40.18 ml D. -0.03 ml
True volume delivered= mass of water/ density of water density of water dependent on temperature or true volume= correction factor + volume delivered C. -0.012+ 40.21= 40.198 approx. 40 .18 ml.
What does it mean to standardize a solution
When you are standardizing a solution, you are determining the exact concentration (molarity) of a solution through a test, usually a titration.
titration curve
a graph of the pH of a solution as a function of the volume of the added titrant
complexometric titration
a titration involving the formation of a complex between metal ions and a reagent such as edta. in this type of titration the end point is marked by a sharp decrease in the concentration of free metal ions
water hardness is a measure of
the concentraiton of all the polyvalent cations dissolved in the water. Water hardness is often defined as the sum of the concentrations of Ca2+ and Mg2+ in water.
total variance equation
total variance = variance due to measuring + variance due to sampling+ variance due to sample preparation
What is sampling dependent on?
upon the use to which the analytical data will be put, upon the accuracy and precision desired, and upon the costs incurred in obtaining suitable samples
potentiometric titration
~ Form of redox titration where no indicator is used. Instead, the electrical potential difference (voltage) is measured using a voltmeter
